Air Test: A general overview to testing and design

1.0 Key Principles

1. The air test procedure is set out in CIBSE TM 23 and the ATTMA TS1

2. An air test involves sealing all ‘normal’ gaps such as vents and pressurising or depressurising the building. The level of fanpower required to maintain the pressure differential indicates the ‘leakiness’ or ‘permeability’ of the building.

3.An  Air test are typically followed by an air test audit (using smoke pencils, for example) to expose and make visible the various air test leakage routes.

4. Where projects comprise large quantities of a single component, component air testing in the laboratory may be appropriate as well as on site element air testing   .

                                  

2.0 Climatic conditions

As mentioned in Chapter 1, the raised pressure differential of 50 Pascals created during an air test is quite small. Whilst this is adequate to overcome most of the common pressure differential anomalies, such a small differential is vulnerable to larger pressure differences created by climatic conditions.

Air tests uk require calm days – i.e. a reading on the Beaufort Scale of 3 or less (3.4 to 5.4 metres per second wind speed at 10 m above ground) This corresponds to a gentle breeze with leaves and small twigs in constant motion. In winter conditions, and on exposed sites therefore an air test may not be possible, although it is often possible to make allowances, so long as these are carefully recorded during the air tests uk .

 

3.0 The Test itself

Guidance on testing buildings for an air test is contained in CIBSE Technical Memorandum TM23 Testing Buildings for Airtightness and in BS EN 13829: 2001. All accredited air tightness testers test to the guidelines contained in the BS EN and within the ATTMA TS1

 

Essentially the air test process is one of pressurising or (less commonly) de-pressurising the inside of the whole building, and of measuring the rate at which air needs to be blown or sucked to maintain that pressure differential; a building suffering from high amounts of air leakage uk during the air test will equalise readily and require a greater measurable effort to maintain the 50 Pascal differential, while a air tight building will easily contain the enforced differential and require little additional input during the air test .

The pressure difference is induced by one or more calibrated fans that are normally mounted within a suitable doorway. An adjustable door panel system, sealed around the edges is used which can also be connected to large external fans via collapsible ductwork if required. The rate of the fan or the volume flow of air through the fan during the air test can be understood as the rate of air entering / escaping throughout the remainder of the building envelope.
Buildings are air tested/air leakage tested in such a way as to recreate ‘normal’ conditions. Doors and windows are closed; trickle ventilators closed, extract fans and such like are closed but not sealed. Internal doors are wedged open. All of this must be actioned prior to the air test /air tightness test
uk

If the building is under construction the air test /air leakage testing uk can be  undertaken during  working hours, but sometimes this is not practical so some scheduling of work needs to be thought through in advance. With all external doors and windows sealed shut, some work becomes impossible (such as work with solvents requiring ventilation) and internal trades are normally ‘sealed in’ for a short time, where they can carry on undisturbed throughout the air test /air tightness test uk

In existing buildings the air tests uk /air tightness tests uk are normally carried out when the building is unoccupied if possible because of the disruption.

 

4.0 Air Leakage Audits

The air test /air tightness test quantifies the rate of air leakage  through the envelope as a whole, but it cannot locate the air leakage paths. Where remedial work is required therefore, air test /air tightness tests are followed by a range of auditing techniques designed to identify the specific places where air leakage is apparent  through the building envelope.

In many cases a simple visual inspection may be sufficient – especially if undertaken by someone with experience of the likely locations of air leakage.

However, most air leakage routes are difficult or impossible to spot without visual aids during the air test /air tightness test. One common technique is to use smoke tracers – smoke pencils or smoke machines. These render the air paths visible in certain situations during the air test . The building may be positively pressurised during the air test /air tightness test and the leaks witnessed externally, or, more usually, negatively pressurised while a smoke pencil is drawn over likely gaps and defects which become visible as the smoke is sucked inwards, during the air test

Another technique, which has certain advantages and disadvantages compared to smoke tracing along with an air test /air tightness test, is the use of an infrared camera during a Thermographic test. Used either externally or internally, these thermographic cameras register the radiant heat levels of surfaces and so are able to ‘see’ for example, where cold air is cooling the fabric around a gap internally, or conversely where warm air is escaping and heating the colder materials on the external face.

To work effectively, there needs to be a recognisable difference between the internal and external ambient temperature, so before any heating has been installed and on a warm summer’s day thermography testing may not be effective. Similarly on warm and sunny days, sunshine on external surfaces can distort the true situation so it is better on such days to wait until early evening. Conversely, rain on external surfaces can be equally distorting of the true thermal situation. However the  Thermographic cameras are useful in identifying problems at high level or difficult to reach areas, and are also very helpful in identifying other construction defects such as poorly installed (or non-existent!) insulation within the fabric.

On larger commercial buildings, the air test /air leakage testing may be undertaken at the same time as ‘standard’ ventilation system commissioning and associated studies, but these are not discussed as part of this guide.

5.0 Component Testing

A distinct aspect of overall air test /air tightness testing is the individual component test. This may be undertaken quite separately, in the laboratory or by the manufacturer of a particular component. Such air testing /air tightness sampling tests may be deemed necessary on a large project where large areas of one particular type of component, for example curtain walling, are to be specified, Insitu element air testing involves isolating the area within a temporary sealed compartment, which is then pressurized during the air test , and the air leakage related to the area of interest assessed. In this way sample areas of a building may be air test /air tightness tested using smaller fans as required.

5.1 Concrete frame and panel 

 

1 Introduction

As thermal insulation levels have risen in the last few years the proportion of energy lost to draughts has increased to the extent that now in some cases around half of all heat losses are due to air leakage across the building fabric. Given that approximately half of all energy used in the UK is in buildings, it is not hard to see that draughts account for a staggering amount of energy - and therefore cost – wastage, this where the air test comes into its own

The situation is such that further increasing thermal insulation levels would be largely unproductive unless air tightness is con­scientiously addressed. Air leakage has been shown to reduce the effectiveness of thermal insulation by up to 70% and so it is clear that if energy efficiency is to be improved in buildings, the next efforts will have to focus on air test /air leakage testing.

Many people make the mistake of thinking that an airtight building is necessarily a ‘stuffy’ building. This is not the case. All buildings have to be ventilated for health and comfort and airtight buildings are no different. An adequate ventilation system (which may well include open able windows as well as fans etc.) has to be planned for every building. The difference will be that a great deal of un­planned air leakage needs to be stemmed, this can be ascertained during the air test

The additional costs of creating an ad­equately airtight building can be negligible, but even where costs are increased e.g. for the air test , these can be more than offset by a reduction in the capital cost of heating and ventilation equipment, not to mention the long term savings in energy.

Given that the vast majority of building stock is existing, a great deal of attention will need to be given, in the foreseeable future, to remedial works to existing buildings, all existing air leakage paths can quickly be found during a air test . This guide specifically in­cludes examples of good and best practice remedial work in terms of air tightness and shows that such works can offer substantial benefits without undue disruption or cost, an air test will have a low impact on site works

Air Pressure Testing hopes to provide practical guidance on how to save energy and costs and protect building fabric. On the basis that prevention is cheaper and easier than cure, one purpose of this guide is to enable Designers to design inherently more robust and durable solutions which avoid costly and time consuming remedial works after the air test after a potential air leakage failure

The general guidance here is firmly focused on the idea of practical design and detailing, and should be read in conjunction with other guidance on sustainable design, energy efficiency and air tight­ness where necessary to provide an overall design framework. The details provided have been fully costed, tested and subjected to a Defects Liability insurance assessment. They are offered as viable alternatives to standard details, and illustrate the possibili­ties that exist. It simply remains for you, the reader, to apply them appropriately in the context of your next project prior to your air test/air tightness test

 Aims of this Guide       

 

• To highlight benefits of air tightness which include both energy and cost efficiency, improved comfort and reduced risks of damage to building fabric

 

• To improve awareness of the need for air tightness in con­struction at design stage

 

   • To promote detailing and specification solutions which cre­ate airtight and efficient buildings thus reducing the need for remedial works after air leakage failure- ‘prevention rather than cure’

 

• To show that new build and remedial air tightness are achiev­able without undue cost penalties to construction works due to multiple air leakage failures, you should be passing after the first air test

 

• .and in this way to help to ‘mainstream’ the good and best practice outlined in the document

 

Target audience

 

This Guide will help all those who wish to improve the airtightness/air leakage rate and energy efficiency of buildings through their construction, e.g:

 

• clients –building owners and users, principal and specialist contractors, interior designers architects and technicians, structural engineers, building service engineers, building surveyors, quantity surveyors/ cost consultants, maintenance and facilities managers, project managers , planning officers and building control officers, funding bodies and their professional advisors, government and non-governmental agencies

 

How to use this Guide

 

This Guide is divided into six sections. The first two sections provide an overview of the issues surrounding air tightness. Sections Three, Four and Five describe the requirements for the design process, the procure­ment and the air leakage/air tightness testing involved in designing for airtight buildings.

 

Section Six provides a number of representative details which have been optimised in terms of preparing for the air test . These are compared with standard details for a variety of construction types, and costed. This section will be primarily of interest to the design team. It should be read in conjunction with sections Three, Four and Five in particular, as all details must be placed in a suitable context.

 

6.0  The Context

 

Key Principles

 

1. Most UK construction is ‘leaky’ and wastes energy and money. Building airtight buildings can save energy and money, improve comfort and reduce the risk of damage to building fabric.

2. Airtight building will NOT mean ‘stuffy’ buildings. Good ventilation is vital for health and comfort - it is the UNPLANNED air leakage that we are aiming to stem.

3. Legislation is slowly catching up with best practice in Scotland, the UK and elsewhere and we can expect a greater emphasis on airtightness in all types of construction in due course.

4. Good and Best Practice Targets of air tightness/air leakage have been set for many types of buildings and are easily achiev­able.

6.1 Infiltration, Ventilation and Airtightness

 

Air infiltration (air leakage) is the uncontrolled flow of air through gaps in the fabric of buildings; this is all the more apparent during the air tes uk. It is driven by wind pressure and temperature differences and as a result is variable, responding in particular to changes in the weather. Infiltration (air leakage) levels are strongly affected by both design decisions and construction quality.

 

Ventilation, on the other hand, is the intended and controlled in­gress and egress of air through buildings, delivering fresh air, and exhausting stale air in combination with the designed heating sys­tem and humidity control, and the fabric of the building itself.

 

Whilst some unwanted air infiltration (air leakage)  will at times aid comfort lev­els, it is not reliable and moreover brings with it a range of signifi­cant disadvantages such as high levels of heat loss, reduction in performance of the installed thermal insulation, poor comfort, poor controllability and risks to the longevity of the building fabric it­self. It cannot be considered an acceptable alternative to designed ventilation. Air Infiltration (air leakage) needs to be reduced as much as possible if we are to create efficient, controllable, comfortable, healthy and durable buildings. This can be achieved by delivering airtight buildings that pass the air test /air leakage tests first time.

 

Air tightness is a term used to describe the ‘leakiness’ of the build­ing fabric. An airtight building will resist most unwanted air infiltra­tion (air leakage) while satisfying its fresh air requirements through a control­led ventilation strategy. Most existing buildings, even those built recently, are far from being airtight and may fail an air test and because of unwanted air infiltration (air leakage) generate huge costs to owners and occupants, in envi­ronmental, financial and health terms. One way of overcoming this making sure the building passes the air test/air leakage

 

It is important to emphasise the distinction between infiltration (air leakage) and ventilation, because while the primary purpose of this document is to show how buildings can be designed and constructed to be airtight and so pass the air test /air leakage test first time, it is equally important to stress that good levels of ventila­tion and a clear ventilation strategy will be required in every case. As the saying goes: ‘build tight, ventilate right.’

 

6.2  Why Build Airtight?

 

Legislation

At a rather prosaic level, the issue is important because it is now part of the Building Regulations in England and Wales concerning non-domestic new buildings over 500 sqm in area, all will now require an air test /air leakage, and is likely to affect a wider range of buildings soon. Whilst the initial targets set for airtightness of buildings are easy to achieve, it is equally likely that once in place, those targets will be ratcheted up to create ever more airtight and efficient buildings in the UK, in line with many of our European neighbours, at present many EU countries have a much higher first air test pass rate.

 

Energy and Cost Saving

 

Typically, the largest heat losses in most buildings are related to levels of thermal insulation, followed by those related to infiltration (air leakage), followed by those related to inefficient plant. Quite rightly therefore, most efforts to save energy and costs have until recently been di­rected at increasing thermal insulation levels. But as these levels have risen, so the relative contribution of infiltration (air leakage) has increased to the point where it can represent around half of all heat loss in a build­ing. In highly insulated buildings, the percentage may be higher.

This is reflected in the fact that total space heating costs in an airtight building may be as much as 40% less than in a leaky one

We are at the stage where it is likely that any further increase in thermal insulation levels would be ineffective until levels of air tight­ness in construction have improved considerably, this is where the air test in invaluable

 

Space Heating System Reduction

Clearly there is potential to reduce the capacity of space heating systems sized to cope with current levels of heat loss if those levels can be reduced by a half or more. Ensuring you achieve a low pass rate during the air test . In addition, airtight buildings are more predictable in terms of environmental control and the capital cost savings of installing smaller heating plant may be augmented by reduced plant room sizes in certain cases and particularly by reduced running costs in the longer term. As well as reducing the need for heating plant, airtight buildings of­fer much greater potential to respond positively to the local external climate through passive, or climate responsive design strategies such as natural ventilation, day lighting, the use of thermal mass and passive solar gain. Energy savings, capital and running costs, along with CO2 emissions can thus be further reduced.

 

Comfort and Control

 

As noted above, airtight buildings are not as affected by variations in external conditions. This makes them easier to control from an Engineer or Designer’s point of view, but it also makes them more comfortable from the point of view of the occupant.

In buildings with high levels of infiltration/air leakage those occupants near draughty windows, for example, will suffer the cold, particularly on windy days, whereas those elsewhere may well suffer from too much heat locally as the system tries to raise the temperature overall. Those who try to achieve comfortable levels through the use of the provided ventilation controls will find these to be rela­tively ineffective, whereas in more airtight buildings greater levels of control and comfort generally are achievable and local control and variation by occupants can have a more direct effect. In one example of an existing superstore, the ambient temperature in the store was raised by 5oC after the store had been sealed after the air test . Complaints by occupants in leaky buildings are common, and remedial measures are usually difficult and expensive, an air test can finds all air leakage paths quickly and effectively

 

 

Deterioration of Fabric

 

Leaky buildings allow cold air in through the construction causing discomfort, they also allow warm (and often moist) air out, causing heat loss. This warm and often moist air can find itself in colder parts of the outer construction where it can cool, and the moisture in the air can condense, leading to a buildup of moisture. This in turn can lead to:

 

decay of organic materials such as timber frames

 

saturation of insulating materials thus reducing their insulative effect (which increases heat loss further)

 

corrosion of metal components

 

frost damage where moisture has accumulated on the cold side of the insulation.

 

6.3  Legislation

 

In England and Wales the relevant regulation on air tightness is contained within Approved Document L1 for dwellings and L2 for non-domestic buildings (2006). There is general encouragement to consider air tightness issues, with a target air permeability for all buildings of 10 m3/hr/m2 envelope area at 50 Pa. In L2, build­ings with a floor area of greater than 500 m2 are required to have a air test if approved details are not used. Further tightening of the regulations are due in 2010.

 

Proposals for changes to the Energy standards were issued to public consultation in March 2006, including guidance that air tightness testing would be required if the calculation of energy performance included air permeability rates lower than 10m3/m2h at 50 Pa.

 

6.4  Measurement

 

A range of units for measuring air tightness/air leakage have been used in the past and this can complicate matters. However, one method only – “air permeability” - is the measure used in European Stan­dards, the new editions of the various UK Building regulations and in CIBSE’s TM23 Air Testing methodology and has been used throughout this document. The Air Permeability is defined as the volume flow in cubic metres of air per hour per square metre of the total building surface area (including the floor) at 50 Pascals pressure differential, expressed in m3/hr/m2 @ 50 Pa.

 

The main difference between the air permeability and previous practice in the UK is the inclusion of the non-exposed ground floor in the calculation of the ‘total surface area’ of the building. The difference between the new measurements and older ones tend only to be marked therefore where there are large volumes and ground floor areas. These new rules must be taken into account during the air test

 

Of the range of measurements used previously, the “Average Air Leakage Rate (or Index)” is similar to the “Air Permeability” except that non-exposed floors are excluded from the measure­ment. Another common expression is the “Air Changes per Hour at 50 Pascals (ACH @ 50 Pa). This is a useful measurement in particular because, when divided by twenty, it gives an approxi­mate value of the natural infiltration rate of the building at normal atmospheric pressure, which can then be used to help size heating and ventilating plant etc.

 

Yet another measurement is the “Equivalent Air Leakage Area” (ELA) at 50, 10 and/or 4 Pascals. This figure gives a representation of the sum of all of the individual cracks, gaps and openings as a single orifice and helps to visualise the scale of the air leakage problem. The main problem of changing the measurement technique is the ability to compare data

The standard pressure differential used is 50 Pascals. This is not in fact a very large pressure differential and corresponds to the pressure exerted by a column of water 5mm high. Compared to the fact that buildings can withstand wind induced pressures of at least 500 Pascals, this seems insignificant, but it is larger than wind induced pressure on a calm day, and by air testing and quoting air leakage figures at 50 Pascals, inaccuracies are reduced and repeatability is improved using this air test method.

 

6.5 Targets

As noted above, the only ‘official’ guidance in the UK applies in England and Wales and relates to non-domestic buildings over 500 sq.m in area. As can be seen from the table below, the target of 10 m3/hr/m2 at 50 Pa. is relatively easily achieved compared to the good and best practice noted in the 2000 document by CIBSE, TM23. This sets out the air tightness/air leakage testing methodology which is the de-facto methodology now followed for a air test in the UK.

 

A number of air tightness experts believe the stated targets are in­adequate when compared with the overwhelming need to address carbon emission reductions, and the potential to do so through air tightness measures. For example, the house illustrated to the right was built in 1992 for the same cost as nearby houses and improved upon the standards noted above by two thirds.

 

Building Type Air Permeability (m3/hr/m2 at 50 Pa)

Good Practice/ Best Practice

Dwellings 10.0/ 5.0

Dwellings (with balanced mech. vent.) 5.0/ 3.0

Offices (naturally ventilated) 7.0/ 3.5

Offices (with balanced mech. vent.) 3.5/ 2.0

Superstores 3.0/ 1.5

Offices (low energy) 3.5/ 2.0

Industrial 10.0/ 2.0

Museum and Archival Storage 1.7/ 1.25

Cold Storage 0.8/ 0.4

 

7.0 Designing for Airtightness

 

7.1 Key Principles

 

1. A Performance Specification is a crucial document for establishing the appropriate targets for airtightness, along with the methodology for achieving it, and the roles and responsibilities of those involved.

 

2. Conceiving of a building in zones and air barriers will help all involved to visualise the task.

 

3. Air barriers must be impermeable, continuous, durable and accessible. They should be supported by positive mechanical seals where possible.

 

4. The simplest solutions will be the most buildable and durable.

 

5. A culture of airtight construction does not yet prevail and until it does, it may be necessary to follow up targets with specific details and specifications, along with guidance on the process of implementing the necessary level of co-ordination and attention to detail.

 

Unlike design for deconstruction (the subject of the first in this series of Guides) and the forthcoming guide on chemi­cal-free design, the design of airtight buildings cannot be left to the specification and details, at least, not until the industry as a whole recognises the need and has sufficiently widespread ex­perience, unfortunately this alone would surely end in an air test failure. For the next few years, it will be necessary not only to provide careful details and performance specification, but also to develop thorough inspection and testing regimes, hence the need for Chapters 7.4 and 7.5 of this guide.

 

7.2  Performance Specification

The Performance specification may be the only document need­ed by the Architect / Designer / Client if the building is to be pro­cured through Design and Build or similar route. However, it is more likely to be part of a suite of documents including detailed drawings.

The performance specification allows appropriate targets to be set for the project, along with a description of how the process is to be conducted, in terms of scheduling, audits and air testing, and potentially remedial works. Given the increasing use of special­ist subcontractors, particularly in larger projects, it is also critical that the performance specification sets out both the responsibil­ity for, and constructive guidance regarding the co-ordination of trades with respect to the final air permeability of the completed envelope.

 

Zones and Barriers

Once appropriate targets have been set for the project, the next task is to identify zones which require greater or lesser airtight­ness uk levels. Ideally, these zones need to be identified on a draw­ing which also identifies the specific air barriers in red.

 


For example an industrial unit with office space is divided into five separate zones, and air barriers are identified as required. Such a drawing, however diagrammatic initially, helps to conceive of the subsequent specification and detailing needs, giving an overview of the problem.

Heated zones need to be kept separate from unheated zones such as roof voids, delivery bays etc. whilst service shafts may require particular attention. Boiler rooms with large flues and in­take vents may need to be separated prior to the air test

 

Entrances are often significant sources of draughts. Lobbies with doors set apart by around 4m, so that one door closes before the second is opened, can be effective, whereas in highly trafficked areas revolving doors are likely to be preferable. Tall buildings, with atria, stairways and service shafts all of which rise through the building can be prone to ‘stack effect’ air movement whereby warm air rises, dragging in cooler air from outside at the lower levels creating more acute air leakage problems. A number of tactics may be employed to reduce the effects, but in any event issues of airtightness are likely to be highlighted in these cases prior to the air test

 

7.3 Design

 

With the zones and air barriers located, it is necessary to design the air barriers themselves.

To be effective, the air barrier must:

 

• be made of suitably air impermeable materials;

 

• be continuous around the envelope or zone

 

• have sufficient strength to withstand any pressures created by wind, stack effect or air control systems

 

• be easily installed

 

• be durable

 

• be accessible for maintenance / replacement if appropriate

 

The last of these is important since there is evidence that the air­tightness of some constructions will tend to decrease over time and in particular the first period after completion.

 

There are a number of strategic measures which can be employed to simplify the business of designing an airtight building. Since service penetrations in and out of a building provide a major source of air leaks, one strategy is to collect all such penetrations into one accessible area, this will drastically improve the air test results

 

In construction types such as steel and timber frame, it is usually wise to employ a specific membrane or layer as the air barrier, rather than rely on sealant between, for example, the sheathing boards. Such a membrane can usually double up as the vapour barrier if used internally and gives the Designer the opportunity to consider and address airtightness explicitly, rather than as a function of other elements. Bear in mind that most membranes are flimsy and will need support in all areas, although there is minimal air pressure during the air test it can still move unsupported membranes this can result in an air test failure.

 

Another strategy is to employ service voids. Creating a service void internally allows for alteration and maintenance of services and fin­ishes without recourse to penetrations through the air barrier. This allows for long term good performance in contrast to membranes which are liable to penetration at all service points, necessitating careful sealing of each and every penetration, in the short term this will help to reduce the air leakage uk  rate and vastly improve your chances of an air test pass, not only initially, but over the years of alterations and maintenance to come.

 

Generally, it is better to conceive of the joints in airtight layers as ‘positively’ connected, anticipating differential movement and de­cay of adhesive or chemical bonds. For example, where different components of a curtain walling system are liable to differential movement, it is clear that a joint whereby the two components are held together with a positive mechanical connection across a compressed gasket is likely to remain airtight longer that a simple butt joint with a mastic sealant between, this attention to detail will improve your air leakage uk rate and the chances of an initial air test pass.

 

Finally it is clear that complex solutions to airtightness are likely to be more prone to poor execution and potentially to greater vul­nerability to differential movement, failure of sealants, dislocation of components and so on. It is important therefore to aim for the simplest solutions to providing a robust airtight layer, using the fewest separate materials, junctions and penetrations, and the easiest installation and maintenance, this will improve the buildablity and improve the chances of a air test /leakage  pass.

 

It is worth making a point of considering each and every specified component with regard not only to its own intrinsic airtightness uk characteristics, but with regard to the connections between it and adjacent components. It is important to provide explicit details and guidance at specific, and particularly tricky detail areas. On design and build contracts it may be necessary to allow for some form of review of proposed solutions and procedures, to try and out design any problematic/complicated junction which will dramatically improve the chance of an air test pass

 

The following provide a few examples whereby airtightness can be simplified at the earliest design stages.

However good the workmanship, blockwork on its own can never be considered airtight. Once plastered, on the other hand, it may be considered extremely airtight, with concern only for those edges and corners where cracking or gaps can appear. This may be contrasted with the more common practice of drylining block walls with plasterboard on battens or dabs, either way when either is built correctly they can form a excellent air test barrier.

 

Design & Detailing for Airtightness

 

Services Zones or Rooms enable a range of services to be collected together before exiting the building, allowing most of the penetrations in the external fabric to be grouped and sealed effectively prior to the air test .

 

Service voids enable cables and pipe­work to be installed and altered without needing to penetrate the air barrier. Note however that if they are not run in con­duit, protection may be needed against subsequent fixings, if not undertaken prior to the air test this will result in an air test failure

 

Positive physical connections are to be preferred over any other joint such as one relying on adhesives. In the timber frame example shown the air barrier membrane is shown lapped and sealed with mastic over a firm background (boards with stud behind) and with a positive mechanical fix - a batten - fixed over the top and through to the stud.

 


Trinsically non-airtight block wall behind, this form of construction typically gives rise to a wide range of air leakage uk paths behind the boards and into floor, partition wall and ceiling cavities. From the perspective of airtightness, dry lining should be avoided unless great care is taken, otherwise it will result in a air test failure

 

Similarly, timber floors are difficult to seal well without a good deal of care. On the continent - and to an increasing extent in the UK at large - concrete floor systems are being used for both ground and first floors (often for other reasons such as acoustics, fire and the desire for underfloor heating) and these are easier to make adequately airtight prior to the air test . Hollow planks however can leak into cavities and require to be sealed at their ends, this will dramatically improve air tightness and the chances of an air test pass

 

One important and often quoted example is the timber first floor connection with a block wall inner leaf. Who is responsible for ensuring absolute airtightness when the timber joists rest on the wall and are infilled between with block and mortar? Presumably the bricklayer, but is it then his fault if the timber is installed at the wrong moisture level and subsequently twists and warps, leaving cracks around every joint? Is it really feasible to attempt to tape or mastic seal around them all, and what if the underside of the ceiling is to be exposed? Far better perhaps, to do away with the joist-onto-wall detail al­together and replace with joist hangers. Increasingly, the de­signer should be seeking solutions which are intrinsically airtight because of the design, rather than continuing as before while ac­cepting an increased use of duck tape and mastic on site! Whilst these may get you through the initial airtightness tests/air tests, they are short term solutions and not likely to lead to the anticipated energy savings for the Client in the long term.

 

A good review of the various materials and components which al­low the Designer to create an air barrier may be found in the BRE Report BR448: Airtightness in Commercial and Public Buildings

 

7.4 Detailed Specification

Beyond the performance specification illustrated earlier, it is im­portant that the issue of air tightness/air tightness testing becomes embedded within the standard specification vocabulary.

Where an equal or approved alternative may be allowed, it is critical that an airtightness performance specification is part and parcel of that equality of performance. For example, it may no longer remain satisfactory merely to specify a membrane, but in addition to specify the fairly precise nature of the sealing, over­lapping and potentially the subsequent layers as well. Simply of­fering a performance specification and ensuring the responsibil­ity resides with the Contractor is all very well, but it is important too to offer solutions that will enable a satisfactory outcome to be achieved and subsequently improve air tightness uk and the chances of an air test pass.

 

Design & Detailing for Airtightness - Implementing Airtightness

In addition to the intrinsic lack of airtight­ness uk, a problem of drylining is that it can create hidden pathways for air and (unfortunately raise the chances of an air test failure), as above, into the void above suspended ceilings and elsewhere throughout the building

 

Timber joists built into a block wall - a poor detail for airtightness. Far better to use joist hangars and avoid the problem. Source. Concrete planks are not free of problems either hollow planks are often left ungrouted where they meet the external wall, which could lead to extensive air leakage internally and subsequently an air test failure.

 

7.5  Implementing Airtightness

 

Key Principles

 

1.       The Contractor or Project Manager must be made responsible for achieving the air tightness levels set. In particular, this will involve careful co-ordination between trades; if this doesn’t happen then an air test /air tightness failure will surely follow

 

2.       Inspection remains an integral part of achieving air tightness and passing the air test .

 

3.       Ideally at least 2 air tests (air tightness tests) will be undertaken; the first when the building is weather tight, and the second air test a couple of weeks or so before handover.

 

4.       Experience suggest that making one person (or team) responsible for air tightness is the most ef­fective way to tackle the issue, this will drastically improve the chances of a air test pass.

 

5.   Remedial air tightness works to existing properties can reap substantial benefits without undue disruption and improve the chances of an air test pass.

 

It is not yet generally possible within the UK to specify that a building shall be airtight and leave it to the Architect or Contrac­tor to sort out. There is not yet a culture of airtight construction, except perhaps, amongst those who construct superstores, these companies (as a whole) pass far more air tests on there fist attempt.

 

The responsibility of the Designer in regards to the airtightness cannot be overestimated, for if airtight buildings are to become main stream, as they are else­where in the world, the techniques must be above all simple and buildable, with most if not all of the ‘tricky’ areas designed out from the start. In this way, such techniques can become ‘second nature’ to Contractors and there is less reliance on potentially adversarial inspection and air test/air tightness testing failures.

 

Ideally too, the Designer will understand the issues sufficient to prepare a sound performance specification – giving achievable targets for air tests/airtightness as well as a clear description of respon­sibilities and procedures, and a clear and practical set of overall and detail drawings, along with a detailed specification.

 

In the meantime, and even with good documents, there is likely to be a need for effort and vigilance by both the Design Team and the Main Contractor or Project Manager on site. This chapter briefly describes this effort, while the next describes in more de­tail the actual air test procedures and auditing techniques used.

 

7.5  Plan of Work

The RIBA Plan of Work provides a framework for the entire de­sign and construction process. The table on the next page allo­cates specific tasks relating to airtightness to each Work Stage to enable a schedule of tasks and responsibilities for the Design team to be prepared according to each project, this will drastically improve the chances of a air test pass.

 

 

7.6  Roles and Responsibilities on Site

 

Designer / Design Team

The responsibilities of the Design Team are detailed on the follow­ing page, showing all stages including site works and beyond. Buildings usually comprise a number of different components, creating a myriad of routes through which air can escape if not carefully sealed at each and every junction. The Designer’s role is to simplify these details to reduce difficulties on site.


It is critical that the purpose of pursuing air tightness is explained so that all concerned understand why they are being asked to attend to these issues. The initial briefing of key personnel at mobilisation stage – whether or not this involves the air tightness specialist – is also critical in determining the approach to con­ducting the works, inspection, air tightness testing and auditing etc, prior to the air test . which will need to be dovetailed into the many other concerns on site.

 

On large projects it may be useful for one member of the Design Team to take special responsibility for the air tightness / air test issues.

 

Contractor

The Main Contractor’s principal responsibility is to deliver the air ­tightness/air test performance overall and the most likely task on any but the smallest jobs will be that of co-ordination between the sub­contractors. The Main Contractor must be clear that he carries responsibility for the overall air tightness and in turn must ensure that all subcontractors are clear about the extent of their respon­sibilities. This is important since there may be some deviation to conventional practice in order for air tightness to be achieved and a air test pass. It is always prudent to place someone in the site team who has experience of air leakage/air tests on their previous projects as their experience  may avert a potential air leakage/air test failure

 

RIBA Work Stage Design Team Tasks

 

A          Appraisal Establish appropriate air permeability rate

 

B          Feasibility / Briefing Note Microclimate

Test existing buildings / building to be refurbished

Identify procedure for review and air testing

 

C          Outline proposals Consider a/t issues in relation to decisions about form of construction

Identify zones and layers

 

D          Detailed Proposals Identify requirement of additional consultants / design by specialists

 

E          Final Proposals Ensure co-ordination between DT to ensure a/t envelope & penetrations

Detailed application of airtight materials, junctions, service penetrations

 

F          Production Info Select sub-contractors for specialist works (incl. testing)

Careful specification of components, membranes, materials

Emphasise methods for airtightness on documentation

Careful specification of components, membranes, materials

Emphasise responsibilities in specification for dealing with ‘loose ends’ between sub-contractor interfaces

 

G          Tender Docum’n Define Contractors’ responsibilities for co-ordinating work sequences

 

H          Tender Action Ensure selected tenders include adequate airtightness procedures

 

J           Mobilisation Brief all involved in areas critical to air infiltration before work starts

Preparation of samples, training, testing and QA procedures

 

K-L       Site Works Co-ordinate inspection with Building Control if required

Ensure inspection of areas to be covered

Ensure audits and testing schedule is adhered to

Ensure design changes do not compromise airtightness performance

 

M         Post Completion Obtain feedback from concerning comfort and energy consumption

Carry out remedial work as required at end of DLP.

 

As with the Design Team, experience suggests that the best (air test ) per­formance has been achieved by Contractors who employ a dedi­cated individual (or team) to carry responsibility for airtightness and the air test , to inspect the works and instruct as required.

 

For Contractors, the issues of airtightness/air leakage uk and the passing of the air test are intimately linked to issues of good or bad workmanship in general and this can make the issue both more sensitive, but also more difficult to control. Even simple buildings are immensely complex and so the most important aspect of all is the creation of an overall culture of care­ful, tidy, accurate and airtight construction, something which can­not be simply forced through with a performance specification.

 

It is easier to specify and draw an airtight detail than to build it, and so the emphasis on inspection and Contractor responsibility has not developed from a prejudice against Contractors, but from a realistic appreciation that this issue cannot be entirely resolved ‘on paper.’ It is genuinely about a culture shift (at least for many in the industry) and this is where the real challenge lies. Once this shift is adopted there will be a far higher percentage of companies passing their air leakage/air tests uk at the first attempt

 

7.7   Inspection

Air leakage uk found in dwellings according to studies undertaken by BRE. The studies offer a range of conclusions, the most significant of which is that the greatest volume of air leakage uk is occurring in areas out with the ‘normal’ consideration of ventilation, through the myriad of cracks and openings all over the building which is described as ‘background air leakage uk, a common cause of air test failures’

Of the background air leakage subsequently investigated, the principal air leakage routes the greatest cause of air test failures  were noted as being:

 

• Plasterboard dry lining on dabs or battens, often linked to routes behind skirtings etc.

 

• Cracks and joints in the main structure; open perpends, shrink­age & settlement cracks

 

• Joists penetrating external walls, esp. inner leaf of cavity walls

 

• Timber floors, under skirtings and between boards

 

• Internal stud walls, at junctions with timber floors and ceilings

 

• Service entries and ducts

 

• Areas of unplastered masonry walls; intermediate floors, be­hind baths, inside service ducts

.


It is perhaps worth mentioning that the BRE results were based on buildings using dry lining on masonry walls and timber floors. Had the masonry walls been plastered, if concrete floors had been used, and if basic airtightness measures were taken, it is likely that the principal problems would occur around service penetrations, and, to a lesser extent, around windows, doors and roof lights. This is the experience of countries where envelope airtightness generally is more developed. As a result they achieve far better first time air leakage/air test results and subsequent air test passes

 

The following table lists many of the most common infiltration problem areas. On larger projects, common problems include:

 

·         Incomplete bulkheads at eaves;

 

·         Gaps where block work abuts to steel columns or beams

 

·         Uncapped cavity walls, at eaves (right) and mid-points where cavity walls change to composite panels

 

·         Gaps along the underside of corrugated roof linings 

 

·         Gaps between block work and steel, and uncapped cavity wall at join with composite panels Common Locations for Inspection (Applicable to all types of Construction)

 

Foundation / Ground Floor

·         Check wall and floor dpcs form an adequate air tightness layer, is a separate layer needed?

 

·         Check gaps at perimeter insulation strips

 

·         Check potential movement gaps between loadbearing structure such as columns and adjacent non- loadbearing slab

 

First and Intermediate Floor Levels

·         Concrete floors: Check joint between the floor and plasterboard to walls

 

·         Check gaps between concrete planks, or beam & blocks are sealed at the wall

 

·         Check voids under floor finishes and service run penetrations

 

·         Timber floors: Check a membrane seal has been incorporated if required

 

·         Check any membrane used is supported between joists

 

Eaves and Verge

·         Check continuity of airtight layer between wall and roof / ceiling

 

Ceiling level beneath the roof

·         Check for separation between deliberate roof ventilation and the conditioned zone

 

·         Check for service penetrations and hatches which pass across the airtight layer

 

Boundaries between different wall envelope systems

·         Check all systems have a dedicated airtightness layer assigned, and that these can be constructed to be continuous across dissimilar elements

 

Windows and Doors

·         Check that the frame to wall junction is properly sealed and continuous with the wall airtight layer, particularly at cills

 

·         Check the windows and doors have appropriate weather seals between the opening unit and the frame

 

Services penetrations

·         Check for proper seals at service entry points, and at points of entry into conditioned zones. These may also require fire protection

 

Main Entrances

·         Check that the whole entrance area is separated from the conditioned zone by an inner airtight layer

 

Lift Shafts, Service Cores, Delivery Areas / Car Park

·         Check these have been separated from conditioned zones with air barriers and draughtproofed access doors

 

Where profile fillers are used poor workmanship is common

 

• Perforated (acoustic) roofs, where the unsealed mineral fibre acoustic layer bridges the eaves of the building, constituting a major leakage point

 

• Gaps where plasterboard or wall linings are incomplete, com­monly above suspended ceilings and to the underside of beams

 

• Incomplete door and window reveals

 

• Services Penetrations into the building, and between zones inside the building

 

Another common issue is porous blockwork, particularly when internal walls are drylined rather than plastered or painted. Where this is likely to be unavoidable, it may be worth requiring blockwork to have an initial air test for air permeability(air leakage), and to have an AP value (by an accredited lab) that is no more than 50% of the target Air Perme­ability/air leakage  uk for the overall building.

 

7.8  Air Testing and Audit Schedule

In many cases to date, an air test /air tightness test has been carried out a week or so before practical completion. If the result is poor – a high rate of air leakage – then a great deal of work suddenly needs to be done, often to areas which have been covered up and the whole business can be both costly and time consuming, just at the point where in many contracts there is already considerable pressure on Contractors.

 

Far better therefore to schedule the air test /air tightness test uk at a time where remedial works are relatively simple to perform. On the other hand, it is important that a air test /air tightness test is undertaken close to han­dover so that the Client and Design Team can be sure that the completed building accords with the performance specification, and so passes the air test first time .

 

Ideally therefore, two Air tests/air tightness tests at least should be carried out. The first Air test /air tightness test uk should be undertaken as soon as a meaningfully air- and weathertight envelope has been installed. Ideally, all air barriers are still accessible and any defects can be readily put right. This air test /air tightness test uk plus the audit techniques which are likely to accompany it, may be used to ensure an acceptable airtightness performance and give a good indication of where subsequent works may ad­vantageously targeted.

.

In this way, the second and final air test /air tightness test uk serves simply to confirm the performance of the building, hopefully at a slightly improved level from the first air test /air tightness test uk without the need for costly and complex operations late in the day.

 

Such air tightness uk /air leakage testing uk schedule is nonetheless costly in itself, but for those who have been involved in such air leakage testing uk schedules, experience suggests that this remains the most cost effective way to deal with the issue. Certainly it is worth avoiding excessive remedial works at the eleventh hour, just prior to the air test works. With a sufficiently good first air test /air tightness test uk per­formance, it may even be possible to dispense with the final air test , if this is deemed acceptable to the Design Team Leader or Cli­ent.

 

It is often the case that the envelope is not sufficiently complete on the due date for the air test /air tightness testing uk. This then necessitates a complex process of temporary sealing of the incomplete areas. It is harder than to ascertain the location of the air leakage and allowances are made which may prove misleading. Experience suggests that this is not ideal and it would be better to put off the air test /air tightness test uk for a week and carry it out when the envelope is complete and ‘as intended’ this will drastically improve the chances of a air test pass.

 

 

On larger projects, more air leakage uk /air tests uk may be needed, or more specific tests of individual areas required. Large projects with multiple units of a similar nature may benefit from either pre-installation component testing, or insitu testing of one installed component to establish acceptable air test /airtightness uk levels early on.

 

7.9  Remedial Airtightness Works

With airtightness testing and a general awareness of airtightness uk issues developing around new build situations, the principal area of concern, as with energy efficiency in general is the existing building stock. In terms of airtightness uk, the UK building stock is considerably worse than comparable northern latitude countries  and there is a good deal of room for improvement, if these improvements do not occur then a large percentage of companies will fail their air test /air leakage tests.

 

Either as a standalone measure or as part of a package of en­ergy efficiency measures generally, there is scope for remedial works to most of the existing UK building stock. Relatively simple measures may in many cases be sufficient, using a wide range of sealants to control air leakage uk. However, it is important that such measures are combined with attention to the ventilation require­ments of buildings where, to date, insufficient ventilation has been ‘augmented’ by infiltration and exfiltration which, if reduced, could lead to other problems and a subsequent air test failure.

 

As with thermal insulation, there is an extent to which controlling some of the air leakage merely diverts the flow of air, inward or outward, to another defect or gap, (this will still result in an air test failure) but there is such scope for improvement that even fairly basic efforts are likely to reap sub­stantial environmental, financial and comfort benefits for owners and occupiers alike.

 

There are many examples of remedial works described in the various publications noted in the references. Some of the more successful measures included carefully sealed secondary glaz­ing installed where old windows had to be kept for conservation purposes, draughtproofing of doors and entranceways generally, and installation of lobbies in well trafficked reception areas, at­tention to draughtproofing of existing windows and targeted use of flexible sealants to ill fitting components and joints between different construction types, will drastically improve your air leakage rate and will should enable  you to pass your air test first time

 

8.       Testing Airtightness

 

Key Principles

 

1.       Air test procedure is set out in CIBSE TM 23 and in BS EN 13829: 2001.

 

2.       An air test / air tightness test uk involves sealing all ‘normal’ gaps such as vents and pressurising or depressuris­ing the building. The level of fanpower required to maintain the pressure differential indicates the ‘leakiness’ or ‘permeability’ of the building.

 

3.       Air  tests uk/ air tightness tests are typically followed by an audit (using smoke pencils, for example) to expose and make visible the various air leakage uk routes during the air test .

 

4.       Where projects comprise large quantities of a single component, component testing in the labora­tory may be appropriate as well as on site element air testing .

 

8.1 Climatic conditions

As mentioned in Chapter 1, the raised pressure differential of 50 Pascals created during an air test /air leakage test uk is quite small. Whilst this is adequate to overcome most of the common pressure dif­ferential anomalies, such a small differential is vulnerable to larg­er pressure differences created by climatic conditions.

 

Air tests uk/ air tightness tests uk require calm days – i.e. a reading on the Beau­fort Scale of 3 or less (3.4 to 5.4 metres per second wind speed at 10 m above ground) This corresponds to a gentle breeze with leaves and small twigs in constant motion. In winter conditions and on exposed sites therefore testing may not be possible, al­though it is often possible to make allowances, so long as these are carefully recorded, during the air test .

 

8.2   The Air Test itself

Essentially the process is one of pressurising the inside of the whole building (during the air test )  and of measuring the rate at which air needs to be blown or sucked to maintain that pressure differential; a building suffering large amounts of air leakage will equalise readily and require a greater measurable effort to maintain the 50 Pascal differential, while a air tight building will easily contain the enforced differential and require little additional input during the air test , this will be easily recognisable within the first couple of minutes during the air test / air tightness test uk

 

The pressure difference is induced by one or more calibrated fans that are normally mounted within a suitable doorway. An adjustable door panel system, sealed around the edges is used which can also be connected to large external fans via collaps­ible ductwork if required. The rate of the fan, or the volume flow of air through the fan can be understood as the rate of air entering / escaping throughout the remainder of the building envelope, this is recorded throughout the air test .

 

Buildings are air tested in such a way as to recreate ‘normal’ condi­tions. Doors and windows are closed, trickle ventilators closed, extract fans and such like are closed but not sealed. Internal doors are wedged open.

 

If the building is under construction the  air test /air leakage testing uk is ideally undertaken out of working hours, but sometimes this is not practical so some scheduling of work needs to be thought through in advance. With all external doors and windows sealed shut, some work becomes impossible (such as work with solvents requiring ventilation) and internal trades are normally ‘sealed in’ for a short time, where they can carry on undisturbed during air test .

In existing buildings, air tests uk/air tightness tests are normally carried out when the building is unoccupied if possible because of the disruption.

 

8.3 Air Leakage Audits

The air test /air tightness test uk quantifies the rate of air leakage uk through the envelope as a whole, but it cannot locate the air leakage uk paths. Where remedial work is required therefore, air test /air tightness tests uk are followed by a range of auditing techniques designed to identify the specific places where air is leaking.

 

In many cases a simple visual inspection may be sufficient – es­pecially if undertaken by someone with experience of the likely locations of leakage, this is a good way of lowering the risk of a air test / air tightness test uk failure

 

However, most leakage routes are difficult or impossible to spot without visual aids. One common technique is to use smoke trac­ers – smoke pencils or smoke machines. These render the air leakage paths visible in certain situations during the air test . The building may be positively pressurised and the air leaks witnessed externally, or, more usually, negatively pressurised while a smoke pencil is drawn over likely gaps and defects which become visible as the smoke is sucked inwards during the air test .

 

Another technique, which has certain advantages and disadvan­tages compared to smoke tracing, is the use of an infrared cam­era by undertaking a thermographic survey, Used either externally or internally, these ther­mographic cameras register the radiant heat levels of surfaces and so are able to ‘see’ for example, where cold air is cooling the fabric around a gap internally, or conversely where warm air is escaping and heating the colder materials on the external face.

 

To work effectively, there needs to be a recognisable difference between the internal and external ambient temperature, so be­fore any heating has been installed and on a warm summer’s day thermography / thermographic surveys may not be effective. Similarly on warm and sunny days, sunshine on external surfaces can distort the true situation so it is better on such days to wait until early evening. Conversely, rain on external surfaces can be equally distorting of the true thermal situation. However, these cameras are useful in identifying problems at high level or difficult to reach areas, and are also very helpful in identifying other construction defects such as poorly installed (or non-existent!) insulation within the fabric.

 

On larger commercial buildings, air tests uk/ air tightness testing may be un­dertaken at the same time as ‘standard’ ventilation system com­missioning and associated studies

 

8.4 Component Testing

A distinct aspect of overall air tightness testing is the individual component air test . This may be undertaken quite separately, in the laboratory or by the manufacturer of a particular component. Such air tests uk may be deemed necessary on a large project where large areas of one particular type of component, for example curtain walling, are to be specified.

 

Insitu element air testing involves isolating the area within a tem­porary sealed compartment, which is then pressurised, and the air leakage related to the area of interest assessed. In this way sample areas of a building may be air tested using smaller fans as required.

 

9. The Details

Caveat

It is important to emphasise the scope and purpose of the following drawings and specifications.

They are included solely to show practitioners the sort of altera­tions that can be made in order to enable buildings to be much more airtight in general.

 

Their purpose is not to offer approved details in any sense, but to illustrate the difference between details and specifications which do not address airtightness issues, and those that do. It is the dif­ferences between the originals and alternatives which is intended to be illustrative, not necessarily the alternatives themselves.

 

The original details have been taken from conventional details and specifications we believe to be broadly representative of their construction types. We hope the principles shown, and the specific references made will assist designers in making similar changes in their own work, but it goes without saying that air test /air testing cannot take responsibility for any work undertaken as a result of the use of these details.

 

Specifically, these details are not intended to show best practice in any sense, nor are they even intended to be up to date. We have striven in the preparation of these details and specifications to keep as close to the original as possible. We have done this in order to show that some quite fundamental alterations – in terms of airtightness - may be made with the minimum of visual or func­tional impact on the original. Where these original details and specifications do not meet current standards or aspirations, the alternatives given are likely to be similarly wanting. To re-iterate, the purpose is not to produce approved details, but to illustrate the process of improvement – in terms of airtightness only – that may be made.

 

Consideration of priorities in airtightness design and specifica­tion is potentially misleading since, in effect, all gaps, cracks or tears let in air and the sealing of one simply redirects infiltration to somewhere else, this becomes all the more apparent during the air test . Like thermal insulation, what is important is the level of continuity generally, not any particular detail on its own. Nonetheless some prioritisation has been attempted in order to help Designers to prioritise their own efforts since not all measures may be necessary.

 


9.1 Steel Frame + Concrete Block Cavity Wall

 

Original Specification

 

Discussion

Because of the largely wet trades involved, one might imagine a masonry construction is  inher­ently more airtight than the dry fixed timber frame and curtain walling construction types. However, insofar as concrete inevitably shrinks as it dries, as mortar beds and perpends are often poorly filled, and due to the differential movement between masonry and the steel frame, the myriad pathways that open up can make masonry buildings extremely susceptible to infiltration, this can lead to an air test failure.

 

To make things worse, construction such as this does not easily lend itself to a simple, single airtight layer which can be applied separately and therefore the need for vigilance, and some care and attention to a number of small but potentially time consuming sealing jobs is high, however these must be undertaken if you are to pass an air test on the first attempt.

 

It would be possible to form an airtight layer inter­nally through the use of an applied membrane and the adoption of a service void. This would have the advantage of allowing for changes in the service or fit-out provision without the risk of damage of compromise of the airtight mem­brane,(this will lead to a air test failure) and for those inclined to this solution.

 

A parge coat and service void could have a similar effect, but the use of plaster internally is a common and effective technique for creating an airtight layer and is preferable in this instance as it is closer to the original detail and will improve the overall air test results

 

HIGH PRIORITY

·         Wet Plaster Finish or Wet plaster coat costs more but provides a better finish overall, as well as significantly improved airtightness across the masonry leaf. Plaster should be extended to all wall areas and not left off in areas which will not be seen,( such as suspended ceilings.

 

·         Membranes Lapped & Sealed2 lines of tape and a positive mechanical fixing by batten ensure laps are sealed for the long term

 

·         Mastic to Skirtings, Linings etc.

 

·         Critical in this detail since the plaster cannot form a continuous layer at these junctions

 

·         Sealed Cavity Closer: Gaps around openings are common so care is needed here to prevent infiltration around the frame and into the cavity

 

·         Vapour Barrier Seal at Eaves: Important here since no effective seal is noted on the original which could lead to excessive airflow at this vulnerable point.

 

 

Costs

The most significant cost implication is associated with the addition of the wet plaster coat to the inner leaf of blockwork. This results in approximately a 60% increase in cost, although the quality of the blockwork is not as critical. This item is also significant in that is changes the ‘look’ of the detail but is probably the highest priority.

 

Otherwise, most of the costs are associated with the additional time, effort and care implicated within the specification and details. Of these, the most significant is the additional labour and materials required for the joining of the vapour barrier in the roof, and sealing it around the perimeter. This work almost certainly more than doubles the cost of the vapour barrier in the original detail, but again, represents a critical factor in reducing air leakage and saves the cost of multiple air test failures.

 

A number of the measures described represent no more than a re-iteration of good practice, such as the sealing of perpends, lapping and sealing of membranes, draught stripping of windows and so on. These may assumed to incur no cost implication, but perhaps one of attention to details (this usually results in first time air test passes) on site.

 

The mastic sealant to skirtings, cills and the like would add about 50% to the costs of these items, though these items represent only a small fraction of the overall costs.

Taping of the insulation boards would depend largely on the board type, but might realistically attract only a marginal cost increase, as would the use of com­pressible foam around the steelwork.

 

MEDIUM PRIORITY

·         Concrete Slab Floors:Concrete slabs form an airtight layer but joints with penetrations such as perimeter blockwork, insulation or structural columns must be sealed.

 

·         Cill to Window Sealing: Double sealed detail which increases the chance of securing an airtight seal at this often overlooked junction

 

·         Compressible Foam between Steel and Blockwork: Potential solution to the inevitable gap which will form here, also sealable with mastic oninside face only.

 

LOW PRIORITY

·         Perpends Fully Filled: Not critical if a wet plaster finish is applied internally, but high priority if they are not.

 

·         T&G and Taped Insulation:Not technically part of the airtight layer, but gaps here simply increase the likelihood of infiltration (air leakage) and are relatively easily sealed.

 

·         Expanding Foam to Gap at Eaves: Not part of the airtight layer but by seal­ing a large gap in the fabric, this reduces the wind pressure driven airflow within the cavity thus reducing the risk of infiltration (air leakage) indirectly.

 


9.2 Index

a) Perpends fully filled

A common problem with blockwork and brickwork buildings is that perpends are not completely

filled and this leads to air flow (air leakage) through the wall this becomes apparent during the air test . To an extent this measure is superceded by both points (aa) and (d), but it is still worth making the point in order to draw attention to this workmanship issue in general, it’s the attention to detail that ensures you pass the air test first time

 

b) Blockwork Maximum Air Permeability by Component Test

An alternative to wet plastering the blockwork on the inner leaf is to require a component air test of the blockwork to satisfy a maximum air permeability of, say, 5m3/hr/m2 or less. On larger

projects, or where wet plastering is unlikely to be effective or desirable, this is one method of

ensuring a reasonable degree of airtightness from the blockwork leaf. These conditions may also be used for the outer leaf but is not as important because it is the inner leaf which is providing the main air barrier for the air test .

 

c) Membrane Lapped and Sealed

Typically membranes are lapped and stapled or tacked, but in order to create airtight layers, it is

important that these laps are rigorously sealed. Best practice in this regard - beyond the correct

use of Manufacturers’ overlap dimensions, proprietary tapes and other accessories - is to run a

layer of double sided tape between the membranes at the overlap and run a tape over the

leading edge of the outer sheet. In addition, since many tapes tend not to last too well, it is

advisable to ensure that laps are made directly over supported areas (i.e. with studs or dwangs

directly behind) and are held down positively with battens fixed through forming a mechanically

tight, as well as an adhesive seal, this will provide an especially strong air tightness seal and will improve the chances of an air test pass

 

d) T&G or Shiplap and Taped Non-Mineral Fibre Insulation

Mineral fibre is permeable to air movement and cannot be counted upon to help in reducing air

leak­age. Extruded polystyrene and other closed cell plastic insulation materials do not suffer from

this and so have the potential to reduce air leakage and improve air tightness in and out of the

building(improving the chance of passing the air test ). However, they are only likely to do so if they are effectively joined at their edges, at corners, openings and around wall ties etc. For this reason, it is likely that t&g or shiplap edge boards (which are available from a number of Manufacturers) will offer better connections, and these can be further augmented by the use of a sealant tape externally.

 

e) Wet Plaster Finish Internally

An alternative to arranging component tests for the blockwork. A simple block finish with 2 coats

of paint which in terms of airtightness is an improvement on a uncoated block wall but is not

sufficient to consider the blockwork airtight in the least. Wet plastering of the blockwork is more

expensive but ensures an airtight masonry leaf, this will improve the chances of a air test . The plaster should extend to all areas of the wall, regardless of whether they will be hidden by suspended ceilings or raised floors. It should extend right to the floor and to undersides of steel beams etc. and where broken by service boxes etc. should be conscientiously filled and sealed.

An alternative which would have a similar effect would be to use a parge coat over the blockwork,

before application of a service void and separate finish layer. The simple wet plaster finish is

closer to the original.

 

f) Mastic Both Edges to Skirtings, Reveal Linings etc.

Where the corner junction behind has been carefully sealed then this measure may not be

required, but in the examples shown on this construction, this particular detail is critical since it

forms an integral part of the airtight layer, particularly where the plaster has to be discontinuous.

 

g) Concrete Slab Floors

Unchanged from the original detail, this is simply to note that concrete slabs form an airtight

barrier and may therefore be considered good practice in this regard. However, no note is made

of the need for care to be taken where the slab meets elements of structure which pass through,

steel columns, for example. At these junctions, a compressible foam strip may be laid around the

steel prior to pour­ing the concrete if practicable, or a mastic sealant may be used subsequent to

the pour to seal the inevitable shrinkage cracks which will form and become air leakage uk paths and will increase the chances of passing the air test

 


h) Insulated and Robust Cavity Closer

A robust and insulated cavity closer enables the cavity to be effectively closed, the gap to be

bridged with insulation without risk of moisture flow between inner and outer and the window to

be securely fixed at the head and jambs if required. The gap between window frames and the

main wall is a no­torious place for infiltration (air leakage uk) and so increases the chances of a air test failure so it is important that this junction is carefully sealed. The flanges of the cavity barrier should be closed against the blockwork faces with a continuous mastic bead between on each flange so that airflow into the cavity from outside or in is prevented, thus resulting in better air tightness uk

 

i) Proprietary Cill with Foam Sealant Internally and Mastic Sealant Externally

In addition to the mechanical fixing of the window frame through the cill piece, it is important that

this fixing is made through a compressible foam strip which is then sealed against air leakage uk

from outside with a mastic type sealant. This gives the Contractors two opportunities to ensure a

completely airtight seal at this particularly vulnerable point, prior to the air test .

 

k) Compressible Foam Strip beneath Steel Beam to Blockwork Top

For reasons of both initial shrinkage and subsequent structural movement, it is to be expected

that a direct connection between a steel beam (or column) and a block wall will open up over time

to form a potential route for infiltration (air leakage). One way to try and reduce this inevitable gap

is to build the blockwork against a compressible foam strip which immediately expands to fill the

gap between and remains flexible thus continuing to fill the gap even after shrinkage and

movement. Since compressible foam strips are not intrinsically airtight, mastic sealant should be

used in addition to form a neat internal joint which should further seal the connection, thus

drastically improving air tightness uk and the chances of passing your air test first time.

 

l) Vapour Barrier Detail at Eaves

Here the vapour barrier is positively sealed to the steel perimeter beam to properly seal the

ceiling vapour - and air - barrier along its edge. Assuming that the steel beam is without

penetrations (a specification note has been added to ensure that this is checked) then as long as

the plaster seal to the underside of the beam is adequate, an airtight layer has been formed

which may be discontinuous in materials but continuous in terms of airtightness uk, this building method should help you pass the air test at the fist attempt.

 

m) Expanding Foam to Large Gap at Eaves

Whilst not strictly part of the airtight layer, this measure reduces the potential wind pressures on

the cavity which in turn reduces the risk of air infiltration through the airtight layer itself. Note also the introduc­tion of a ply layer above to support the insulation (nothing is noted as doing so in the

original detail) but significantly against which the foam can create a firm seal which should

drastically improve air tightness uk and a air test pass

 

n) Mastic Sealant to Joints

Additional notes to seal connections between dissimilar materials which are likely to provide

routeways for airflow (air leakage) unless conscientiously sealed.

 

o) Draughtstripping to Windows and Doors

Most commercially available joinery, metal or plastic windows and doors will be adequately

draughtstripped but it is important to explicitly ensure that this is the case, and that seals

(preferably tubular rubber / epdm type) are accessible and can be easily replaced should they

begin to fail to adequately seal when closed.

 

9.3 Timber Frame with Con­crete Block Outer Leaf

Original Specification

Discussion

Despite the inherently dry fixed nature of timber frame construction, it offers good opportunities to ensure airtightness uk because of the existing convention of using vapour control layers internal to the insulation and breather membranes externally. This gives the Designer two layers with which to work to form a robust airtight envelope overall, and without introducing any significant or new component. The outer layer of blockwork (or brick, or dry cladding of any type) need not perform any major role in the airtightness strategy, and should not affect the air test result.

 

Although there are a large number of small adjust­ments to conventional practice outlined, none of these, except perhaps the addition of the service void and backing board involve any major shift in construc­tion process. Experience suggests that such changes are readily made and subsumed within the standard details and specification clauses of the practice.

 

More tricky is the need to convey the need for greater effort, co-ordination, care and vigilance to Contractors for whom there is little to be gained from the good practice noted, and quite a lot to be lost in terms of potentially time consuming additional tasks. In the short term it is important to emphasise the additional co-ordination and tasks to Contractors at the time of tendering so that these are not overlooked and the extra effort can be adequately assessed.

 

HIGH PRIORITY

a) Continuity of Layer / Co-ordination of Trades

General measures to ensure tradesmen are aware of the need for air tightness that all involved are conscientious and rigorous, and that someone is responsible for co-ordination between trades prior to the air test

 

b) Service Void

Use of a service void means most if not all penetrations through the vapour control and airtight layer can be avoided.

 

c) Joist Hangers

Use of Joist hangers avoids the common problems of air infiltration where joists are built into the inner leaf

 

d) Membrane to Floor Perimeter Beams

Slightly awkward solution for solving the problems of discontinuity at this area which is nearly impossible to solve otherwise.

 

e) Flexible Foam around Joinery

Gaps around openings are common and neat, effective solutions can be difficult, careful use of flexible foam enables effec­tive and durable seals to be formed.

 

f) Continuous Layer Over Partitions

High priority because of the high potential exfiltration rates and condensation risks at this point

 

g) Backing Boards

Use of backing boards makes installation of the membrane easy and thus less prone to poor workmanship and subsequent air test failure.

 

MEDIUM PRIORITY

a) Membranes Lapped & Sealed

2 lines of tape and a positive mechanical fixing by batten ensure laps are sealed for the long term

 


Costs

Not surprisingly, the addition of the service voids adds considerably to the costs of both the walls and ceilings. Of course, such costs say nothing of the increased ease of services installation, nor of the long term benefits of a much greater access for upgrading and alterations.

Nonetheless, the addition of the OSB and battens forming the service void in the walls adds approxi­mately 35% to the cost of the external wall. Mechani­cally fixing the vapour barrier to the floor and taping would add approximately 4% to the overall wall cost in addition.

Adding the service void to the ceiling would represent an approximate 130% increase in cost over just the 2 layers of plasterboard. But again, services instal­lation would be easier.

 

The additional work associated with the breather membrane would incur a similar additional cost, but may not be a priority if the internal vapour barrier is well installed.

The mastic sealing of the skirting boards would increase the cost of their installation by about 50%, although these represent only small costs overall, the use of polythene strips at the floor and eaves, and the use of foam around the windows would attract only a marginal cost increase.

The use of flexible insulation need not attract any increase in cost if a common, economical type was chosen. Remember all of the above can be far more economical that not passing the air test and therefore resulting in costly LED’s

 

MEDIUM PRIORITY

a) Joinery Draughtstripping

Tubular seals are probably the best option.it is important that they can be easily ac­cessed for maintenance and replacement.

 

b) Continuity at Openings

Continuity between the framing sealant (m) and the membrane can be tricky and care is needed to ensure a good, durable seal.

 

c) Seal Loft Hatches

Unsealed loft hatches may contribute to air leakage, so worth some care.

 

d) Plasterboard Penetrations

If the airtight layers are sound then this should not matter, but still worth attention.

 

e) Flexible Not Rigid Insulation

Flexible Insulation provides a better fill between studs, rafters etc.

 

LOW PRIORITY

a) Continuity Behind Lintols

An extra strip of membrane to form a con­tinuous layer when the main one is lapped over the cavity barrier, also fill behind lintol.

 

b) Mastic to Skirting’s, Linings, Cornices

Not necessary if the airtight layer is sound

 

c) Air Barrier to Ceiling

High Priority in separating floors

 

d) Laying Tape to Plasterboard Junctions

 

e) Wall Tie Fixings

 

f) Top Runner Strip Seal

 

g) Airtight Service Boxes

 

h) Corrosion Resistant Fixings

 


9.3 Index

e) Wall Tie Fixings to Timber Frame

The breather membrane is not the main air barrier, but it is nonetheless a useful ally in reducing air leakage uk through the construction generally. Ensure that wall tie fixings do not lead to damage to the membrane (as this will lead to large amounts of air leakage uk, and a subsequent air test failure) ,  ideally, by taping over the area of membrane at which the tie is fixed.

 

(b) Use of Corrosion Resistant Staples or Fixings

Non-corrosion resistant fixings to external breather membrane can corrode to a point where they fail, allowing the membrane to come loose, often creating a small hole in the membrane and reducing the effectiveness of the membrane as an airtight layer, this will allow for air leakage uk and a probable air test failure. Copper is non-corrosive but can affect polyethyl­ene, whereas stainless steel has no effect on polyethlene and so should be preferred.

 

(c) Membranes to be Lapped and Sealed

Typically both internal and external membranes are lapped and stapled or tacked, but in order to create airtight layers, it is important that these laps are rigorously sealed, this will ensure a air test pass. Best practice in this regard - beyond the correct use of Manufacturers’ overlap dimensions, proprietary tapes and other acces­sories - is to run a layer of double sided tape between the membranes at the overlap and run a tape over the leading edge of the outer sheet. In addition, since many tapes tend not to last too well, it is advisable to ensure that laps are made directly over supported areas (i.e. with studs directly behind) and are held down positively with battens fixed through forming a mechanically tight, as well as an adhesive seal. This requires consideration of lap positions early on if extra framing or subsequent battening is needed.

 

(d) Ensure Continuity of Membrane / Co-ordination of Trades 

Whilst this is easy to achieve across large, flat areas, it is more difficult at the many awkward angles, junctions, corners and so on a typical site. There is no specific guidance except to ensure that those responsible for installation of the membrane are rigorous and conscientious in their attention to all of the inevitable nooks and crannies, and that the person responsible for co-ordination is equally attentive, particularly when the junctions are between separate forms of joint and separate trades. Having someone experienced on previous projects that required air tightness uk tests is a huge advantage and dramatically improves the chance of a air test pass

 

(e) Ensure Membrane is taken into Opening Reveals, Taped and Sealed and Made Continuous with Opening Seals

it is typical at openings in timber frame buildings to allow the membrane to run across the opening initially, then form a star cut into the opening, folding over the sections of membrane and trimming as necessary. In these cases, there are inevitable gaps in the airtight layer at the corners of the opening, and it is important to ensure that these are made good before subsequent installation of joinery etc and the following air test .

 

(f) Fix Airtight Membranes to Firm Backing Boards

In conventional timber frame construction, vapour barriers are fixed across studwork, usually after the installation of insulation and prior to the fixing of the internal lining. Equally external breather membranes are sometimes installed across gaps between rafters or studs. In both cases membranes are susceptible to pressures from both sides, leading to the membrane breaking free of its fixing and creating holes in the airtight layer. Ideally, membranes should be fixed against a firm backing board by way of protection against damage of this nature, this in turn will improve the chances of passing the initial air test .

 

(g) Service Void 

The principal advantage of a service void is related to functionality and maintenance over time, but a secondary advantage which relates directly to airtightness uk is that since all services may be incorpo­rated within, that is, on the inside of the vapour control layer, there is no need to penetrate the layer at each and every service installation, thus significantly cutting down on the myriad potential gaps that are typically formed and either left, as this will surely lead to a air test failure or made good which is time consuming and costly.

 

(h) Laying Tape at Plasterboard Junctions

Using laying tape at junctions makes the formation of an airtight junction both conscious and relatively easy, even allowing for subsequent shrinkage and cracking of the skim layer.

 

(i) Airtight Service Boxes 

Developed in Canada where airtight construction is more advanced, these service boxes are fitted with gaskets and a flange surround allowing for an airtight seal at all openings in the lining.

 

 (j) Mastic Both Edges to Skirtings, Reveal Linings, Cornices etc.

Where the corner junction behind has been carefully sealed then this measure may not be required. In addition to the nail or screw fixing, a mastic seal both edges aid’s efforts to guard against infiltration/air leakage (which will increase the chances of passing a air test ) but it makes removal and alterations more difficult.

 

(k) Ensure Continuity of Membrane behind and around Lintols 

It is likely that to achieve this requires two separate measures. First the breather membrane needs to be continuous and extend into the opening, thus a second strip should be affixed to the wall and lapped and sealed to the main membrane which must lap over the lintol or cavity barrier etc. Second, it is likely that gaps could form between the top, outer edge of the joinery and the lower, inner edge of the lintol, leading to a cavity behind the lintol. This cavity should be filled with expanding foam or mineral wool and if possible the gap filled, probably with a mastic sealant, this will drastically improve the chances of passing your air test at the first attempt

 

(l) Flexible Foam Sealant around Joinery Insertions

Gaps around openings are one of the most common of infiltration paths. They range from 0 to 20mm, which is too large to be filled by mastic. Compressible flexible foams are ideal for this application. En­sure that the airtight membrane meets the seal on both sides to maintain the airtight layer overall, and subsequently pass the air test

 

(m) Draughtstripping of Openings in Joinery

Draughtstripping of joinery comes in many forms. It appears that synthetic rubber or elastomeric tubular seals work well, creating good seals with minimal compression, depending on the size of the gap. It is important that seals are unaffected by paintwork and subsequent decoration, or are easily acces­sible and removable. This is important so that seals can be replaced as they start to fail to maintain the airtight layer.

 

(n) Seal all Penetrations in Plasterboard / Internal Lining 

Even with the use of airtight outlet boxes there will be inevitable penetrations such as ceiling pendants, pull cords, recessed fittings etc. which must be made good manually, typically with mastic, otherwise you may end up failing your air test

 

(o) Seal Loft Hatches

Generally, this involves a continuous bead of mastic to the underside flange, and, depending on the design, the use of compressed and flexible foam, or mineral fibre etc. above. Please note in our experience this is a common area for air leakage, and a major cause of air test failures

 

(p) Use of Joist Hangars as Opposed to Built-in Joists

The original specification here is already good practice, that is, the use of joist hangars which sidestep the problems of joist movement and shrinkage allowing infiltration and airflow within the floor voids, another major cause of air test failures

 

(q) Membrane Strip to Inner Face of Floor perimeter Beams 

100 gauge polythene or similar fixed to the inner face of the perimeter beams early on in the framing process can lapped and sealed to the internal vapour control layer typically installed a good deal later, so that a continuous internal vapour control and airtight layer may be effectively created.

 

(r) Continuity of Membrane to Ceiling over Partition Walls 

ideally this would comprise a continuous membrane affixed before the partitions are installed. However it is more likely that partitions are installed before, therefore such a layer would require strips to be fixed to the partition top runners to be later lapped and sealed to the ceiling vapour control layer.

 

(s) Flexible, Rather than Rigid Insulation 

Rigid insulation between joists, studs or trusses generally has to be cut to fit and this is never 100% accurate, leading to myriad gaps and routes for airflow. Flexible insulation avoids this problem and improves the chances of passing your air test at the first attempt.

 

(t) Top Runner Strip Seal

The use of this strip, lapped and sealed with subsequent membranes both sides prevents air infiltration into the wall itself from the ventilated eaves area, thus ensuring continuity of the airtight layer, which should help you to achieve an air test

 

(t) Air Barrier to Ceilings 

In ceilings within dwellings of the same occupancy, this is unlikely to be useful, but in separating floors, it is extremely important that an air barrier is included in the floor and ceiling make-up. Noted here by way of a reminder.

 

9.4 Steel Frame + Glazed

Façade

 

Discussion

It is important to be confident that the curtain walling manufacturer, supplier and installers all share an ex­plicit commitment to producing an airtight wall overall, as it will be very difficult for the Main Contractor to ensure a continuous airtight fabric if this element is not firmly ‘tied down’ before the start on site, this is one of the main causes for air test failure

 

The focus of concern then falls to all the various cor­ers and perimeters where the system meets other construction elements and here both Designer and Contractor need to have carefully considered in detail each occurrence and made adequate provision, to avoid large amounts of ad hoc remedial work, during the air test .

 

The roof membrane must be carefully sealed and the perimeter condition considered so that a continuous and positive connection can be made. Note this is another major cause of air test failure

 

HIGH PRIORITY

(a) Curtain Walling Performance Spec.

Since this represents the largest area exposed to wind it is important that the performance specification is adequate and that the components are conscientiously installed

 

(b) Mastic Perimeter Seals

With the main curtain walling components installed and airtight, the next most signifi­cant air leakage route is likely to be the pe­rimeter seals. Both mastic and membrane seals are valuable in this regard. Note this is another major cause of air test failure

 

(c) Membrane Perimeter Seals

With the main curtain walling components installed and airtight, the next most signifi­cant air leakage route is likely to be the pe­rimeter seals. Both mastic and membrane seals are valuable in this regard. Note as above this is another major cause of air test failure

 

 

(e) Roof Membrane Sealing

Any leakage in the roof membrane or at the roof / wall junction could be serious in terms of both energy waste and risk of moisture related damage to the roof build-up, so this detail is important. By properly preparing for your air test this will alleviate any of these future problems

 

MEDIUM PRIORITY

(h) Plates Added to Beam

Because of the difficulty in forming an adequate seal to protruding beams, this is likely to be a major source of air leakage in the long term so designed, rather than ad hoc site measures to reduce air infiltration are important.

 

(f) T&G and Taped Insulation

Potentially a minor issue, but given higher priority becasue it is relatively easy to solve and reduce airtightness/air test  uk failure and thermal insula­tion related risks.


 

Costs

It is difficult to ascertain any meaningful cost implica­tions with this detail because of the variety of curtain walling systems available.

 

The measures outlined are fairly standard in most installations and should in all cases represent no more than a re-iteration of good or best practice. However, they could attract an additional cost where one particular system did not address airtightness and the subsequent air test in one way or another.

Measures such as the additional efforts associated with air barriers at the separating floor, eaves and flor / wall junction might attract additional costs over that aspect of the original detail by approximately 30% largely because of the additional labour and attention required. However these costs are still cheaper than suffering costly LED’s for not passing the air test

 

LOW PRIORITY

(g) Membrane Seal between Floors

The existing detail should provide a rea­sonable degree of airtightness, but this measure will make the task conscious and affect a greater degree of separation.

 

(d) Foam Filler

Should not be required if the measures in (b) and (c) are completed, but an additional measure that also has value in provid­ing a backing to a continuous mastic seal internally.


 

9.4 Index

(a) Airtight Performance Specification for Curtain Walling

The de facto standard for curtain walling air permeability that most curtain walling manufacturers comply with is the CWCT (Centre for window and cladding technology) ‘Standard and Guide to Good Practice for Curtain Walling’. This specifies a maximum air permeability of 1.5 m3 / hour/m2 @600pascals for an area of fixed glazing, and 2m3/hr/linear metre of joint for opening panels. This is the same as the British Standard BS EN 12152:2002, category A4. However, the BS has a further category, AE that achieves 1.5m3 /hour /m2 at a pressure differential of more than 600pascals. Specification of this ‘exceptional category may be possible but it may mean a reduction in choice as this is a more stringent level of air testing . The rule is: If wind load up to 2400kn then curtain walling to be tested to 600 pascals. If wind load greater than 2400kn then test to wind load/4, e.g if 4000kn, test curtain wall to 1000 pascals.

Maximising airtightness can be done by having vulcanised welded joints to gaskets within the curtain wall frame, instead of usual mitred ones. This should ensure that the unit its self is airtight, although it is an expensive option, it will help you to pass the air test

 

(b) Mastic Bedded Fixings

Where membranes and components are connected, it is often possible for thin - and often more or less invisible gaps to be left between the joint. A continuous mastic seal used along the line of any such mechanical fixing ensures that any minor cracks like this are completely sealed.

 

(c) Additional Membrane Seal at Junction

Some Manufacturers (eg Schuco) supply as part of their system an EPDM perimeter gasket seal that should be tied into vertical DPM. Angle at jambs and loose dpm to wrap ensure good seal with EPDM. This is a particularly good way to ensure airtightness and the chances of a air test pass. At these critical junctions because it requires a conscious task (sealing the membrane) to ensure all ‘loose ends’ are firmly fixed, as opposed to leaving the airtightness to be achieved through the use of applied sealants.

 

(d) Foam Filler to Internal Joint

Assuming that the seal mentioned above is installed correctly this should not be required, but such a seal acts as an additional check against air leakage uk and could be used as a backing strip against which to seal a continuous mastic seal internally.

 

(e) Membranes to be Lapped and Sealed

Best practice in this regard - beyond the correct use of Manufacturers’ overlap dimensions, proprietary tapes and other accessories - is to run a layer of double sided tape between the membranes at the overlap and run a tape over the leading edge of the outer sheet. In addition, since many tapes tend not to last too well, it is advisable to ensure that laps are made directly over supported areas (i.e. with solid materials directly behind) and are held down positively with battens fixed through, or some other ‘positive’ connection forming a mechanically tight, as well as an adhesive seal. This may require consideration of lap positions early on.

 

(f) T&G Jointed and Taped Rigid Insulation

Butt jointed insulation, even if installed firmly may be subject to movement during the course of con­struction and over time, and is unlikely to offer a continuous insulation layer in the long term. Using t&g slabs overcomes some of this problem and taping the slabs ensures that air leakage paths cannot form between the minute, but inevitable cracks between the units, which can later lead to air test failure

 

(g) Additional Airtight Membrane

It is likely that the fireproof stopping will not be able to create an adequately airtight seal and so this measure ensures the task is performed consciously. Using a simple polythene membrane and forming positive connections to the underside of the slab and the top of the curtain walling ensures an airtight seal between floors, and at the vulnerable connection of curtain walling to spandrel panels. One of the main causes of air test failures


 

 (h) Localised Welding of Plates of Beam

It is practically very difficult to form an airtight seal perpendicular to an ‘I’ beam or similar, expanding foam tends to be used because no ‘built’ connection appears workable, nor cost effective. Such ad hoc seals are unlikely to last in the long term.

Ideally plates should be welded to the beam such that there is no air route along the length of the beam (a plate welded perpendicular to the web and extending between the two flanges) and such that airtight seals are easily formed around the beam as it passes the airtight layer. Side plates fixed between flanges form a sort of localised rectangular section which is more easily sealed. This makes the task more readily achieved on site, and more durable in the long term


 

9.5 Refurbishment of Masonry Building

Discussion

If the existing masonry fabric of a refurbished building is in good condition, it is potentially simple to render it relatively airtight if the details proposed - particularly the use of service voids - are followed. All the work can be carried out internally and is simple to install and check and will drastically improve the chances of a air test pass

In addition there is no cavity in this form of construction and this means there are fewer opportunities for undetected airways.

 

It goes without saying that any cracks or damage to the existing fabric should be made good before installation of the internal frame, otherwise this may help lead to a air test failure

If there is enough space, it might be best to retain all existing lath and plaster on ceilings and walls, ensure that it is effectively sealed, and work inwards from there. Experience suggests that lath and plaster itself is fairly airtight and removing it merely creates more waste. One potential disadvantage is that in keeping the existing plaster, it may not be possible to access the gaps behind which may run into floor voids and partitions creating air leakage uk paths throughout the building.

 

A number of reviewers of this Guide commented that it is more common to maintain a cavity between the existing wall and any new-build internal leaf. The alternative proposed keeps to the same format as the original, but the advantages of the use of a cavity are well understood.

 

HIGH PRIORITY

(a) Membranes Lapped & Sealed

With only one membrane to ensure airtight­ness it is crucial that laps and junctions are conscientiously sealed.

 

(b) Service Void

Use of a service void means most if not all penetrations through the vapour control and airtight layer can be avoided.

 

(c) Joinery Edge Sealing Batten

If the membrane generally is well sealed, the only other major area for air infiltration uk is the openings and the gap between the frame and masonry. If the windows can be effectively sealed by (medium priority e) then this measure is not necessary.

 

(d) Joinery Draughtstripping

A particular issue with sash and case windows. It is important that seals can be easily accessed for maintenance and replacement.

 

MEDIUM PRIORITY

(e) Flexible Foam around Joinery

Gaps around openings are common and neat, effective solutions can be difficult, careful use of flexible foam enables effec­tive and durable seals to be formed. If this can be effectively achieved with the sash and case window then (f) is not necessary.

 

(f) Continuity at Openings

Continuity between the framing sealant (m) and the membrane can be tricky and care is needed to ensure a good, durable seal.

 

(g) Backing Boards

Use of backing boards makes installation of the membrane easy and thus less prone to poor workmanship and subsequent failure.


 

Costs

The retention of the ceiling lath and plaster saves approximately 24% of the costs of that element, while the addition of the service void and vapour check represents a 18% cost increase, thus, without the addition of the breather membrane over the ceiling joists (a medium priority measure) there is a cost saving to complement the increase in ease and cost of services installation.

The breather membrane represents a 13% increase in cost and therefore tips the balance of the ceiling cost overall. However this is still cheaper than repercussions due to failing your air test

 

The addition of the OSB backing board and service void to the walls constitutes around a 46% increase in cost of the wall, the majority of which (33%) is made up by the OSB, so perhaps a cheaper, yet firm back­ing board might alleviate the cost burden. The addition of the OSB layer to form a service void beneath the floor boards would add approximately one third to the cost of the original detail. Again this is still cheaper than repercussions due to failing your air test

 

 

Double mastic sealing of the skirting boards adds approximately 50% to their installation cost, although their overall costs are small in the overall picture.The sealing of the vapour control layer above and below the intermediate floor should not attract any additional cost if assumed to be part of a standard, if careful installation. Measures to help seal around the window would add marginally to a standard installation cost.

 

MEDIUM PRIORITY

(a) Batten Seal at Corners

A version of (b) but worth particular men­tion as these junctions are particularly important to seal well.

 

(b) Keep Existing Lath and Plaster

Really a version of (e) except in this case we suggest retaining the existing firm base of lath and plaster against which to affix the vapour control layer.

 

(c) Plasterboard Penetrations

If the airtight layers are sound then this should not matter, but still worth attention.

 

(d) Membrane Over Roof Insulation

Protects installed insulation from disrup­tion and provides a secondary layer at this important area.

 

LOW PRIORITY

(a) Silicone to Joinery Externally

Should be standard practice, but forms useful role in airtightness as well as weath­erproofing.

 

(b) Mastic to Skirtings, Linings, Cornices

Not necessary if the airtight layer is sound but worth attention in these examples.

 


9.6 Index

(a) Breather Membrane over Insulation 

In well ventilated loft areas, loose insulation may become dislodged by air movement. This precau­tionary measure ensures that the initial fully fitting installation of batts against joists etc is maintained over time, reduces dirt and debris entering and provides an additional airtight layer (which is useful since the loft is ventilated) whilst allowing for vapour egress into the ventilated space.

 

(b) Membranes to be Lapped and Sealed

In order to create airtight layers, it is important that laps are rigorously sealed. Best practice in this regard - beyond the correct use of Manufacturers’ overlap dimensions, proprietary tapes and other accessories - is to run a layer of double sided tape between the membranes at the overlap and run a tape over the leading edge of the outer sheet. In addition, since many tapes tend not to last too well, it is advisable to ensure that laps are made directly over supported areas (i.e. with studs or dwangs directly behind) and are held down positively with battens fixed through forming a mechanically tight, as well as an adhesive seal. This requires consideration of lap positions early on if extra framing or subsequent battening is needed.

 

(c) Vapour Control Layer over Existing Lath and Plaster 

Rather than remove the existing lath and plaster ceiling, this detail saves a little money, time and resources by reusing the existing ceiling as a backing to the installation of the vapour control layer (refer also (e)) Plaster need not be repaired if damage is localised and does not threaten the integrity of the vapour control layer.

 

(d) Service Void 

The principal advantage of a service void is related to functionality and maintenance of services over time, but a secondary advantage which relates directly to airtightness is that since all services may be incorporated within, that is, on the inside of the vapour control layer, there is no need to penetrate the layer at each and every service installation, thus significantly cutting down on the myriad potential gaps that are typically formed and either left, or made good which is time consuming and costly, and could lead to a air test failure

 

(e) Fix Airtight Membranes to Firm Backing Boards 

In many situations membranes required for vapour control and airtightness are installed unsupported and are thus susceptible to pressures from both sides, leading to the membrane breaking free of its fixing and creating holes in the airtight layer. Ideally, membranes should be fixed against a firm back­ing board by way of protection against damage of this nature.

 

(f) Mastic Both Edges to Skirtings, Reveal Linings etc. 

Where the corner junction behind has been carefully sealed then this measure may not be required, In the examples shown on this construction, this particular detail is not critical but is nonetheless valu­able in helping to ensure a good seal at all points.

 

(g) Airtight Layer Taken Behind Batten at Corners 

As noted in (b) above, the best airtight seal is a positive and mechanical one such as shown here whereby at corners and edges, a membrane is not only lapped and taped against the adjoining sur­face, but held firm by a batten fixed through. This overcomes any potential adhesive failures or tears in staples or tacks etc. In the ceiling junction where the plasterboard layer must be continuous for reasons of fire spread prevention, it is also advisable to install laying tape at the junction between the plasterboard and the wall to ensure an airtight seal here also, thus improving the chances of a air test pass

 

(h) Seal all Penetrations in Plasterboard / Internal Lining

Even with the use of airtight outlet boxes there will be inevitable penetrations such as ceiling pen­dants, pull cords, recessed fittings etc. which must be made good manually, typically with mastic, and in this case, with a suitably fireproof mastic to maintain the fire barrier.


 

(i) Ensure Membrane is taken into Opening Reveals, Taped and Sealed and Made Continuous with Opening Seals, it is typical at openings to allow the membrane to run across the opening initially, then form a star cut into the opening, folding over the sections of membrane and trimming as necessary. In these cases, there are inevitable gaps in the airtight layer at the corners of the opening, and it is important to ensure that these are made good before subsequent installation of joinery etc.

 

(j) Sealing Batten 

This detail may be considered as an alternative, or ideally as an additional measure with (k). Since it is possible that replacement sash and case windows cannot be easily sealed around their perimeter (they are often ‘open’ around the outer edge) it may be necessary to use this detail which creates the airtight seal on the inside of the frame rather than ‘in line’ with the frame as noted below.

 

(k) Flexible Foam Sealant around Joinery Insertions

Gaps around openings are one of the most common of air infiltration uk paths. They range from 0 to 20mm, which is too large to be filled by mastic. Compressible flexible foams are ideal for this application. En­sure that the airtight membrane meets the seal on both sides to maintain the airtight layer overall and imrove the chances of a air test pass.

 

(l) Draughtstripping of Openings in Joinery 

Draughtstripping of joinery comes in many forms. It appears that synthetic rubber or elastomeric tubular seals work well, creating good seals with minimal compression. It is important that seals are unaffected by paintwork and subsequent decoration, or are easily accessible and removable. This is important so that seals can be replaced as they start to fail to maintain the airtight layer. Brush seals are likely to be used in sash and case windows.

 

(m) Silicone Sealant to External Window Surround 

Some form of neat and potentially paintable edge seal will be required externally

.

(n) Breather Membrane Instead of Netlon 

Notwithstanding the air barrier placed above, mineral wool is permeable to air movement and so replacing the netlon with a vapour permeable but airtight breather membrane reduces air movement in the insulation, improving insulation levels and reducing the risk of air leakage uk and air test failures generally.


 

9.7 Concrete Frame and

Panel

Discussion

Concrete panel construction represents a potentially good airtight form of construction. This is because the panels themselves are essentially airtight and being large, have fewer gaps which must be sealed. Being fairly predictable in terms of thermal and structural movement they are easy to seal well, and the only areas of concern then are the service penetrations and junctions with openings. With care and attention in these areas, a very good overall airtight external envelope is easily within reach. Having said that, in some early examples of this build­ing type, the sealants between panels have failed, highlighting the vulnerability of the system to such air test failure and the importance of correct specification and application.

 

A number of systems are available but the principles outlined for the improvement of the system chosen are widely applicable. Where two leafs of concrete panel are used, it is unlikely that the outer layer will be used as a rain ­screen layer, but this is sometimes done, and in these cases the airtightness of the internal layer of panels becomes critical, and may be augmented by the ap­plication of a vapour control and airtight membrane on the inner face of the insulation, applied to the panels before the insulation is installed. Guidance on the application of this membrane, and on poten­tially more airtight forms of insulation may be found

In Sweden, concrete panels are sometimes sealed to each other using polyurethane foam which is claimed to increase the airtightness levels and subsequent air test passes , but there does not appear to be any evidence of this form of sealant in the UK.

 

HIGH PRIORITY

(a) Integral Beam and Internal Panel

Important because this reduces the number of joints and simplifies construction.

 

(b) Sealing of all Penetrations

Care and attention to detail at all services and other penetrations is vital, most pres­sure tested panel buildings suffer air leakage uk at these locations, if these are not sealed it will result in a air test failure

 

(d) Sealing around Windows

The other major source of air leakage uk in con­crete panel buildings apart from (b) above, care and attention to detail along all joints needed.

 

(c) Screed Edge Strip and Seal

Ensures that air does not leak between floors around the perimeter and at other floor penetrations and breaks in the screed, another major cause of air test failure

 

MEDIUM PRIORITY

(a) Accessible Draughtstripping

It is important that the draughtstripping is accessible since it is likely that it will not last as long as the windows themselves and require replacement.

 

(b) Membrane around Windows

Required for vapour and air leakage con­trol, this also required attention and inspec­tion and can be seen as complementary to the mastic / silicon sealants

 

(c) Double Silicon Seal to External Panels

Double silicon sealant lines in the external panels is normally standard practice, and is typically good enough to ensure that the outer panels provide an effective airtight seal throughout, and help towards passing your air test

 


Costs

The alternative specification highlights best practice installation and should not incur any additional costs. The design of the panel construction system itself would dictate any cost difference.

 

LOW PRIORITY

(h) Membrane Under Roof Insulation

May not be required if the screed below is fully sealed against vapour and air flow, but given the typical number of penetrations in a commercial roof screed, the addition of a dedicated membrane may be considered advisable



1.0 Key Principles

1. The air test procedure is set out in CIBSE TM 23 and the ATTMA TS1

2. A air test involves sealing all ‘normal’ gaps such as vents and pressurising or depressurising the building. The level of fanpower required to maintain the pressure differential indicates the ‘leakiness’ or ‘permeability’ of the building.

3. Air test are typically followed by an air test audit (using smoke pencils, for example) to expose and make visible the various air test leakage routes.

4. Where projects comprise large quantities of a single component, component air testing in the laboratory may be appropriate as well as on site element air testing   .

                                  

2.0 Climatic conditions

As mentioned in Chapter 1, the raised pressure differential of 50 Pascals created during an air test is quite small. Whilst this is adequate to overcome most of the common pressure differential anomalies, such a small differential is vulnerable to larger pressure differences created by climatic conditions.

Air tests uk require calm days – i.e. a reading on the Beaufort Scale of 3 or less (3.4 to 5.4 metres per second wind speed at 10 m above ground) This corresponds to a gentle breeze with leaves and small twigs in constant motion. In winter conditions, and on exposed sites therefore an air test may not be possible, although it is often possible to make allowances, so long as these are carefully recorded during the air tests uk .

 

3.0 The Test itself

Guidance on testing buildings for an air test is contained in CIBSE Technical Memorandum TM23 Testing Buildings for Airtightness and in BS EN 13829: 2001. All accredited air tightness testers test to the guidelines contained in the BS EN and within the ATTMA TS1

 

Essentially the air test process is one of pressurising or (less commonly) de-pressurising the inside of the whole building, and of measuring the rate at which air needs to be blown or sucked to maintain that pressure differential; a building suffering from high amounts of air leakage uk during the air test will equalise readily and require a greater measurable effort to maintain the 50 Pascal differential, while a air tight building will easily contain the enforced differential and require little additional input during the air test .

The pressure difference is induced by one or more calibrated fans that are normally mounted within a suitable doorway. An adjustable door panel system, sealed around the edges is used which can also be connected to large external fans via collapsible ductwork if required. The rate of the fan or the volume flow of air through the fan during the air test can be understood as the rate of air entering / escaping throughout the remainder of the building envelope.
Buildings are air tested/air leakage tested in such a way as to recreate ‘normal’ conditions. Doors and windows are closed; trickle ventilators closed, extract fans and such like are closed but not sealed. Internal doors are wedged open. All of this must be actioned prior to the air test /air tightness test
uk

If the building is under construction the air test /air leakage testing uk can be  undertaken during  working hours, but sometimes this is not practical so some scheduling of work needs to be thought through in advance. With all external doors and windows sealed shut, some work becomes impossible (such as work with solvents requiring ventilation) and internal trades are normally ‘sealed in’ for a short time, where they can carry on undisturbed throughout the air test /air tightness test uk

In existing buildings the air tests uk /air tightness tests uk are normally carried out when the building is unoccupied if possible because of the disruption.

 

4.0 Air Leakage Audits

The air test /air tightness test quantifies the rate of air leakage  through the envelope as a whole, but it cannot locate the air leakage paths. Where remedial work is required therefore, air test /air tightness tests are followed by a range of auditing techniques designed to identify the specific places where air leakage is apparent  through the building envelope.

In many cases a simple visual inspection may be sufficient – especially if undertaken by someone with experience of the likely locations of air leakage.

However, most air leakage routes are difficult or impossible to spot without visual aids during the air test /air tightness test. One common technique is to use smoke tracers – smoke pencils or smoke machines. These render the air paths visible in certain situations during the air test . The building may be positively pressurised during the air test /air tightness test and the leaks witnessed externally, or, more usually, negatively pressurised while a smoke pencil is drawn over likely gaps and defects which become visible as the smoke is sucked inwards, during the air test

Another technique, which has certain advantages and disadvantages compared to smoke tracing along with an air test /air tightness test, is the use of an infrared camera during a Thermographic test. Used either externally or internally, these thermographic cameras register the radiant heat levels of surfaces and so are able to ‘see’ for example, where cold air is cooling the fabric around a gap internally, or conversely where warm air is escaping and heating the colder materials on the external face.

To work effectively, there needs to be a recognisable difference between the internal and external ambient temperature, so before any heating has been installed and on a warm summer’s day thermography testing may not be effective. Similarly on warm and sunny days, sunshine on external surfaces can distort the true situation so it is better on such days to wait until early evening. Conversely, rain on external surfaces can be equally distorting of the true thermal situation. However the  Thermographic cameras are useful in identifying problems at high level or difficult to reach areas, and are also very helpful in identifying other construction defects such as poorly installed (or non-existent!) insulation within the fabric.

On larger commercial buildings, the air test /air leakage testing may be undertaken at the same time as ‘standard’ ventilation system commissioning and associated studies, but these are not discussed as part of this guide.

5.0 Component Testing

A distinct aspect of overall air test /air tightness testing is the individual component test. This may be undertaken quite separately, in the laboratory or by the manufacturer of a particular component. Such air testing /air tightness sampling tests may be deemed necessary on a large project where large areas of one particular type of component, for example curtain walling, are to be specified, Insitu element air testing involves isolating the area within a temporary sealed compartment, which is then pressurized during the air test , and the air leakage related to the area of interest assessed. In this way sample areas of a building may be air test /air tightness tested using smaller fans as required.

5.1 Concrete frame and panel 

 

1 Introduction

As thermal insulation levels have risen in the last few years the proportion of energy lost to draughts has increased to the extent that now in some cases around half of all heat losses are due to air leakage across the building fabric. Given that approximately half of all energy used in the UK is in buildings, it is not hard to see that draughts account for a staggering amount of energy - and therefore cost – wastage, this where the air test comes into its own

The situation is such that further increasing thermal insulation levels would be largely unproductive unless air tightness is con­scientiously addressed. Air leakage has been shown to reduce the effectiveness of thermal insulation by up to 70% and so it is clear that if energy efficiency is to be improved in buildings, the next efforts will have to focus on air test /air leakage testing.

Many people make the mistake of thinking that an airtight building is necessarily a ‘stuffy’ building. This is not the case. All buildings have to be ventilated for health and comfort and airtight buildings are no different. An adequate ventilation system (which may well include open able windows as well as fans etc.) has to be planned for every building. The difference will be that a great deal of un­planned air leakage needs to be stemmed, this can be ascertained during the air test

The additional costs of creating an ad­equately airtight building can be negligible, but even where costs are increased e.g. for the air test , these can be more than offset by a reduction in the capital cost of heating and ventilation equipment, not to mention the long term savings in energy.

Given that the vast majority of building stock is existing, a great deal of attention will need to be given, in the foreseeable future, to remedial works to existing buildings, all existing air leakage paths can quickly be found during a air test . This guide specifically in­cludes examples of good and best practice remedial work in terms of air tightness and shows that such works can offer substantial benefits without undue disruption or cost, an air test will have a low impact on site works

Air Pressure Testing hopes to provide practical guidance on how to save energy and costs and protect building fabric. On the basis that prevention is cheaper and easier than cure, one purpose of this guide is to enable Designers to design inherently more robust and durable solutions which avoid costly and time consuming remedial works after the air test after a potential air leakage failure

The general guidance here is firmly focused on the idea of practical design and detailing, and should be read in conjunction with other guidance on sustainable design, energy efficiency and air tight­ness where necessary to provide an overall design framework. The details provided have been fully costed, tested and subjected to a Defects Liability insurance assessment. They are offered as viable alternatives to standard details, and illustrate the possibili­ties that exist. It simply remains for you, the reader, to apply them appropriately in the context of your next project prior to your air test/air tightness test

 Aims of this Guide       

 

• To highlight benefits of air tightness which include both energy and cost efficiency, improved comfort and reduced risks of damage to building fabric

 

• To improve awareness of the need for air tightness in con­struction at design stage

 

   • To promote detailing and specification solutions which cre­ate airtight and efficient buildings thus reducing the need for remedial works after air leakage failure- ‘prevention rather than cure’

 

• To show that new build and remedial air tightness are achiev­able without undue cost penalties to construction works due to multiple air leakage failures, you should be passing after the first air test

 

• .and in this way to help to ‘mainstream’ the good and best practice outlined in the document

 

Target audience

 

This Guide will help all those who wish to improve the airtightness/air leakage rate and energy efficiency of buildings through their construction, e.g:

 

• clients –building owners and users, principal and specialist contractors, interior designers architects and technicians, structural engineers, building service engineers, building surveyors, quantity surveyors/ cost consultants, maintenance and facilities managers, project managers , planning officers and building control officers, funding bodies and their professional advisors, government and non-governmental agencies

 

How to use this Guide

 

This Guide is divided into six sections. The first two sections provide an overview of the issues surrounding air tightness. Sections Three, Four and Five describe the requirements for the design process, the procure­ment and the air leakage/air tightness testing involved in designing for airtight buildings.

 

Section Six provides a number of representative details which have been optimised in terms of preparing for the air test . These are compared with standard details for a variety of construction types, and costed. This section will be primarily of interest to the design team. It should be read in conjunction with sections Three, Four and Five in particular, as all details must be placed in a suitable context.

 

6.0  The Context

 

Key Principles

 

1. Most UK construction is ‘leaky’ and wastes energy and money. Building airtight buildings can save energy and money, improve comfort and reduce the risk of damage to building fabric.

2. Airtight building will NOT mean ‘stuffy’ buildings. Good ventilation is vital for health and comfort - it is the UNPLANNED air leakage that we are aiming to stem.

3. Legislation is slowly catching up with best practice in Scotland, the UK and elsewhere and we can expect a greater emphasis on airtightness in all types of construction in due course.

4. Good and Best Practice Targets of air tightness/air leakage have been set for many types of buildings and are easily achiev­able.

6.1 Infiltration, Ventilation and Airtightness

 

Air infiltration (air leakage) is the uncontrolled flow of air through gaps in the fabric of buildings; this is all the more apparent during the air tes uk. It is driven by wind pressure and temperature differences and as a result is variable, responding in particular to changes in the weather. Infiltration (air leakage) levels are strongly affected by both design decisions and construction quality.

 

Ventilation, on the other hand, is the intended and controlled in­gress and egress of air through buildings, delivering fresh air, and exhausting stale air in combination with the designed heating sys­tem and humidity control, and the fabric of the building itself.

 

Whilst some unwanted air infiltration (air leakage)  will at times aid comfort lev­els, it is not reliable and moreover brings with it a range of signifi­cant disadvantages such as high levels of heat loss, reduction in performance of the installed thermal insulation, poor comfort, poor controllability and risks to the longevity of the building fabric it­self. It cannot be considered an acceptable alternative to designed ventilation. Air Infiltration (air leakage) needs to be reduced as much as possible if we are to create efficient, controllable, comfortable, healthy and durable buildings. This can be achieved by delivering airtight buildings that pass the air test /air leakage tests first time.

 

Air tightness is a term used to describe the ‘leakiness’ of the build­ing fabric. An airtight building will resist most unwanted air infiltra­tion (air leakage) while satisfying its fresh air requirements through a control­led ventilation strategy. Most existing buildings, even those built recently, are far from being airtight and may fail an air test and because of unwanted air infiltration (air leakage) generate huge costs to owners and occupants, in envi­ronmental, financial and health terms. One way of overcoming this making sure the building passes the air test/air leakage

 

It is important to emphasise the distinction between infiltration (air leakage) and ventilation, because while the primary purpose of this document is to show how buildings can be designed and constructed to be airtight and so pass the air test /air leakage test first time, it is equally important to stress that good levels of ventila­tion and a clear ventilation strategy will be required in every case. As the saying goes: ‘build tight, ventilate right.’

 

6.2  Why Build Airtight?

 

Legislation

At a rather prosaic level, the issue is important because it is now part of the Building Regulations in England and Wales concerning non-domestic new buildings over 500 sqm in area, all will now require an air test /air leakage, and is likely to affect a wider range of buildings soon. Whilst the initial targets set for airtightness of buildings are easy to achieve, it is equally likely that once in place, those targets will be ratcheted up to create ever more airtight and efficient buildings in the UK, in line with many of our European neighbours, at present many EU countries have a much higher first air test pass rate.

 

Energy and Cost Saving

 

Typically, the largest heat losses in most buildings are related to levels of thermal insulation, followed by those related to infiltration (air leakage), followed by those related to inefficient plant. Quite rightly therefore, most efforts to save energy and costs have until recently been di­rected at increasing thermal insulation levels. But as these levels have risen, so the relative contribution of infiltration (air leakage) has increased to the point where it can represent around half of all heat loss in a build­ing. In highly insulated buildings, the percentage may be higher.

This is reflected in the fact that total space heating costs in an airtight building may be as much as 40% less than in a leaky one

We are at the stage where it is likely that any further increase in thermal insulation levels would be ineffective until levels of air tight­ness in construction have improved considerably, this is where the air test in invaluable

 

Space Heating System Reduction

Clearly there is potential to reduce the capacity of space heating systems sized to cope with current levels of heat loss if those levels can be reduced by a half or more. Ensuring you achieve a low pass rate during the air test . In addition, airtight buildings are more predictable in terms of environmental control and the capital cost savings of installing smaller heating plant may be augmented by reduced plant room sizes in certain cases and particularly by reduced running costs in the longer term. As well as reducing the need for heating plant, airtight buildings of­fer much greater potential to respond positively to the local external climate through passive, or climate responsive design strategies such as natural ventilation, day lighting, the use of thermal mass and passive solar gain. Energy savings, capital and running costs, along with CO2 emissions can thus be further reduced.

 

Comfort and Control

 

As noted above, airtight buildings are not as affected by variations in external conditions. This makes them easier to control from an Engineer or Designer’s point of view, but it also makes them more comfortable from the point of view of the occupant.

In buildings with high levels of infiltration/air leakage those occupants near draughty windows, for example, will suffer the cold, particularly on windy days, whereas those elsewhere may well suffer from too much heat locally as the system tries to raise the temperature overall. Those who try to achieve comfortable levels through the use of the provided ventilation controls will find these to be rela­tively ineffective, whereas in more airtight buildings greater levels of control and comfort generally are achievable and local control and variation by occupants can have a more direct effect. In one example of an existing superstore, the ambient temperature in the store was raised by 5oC after the store had been sealed after the air test . Complaints by occupants in leaky buildings are common, and remedial measures are usually difficult and expensive, an air test can finds all air leakage paths quickly and effectively

 

 

Deterioration of Fabric

 

Leaky buildings allow cold air in through the construction causing discomfort, they also allow warm (and often moist) air out, causing heat loss. This warm and often moist air can find itself in colder parts of the outer construction where it can cool, and the moisture in the air can condense, leading to a buildup of moisture. This in turn can lead to:

 

decay of organic materials such as timber frames

 

saturation of insulating materials thus reducing their insulative effect (which increases heat loss further)

 

corrosion of metal components

 

frost damage where moisture has accumulated on the cold side of the insulation.

 

6.3  Legislation

 

In England and Wales the relevant regulation on air tightness is contained within Approved Document L1 for dwellings and L2 for non-domestic buildings (2006). There is general encouragement to consider air tightness issues, with a target air permeability for all buildings of 10 m3/hr/m2 envelope area at 50 Pa. In L2, build­ings with a floor area of greater than 500 m2 are required to have a air test if approved details are not used. Further tightening of the regulations are due in 2010.

 

Proposals for changes to the Energy standards were issued to public consultation in March 2006, including guidance that air tightness testing would be required if the calculation of energy performance included air permeability rates lower than 10m3/m2h at 50 Pa.

 

6.4  Measurement

 

A range of units for measuring air tightness/air leakage have been used in the past and this can complicate matters. However, one method only – “air permeability” - is the measure used in European Stan­dards, the new editions of the various UK Building regulations and in CIBSE’s TM23 Air Testing methodology and has been used throughout this document. The Air Permeability is defined as the volume flow in cubic metres of air per hour per square metre of the total building surface area (including the floor) at 50 Pascals pressure differential, expressed in m3/hr/m2 @ 50 Pa.

 

The main difference between the air permeability and previous practice in the UK is the inclusion of the non-exposed ground floor in the calculation of the ‘total surface area’ of the building. The difference between the new measurements and older ones tend only to be marked therefore where there are large volumes and ground floor areas. These new rules must be taken into account during the air test

 

Of the range of measurements used previously, the “Average Air Leakage Rate (or Index)” is similar to the “Air Permeability” except that non-exposed floors are excluded from the measure­ment. Another common expression is the “Air Changes per Hour at 50 Pascals (ACH @ 50 Pa). This is a useful measurement in particular because, when divided by twenty, it gives an approxi­mate value of the natural infiltration rate of the building at normal atmospheric pressure, which can then be used to help size heating and ventilating plant etc.

 

Yet another measurement is the “Equivalent Air Leakage Area” (ELA) at 50, 10 and/or 4 Pascals. This figure gives a representation of the sum of all of the individual cracks, gaps and openings as a single orifice and helps to visualise the scale of the air leakage problem. The main problem of changing the measurement technique is the ability to compare data

The standard pressure differential used is 50 Pascals. This is not in fact a very large pressure differential and corresponds to the pressure exerted by a column of water 5mm high. Compared to the fact that buildings can withstand wind induced pressures of at least 500 Pascals, this seems insignificant, but it is larger than wind induced pressure on a calm day, and by air testing and quoting air leakage figures at 50 Pascals, inaccuracies are reduced and repeatability is improved using this air test method.

 

6.5 Targets

As noted above, the only ‘official’ guidance in the UK applies in England and Wales and relates to non-domestic buildings over 500 sq.m in area. As can be seen from the table below, the target of 10 m3/hr/m2 at 50 Pa. is relatively easily achieved compared to the good and best practice noted in the 2000 document by CIBSE, TM23. This sets out the air tightness/air leakage testing methodology which is the de-facto methodology now followed for a air test in the UK.

 

A number of air tightness experts believe the stated targets are in­adequate when compared with the overwhelming need to address carbon emission reductions, and the potential to do so through air tightness measures. For example, the house illustrated to the right was built in 1992 for the same cost as nearby houses and improved upon the standards noted above by two thirds.

 

Building Type Air Permeability (m3/hr/m2 at 50 Pa)

Good Practice/ Best Practice

Dwellings 10.0/ 5.0

Dwellings (with balanced mech. vent.) 5.0/ 3.0

Offices (naturally ventilated) 7.0/ 3.5

Offices (with balanced mech. vent.) 3.5/ 2.0

Superstores 3.0/ 1.5

Offices (low energy) 3.5/ 2.0

Industrial 10.0/ 2.0

Museum and Archival Storage 1.7/ 1.25

Cold Storage 0.8/ 0.4

 

7.0 Designing for Airtightness

 

7.1 Key Principles

 

1. A Performance Specification is a crucial document for establishing the appropriate targets for airtightness, along with the methodology for achieving it, and the roles and responsibilities of those involved.

 

2. Conceiving of a building in zones and air barriers will help all involved to visualise the task.

 

3. Air barriers must be impermeable, continuous, durable and accessible. They should be supported by positive mechanical seals where possible.

 

4. The simplest solutions will be the most buildable and durable.

 

5. A culture of airtight construction does not yet prevail and until it does, it may be necessary to follow up targets with specific details and specifications, along with guidance on the process of implementing the necessary level of co-ordination and attention to detail.

 

Unlike design for deconstruction (the subject of the first in this series of Guides) and the forthcoming guide on chemi­cal-free design, the design of airtight buildings cannot be left to the specification and details, at least, not until the industry as a whole recognises the need and has sufficiently widespread ex­perience, unfortunately this alone would surely end in an air test failure. For the next few years, it will be necessary not only to provide careful details and performance specification, but also to develop thorough inspection and testing regimes, hence the need for Chapters 7.4 and 7.5 of this guide.

 

7.2  Performance Specification

The Performance specification may be the only document need­ed by the Architect / Designer / Client if the building is to be pro­cured through Design and Build or similar route. However, it is more likely to be part of a suite of documents including detailed drawings.

The performance specification allows appropriate targets to be set for the project, along with a description of how the process is to be conducted, in terms of scheduling, audits and air testing, and potentially remedial works. Given the increasing use of special­ist subcontractors, particularly in larger projects, it is also critical that the performance specification sets out both the responsibil­ity for, and constructive guidance regarding the co-ordination of trades with respect to the final air permeability of the completed envelope.

 

Zones and Barriers

Once appropriate targets have been set for the project, the next task is to identify zones which require greater or lesser airtight­ness uk levels. Ideally, these zones need to be identified on a draw­ing which also identifies the specific air barriers in red.

 


For example an industrial unit with office space is divided into five separate zones, and air barriers are identified as required. Such a drawing, however diagrammatic initially, helps to conceive of the subsequent specification and detailing needs, giving an overview of the problem.

Heated zones need to be kept separate from unheated zones such as roof voids, delivery bays etc. whilst service shafts may require particular attention. Boiler rooms with large flues and in­take vents may need to be separated prior to the air test

 

Entrances are often significant sources of draughts. Lobbies with doors set apart by around 4m, so that one door closes before the second is opened, can be effective, whereas in highly trafficked areas revolving doors are likely to be preferable. Tall buildings, with atria, stairways and service shafts all of which rise through the building can be prone to ‘stack effect’ air movement whereby warm air rises, dragging in cooler air from outside at the lower levels creating more acute air leakage problems. A number of tactics may be employed to reduce the effects, but in any event issues of airtightness are likely to be highlighted in these cases prior to the air test

 

7.3 Design

 

With the zones and air barriers located, it is necessary to design the air barriers themselves.

To be effective, the air barrier must:

 

• be made of suitably air impermeable materials;

 

• be continuous around the envelope or zone

 

• have sufficient strength to withstand any pressures created by wind, stack effect or air control systems

 

• be easily installed

 

• be durable

 

• be accessible for maintenance / replacement if appropriate

 

The last of these is important since there is evidence that the air­tightness of some constructions will tend to decrease over time and in particular the first period after completion.

 

There are a number of strategic measures which can be employed to simplify the business of designing an airtight building. Since service penetrations in and out of a building provide a major source of air leaks, one strategy is to collect all such penetrations into one accessible area, this will drastically improve the air test results

 

In construction types such as steel and timber frame, it is usually wise to employ a specific membrane or layer as the air barrier, rather than rely on sealant between, for example, the sheathing boards. Such a membrane can usually double up as the vapour barrier if used internally and gives the Designer the opportunity to consider and address airtightness explicitly, rather than as a function of other elements. Bear in mind that most membranes are flimsy and will need support in all areas, although there is minimal air pressure during the air test it can still move unsupported membranes this can result in an air test failure.

 

Another strategy is to employ service voids. Creating a service void internally allows for alteration and maintenance of services and fin­ishes without recourse to penetrations through the air barrier. This allows for long term good performance in contrast to membranes which are liable to penetration at all service points, necessitating careful sealing of each and every penetration, in the short term this will help to reduce the air leakage uk  rate and vastly improve your chances of an air test pass, not only initially, but over the years of alterations and maintenance to come.

 

Generally, it is better to conceive of the joints in airtight layers as ‘positively’ connected, anticipating differential movement and de­cay of adhesive or chemical bonds. For example, where different components of a curtain walling system are liable to differential movement, it is clear that a joint whereby the two components are held together with a positive mechanical connection across a compressed gasket is likely to remain airtight longer that a simple butt joint with a mastic sealant between, this attention to detail will improve your air leakage uk rate and the chances of an initial air test pass.

 

Finally it is clear that complex solutions to airtightness are likely to be more prone to poor execution and potentially to greater vul­nerability to differential movement, failure of sealants, dislocation of components and so on. It is important therefore to aim for the simplest solutions to providing a robust airtight layer, using the fewest separate materials, junctions and penetrations, and the easiest installation and maintenance, this will improve the buildablity and improve the chances of a air test /leakage  pass.

 

It is worth making a point of considering each and every specified component with regard not only to its own intrinsic airtightness uk characteristics, but with regard to the connections between it and adjacent components. It is important to provide explicit details and guidance at specific, and particularly tricky detail areas. On design and build contracts it may be necessary to allow for some form of review of proposed solutions and procedures, to try and out design any problematic/complicated junction which will dramatically improve the chance of an air test pass

 

The following provide a few examples whereby airtightness can be simplified at the earliest design stages.

However good the workmanship, blockwork on its own can never be considered airtight. Once plastered, on the other hand, it may be considered extremely airtight, with concern only for those edges and corners where cracking or gaps can appear. This may be contrasted with the more common practice of drylining block walls with plasterboard on battens or dabs, either way when either is built correctly they can form a excellent air test barrier.

 

Design & Detailing for Airtightness

 

Services Zones or Rooms enable a range of services to be collected together before exiting the building, allowing most of the penetrations in the external fabric to be grouped and sealed effectively prior to the air test .

 

Service voids enable cables and pipe­work to be installed and altered without needing to penetrate the air barrier. Note however that if they are not run in con­duit, protection may be needed against subsequent fixings, if not undertaken prior to the air test this will result in an air test failure

 

Positive physical connections are to be preferred over any other joint such as one relying on adhesives. In the timber frame example shown the air barrier membrane is shown lapped and sealed with mastic over a firm background (boards with stud behind) and with a positive mechanical fix - a batten - fixed over the top and through to the stud.

 


Trinsically non-airtight block wall behind, this form of construction typically gives rise to a wide range of air leakage uk paths behind the boards and into floor, partition wall and ceiling cavities. From the perspective of airtightness, dry lining should be avoided unless great care is taken, otherwise it will result in a air test failure

 

Similarly, timber floors are difficult to seal well without a good deal of care. On the continent - and to an increasing extent in the UK at large - concrete floor systems are being used for both ground and first floors (often for other reasons such as acoustics, fire and the desire for underfloor heating) and these are easier to make adequately airtight prior to the air test . Hollow planks however can leak into cavities and require to be sealed at their ends, this will dramatically improve air tightness and the chances of an air test pass

 

One important and often quoted example is the timber first floor connection with a block wall inner leaf. Who is responsible for ensuring absolute airtightness when the timber joists rest on the wall and are infilled between with block and mortar? Presumably the bricklayer, but is it then his fault if the timber is installed at the wrong moisture level and subsequently twists and warps, leaving cracks around every joint? Is it really feasible to attempt to tape or mastic seal around them all, and what if the underside of the ceiling is to be exposed? Far better perhaps, to do away with the joist-onto-wall detail al­together and replace with joist hangers. Increasingly, the de­signer should be seeking solutions which are intrinsically airtight because of the design, rather than continuing as before while ac­cepting an increased use of duck tape and mastic on site! Whilst these may get you through the initial airtightness tests/air tests, they are short term solutions and not likely to lead to the anticipated energy savings for the Client in the long term.

 

A good review of the various materials and components which al­low the Designer to create an air barrier may be found in the BRE Report BR448: Airtightness in Commercial and Public Buildings

 

7.4 Detailed Specification

Beyond the performance specification illustrated earlier, it is im­portant that the issue of air tightness/air tightness testing becomes embedded within the standard specification vocabulary.

Where an equal or approved alternative may be allowed, it is critical that an airtightness performance specification is part and parcel of that equality of performance. For example, it may no longer remain satisfactory merely to specify a membrane, but in addition to specify the fairly precise nature of the sealing, over­lapping and potentially the subsequent layers as well. Simply of­fering a performance specification and ensuring the responsibil­ity resides with the Contractor is all very well, but it is important too to offer solutions that will enable a satisfactory outcome to be achieved and subsequently improve air tightness uk and the chances of an air test pass.

 

Design & Detailing for Airtightness - Implementing Airtightness

In addition to the intrinsic lack of airtight­ness uk, a problem of drylining is that it can create hidden pathways for air and (unfortunately raise the chances of an air test failure), as above, into the void above suspended ceilings and elsewhere throughout the building

 

Timber joists built into a block wall - a poor detail for airtightness. Far better to use joist hangars and avoid the problem. Source. Concrete planks are not free of problems either hollow planks are often left ungrouted where they meet the external wall, which could lead to extensive air leakage internally and subsequently an air test failure.

 

7.5  Implementing Airtightness

 

Key Principles

 

5.       The Contractor or Project Manager must be made responsible for achieving the air tightness levels set. In particular, this will involve careful co-ordination between trades; if this doesn’t happen then an air test /air tightness failure will surely follow

 

6.       Inspection remains an integral part of achieving air tightness and passing the air test .

 

7.       Ideally at least 2 air tests (air tightness tests) will be undertaken; the first when the building is weather tight, and the second air test a couple of weeks or so before handover.

 

8.       Experience suggest that making one person (or team) responsible for air tightness is the most ef­fective way to tackle the issue, this will drastically improve the chances of a air test pass.

 

5.   Remedial air tightness works to existing properties can reap substantial benefits without undue disruption and improve the chances of an air test pass.

 

It is not yet generally possible within the UK to specify that a building shall be airtight and leave it to the Architect or Contrac­tor to sort out. There is not yet a culture of airtight construction, except perhaps, amongst those who construct superstores, these companies (as a whole) pass far more air tests on there fist attempt.

 

The responsibility of the Designer in regards to the airtightness cannot be overestimated, for if airtight buildings are to become main stream, as they are else­where in the world, the techniques must be above all simple and buildable, with most if not all of the ‘tricky’ areas designed out from the start. In this way, such techniques can become ‘second nature’ to Contractors and there is less reliance on potentially adversarial inspection and air test/air tightness testing failures.

 

Ideally too, the Designer will understand the issues sufficient to prepare a sound performance specification – giving achievable targets for air tests/airtightness as well as a clear description of respon­sibilities and procedures, and a clear and practical set of overall and detail drawings, along with a detailed specification.

 

In the meantime, and even with good documents, there is likely to be a need for effort and vigilance by both the Design Team and the Main Contractor or Project Manager on site. This chapter briefly describes this effort, while the next describes in more de­tail the actual air test procedures and auditing techniques used.

 

7.5  Plan of Work

The RIBA Plan of Work provides a framework for the entire de­sign and construction process. The table on the next page allo­cates specific tasks relating to airtightness to each Work Stage to enable a schedule of tasks and responsibilities for the Design team to be prepared according to each project, this will drastically improve the chances of a air test pass.

 

 

7.6  Roles and Responsibilities on Site

 

Designer / Design Team

The responsibilities of the Design Team are detailed on the follow­ing page, showing all stages including site works and beyond. Buildings usually comprise a number of different components, creating a myriad of routes through which air can escape if not carefully sealed at each and every junction. The Designer’s role is to simplify these details to reduce difficulties on site.


It is critical that the purpose of pursuing air tightness is explained so that all concerned understand why they are being asked to attend to these issues. The initial briefing of key personnel at mobilisation stage – whether or not this involves the air tightness specialist – is also critical in determining the approach to con­ducting the works, inspection, air tightness testing and auditing etc, prior to the air test . which will need to be dovetailed into the many other concerns on site.

 

On large projects it may be useful for one member of the Design Team to take special responsibility for the air tightness / air test issues.

 

Contractor

The Main Contractor’s principal responsibility is to deliver the air ­tightness/air test performance overall and the most likely task on any but the smallest jobs will be that of co-ordination between the sub­contractors. The Main Contractor must be clear that he carries responsibility for the overall air tightness and in turn must ensure that all subcontractors are clear about the extent of their respon­sibilities. This is important since there may be some deviation to conventional practice in order for air tightness to be achieved and a air test pass. It is always prudent to place someone in the site team who has experience of air leakage/air tests on their previous projects as their experience  may avert a potential air leakage/air test failure

 

RIBA Work Stage Design Team Tasks

 

A          Appraisal Establish appropriate air permeability rate

 

B          Feasibility / Briefing Note Microclimate

Test existing buildings / building to be refurbished

Identify procedure for review and air testing

 

C          Outline proposals Consider a/t issues in relation to decisions about form of construction

Identify zones and layers

 

D          Detailed Proposals Identify requirement of additional consultants / design by specialists

 

E          Final Proposals Ensure co-ordination between DT to ensure a/t envelope & penetrations

Detailed application of airtight materials, junctions, service penetrations

 

F          Production Info Select sub-contractors for specialist works (incl. testing)

Careful specification of components, membranes, materials

Emphasise methods for airtightness on documentation

Careful specification of components, membranes, materials

Emphasise responsibilities in specification for dealing with ‘loose ends’ between sub-contractor interfaces

 

G          Tender Docum’n Define Contractors’ responsibilities for co-ordinating work sequences

 

H          Tender Action Ensure selected tenders include adequate airtightness procedures

 

J           Mobilisation Brief all involved in areas critical to air infiltration before work starts

Preparation of samples, training, testing and QA procedures

 

K-L       Site Works Co-ordinate inspection with Building Control if required

Ensure inspection of areas to be covered

Ensure audits and testing schedule is adhered to

Ensure design changes do not compromise airtightness performance

 

M         Post Completion Obtain feedback from concerning comfort and energy consumption

Carry out remedial work as required at end of DLP.

 

As with the Design Team, experience suggests that the best (air test ) per­formance has been achieved by Contractors who employ a dedi­cated individual (or team) to carry responsibility for airtightness and the air test , to inspect the works and instruct as required.

 

For Contractors, the issues of airtightness/air leakage uk and the passing of the air test are intimately linked to issues of good or bad workmanship in general and this can make the issue both more sensitive, but also more difficult to control. Even simple buildings are immensely complex and so the most important aspect of all is the creation of an overall culture of care­ful, tidy, accurate and airtight construction, something which can­not be simply forced through with a performance specification.

 

It is easier to specify and draw an airtight detail than to build it, and so the emphasis on inspection and Contractor responsibility has not developed from a prejudice against Contractors, but from a realistic appreciation that this issue cannot be entirely resolved ‘on paper.’ It is genuinely about a culture shift (at least for many in the industry) and this is where the real challenge lies. Once this shift is adopted there will be a far higher percentage of companies passing their air leakage/air tests uk at the first attempt

 

7.7   Inspection

Air leakage uk found in dwellings according to studies undertaken by BRE. The studies offer a range of conclusions, the most significant of which is that the greatest volume of air leakage uk is occurring in areas out with the ‘normal’ consideration of ventilation, through the myriad of cracks and openings all over the building which is described as ‘background air leakage uk, a common cause of air test failures’

Of the background air leakage subsequently investigated, the principal air leakage routes the greatest cause of air test failures  were noted as being:

 

• Plasterboard dry lining on dabs or battens, often linked to routes behind skirtings etc.

 

• Cracks and joints in the main structure; open perpends, shrink­age & settlement cracks

 

• Joists penetrating external walls, esp. inner leaf of cavity walls

 

• Timber floors, under skirtings and between boards

 

• Internal stud walls, at junctions with timber floors and ceilings

 

• Service entries and ducts

 

• Areas of unplastered masonry walls; intermediate floors, be­hind baths, inside service ducts

.


It is perhaps worth mentioning that the BRE results were based on buildings using dry lining on masonry walls and timber floors. Had the masonry walls been plastered, if concrete floors had been used, and if basic airtightness measures were taken, it is likely that the principal problems would occur around service penetrations, and, to a lesser extent, around windows, doors and roof lights. This is the experience of countries where envelope airtightness generally is more developed. As a result they achieve far better first time air leakage/air test results and subsequent air test passes

 

The following table lists many of the most common infiltration problem areas. On larger projects, common problems include:

 

·         Incomplete bulkheads at eaves;

 

·         Gaps where block work abuts to steel columns or beams

 

·         Uncapped cavity walls, at eaves (right) and mid-points where cavity walls change to composite panels

 

·         Gaps along the underside of corrugated roof linings  

 

·         Gaps between block work and steel, and uncapped cavity wall at join with composite panels Common Locations for Inspection (Applicable to all types of Construction)

 

Foundation / Ground Floor

·         Check wall and floor dpcs form an adequate air tightness layer, is a separate layer needed?

 

·         Check gaps at perimeter insulation strips

 

·         Check potential movement gaps between loadbearing structure such as columns and adjacent non- loadbearing slab

 

First and Intermediate Floor Levels

·         Concrete floors: Check joint between the floor and plasterboard to walls

 

·         Check gaps between concrete planks, or beam & blocks are sealed at the wall

 

·         Check voids under floor finishes and service run penetrations

 

·         Timber floors: Check a membrane seal has been incorporated if required

 

·         Check any membrane used is supported between joists

 

Eaves and Verge

·         Check continuity of airtight layer between wall and roof / ceiling

 

Ceiling level beneath the roof

·         Check for separation between deliberate roof ventilation and the conditioned zone

 

·         Check for service penetrations and hatches which pass across the airtight layer

 

Boundaries between different wall envelope systems

·         Check all systems have a dedicated airtightness layer assigned, and that these can be constructed to be continuous across dissimilar elements

 

Windows and Doors

·         Check that the frame to wall junction is properly sealed and continuous with the wall airtight layer, particularly at cills

 

·         Check the windows and doors have appropriate weather seals between the opening unit and the frame

 

Services penetrations

·         Check for proper seals at service entry points, and at points of entry into conditioned zones. These may also require fire protection

 

Main Entrances

·         Check that the whole entrance area is separated from the conditioned zone by an inner airtight layer

 

Lift Shafts, Service Cores, Delivery Areas / Car Park

·         Check these have been separated from conditioned zones with air barriers and draughtproofed access doors

 

Where profile fillers are used poor workmanship is common

 

• Perforated (acoustic) roofs, where the unsealed mineral fibre acoustic layer bridges the eaves of the building, constituting a major leakage point

 

• Gaps where plasterboard or wall linings are incomplete, com­monly above suspended ceilings and to the underside of beams

 

• Incomplete door and window reveals

 

• Services Penetrations into the building, and between zones inside the building

 

Another common issue is porous blockwork, particularly when internal walls are drylined rather than plastered or painted. Where this is likely to be unavoidable, it may be worth requiring blockwork to have an initial air test for air permeability(air leakage), and to have an AP value (by an accredited lab) that is no more than 50% of the target Air Perme­ability/air leakage  uk for the overall building.

 

7.8  Air Testing and Audit Schedule

In many cases to date, an air test /air tightness test has been carried out a week or so before practical completion. If the result is poor – a high rate of air leakage – then a great deal of work suddenly needs to be done, often to areas which have been covered up and the whole business can be both costly and time consuming, just at the point where in many contracts there is already considerable pressure on Contractors.

 

Far better therefore to schedule the air test /air tightness test uk at a time where remedial works are relatively simple to perform. On the other hand, it is important that a air test /air tightness test is undertaken close to han­dover so that the Client and Design Team can be sure that the completed building accords with the performance specification, and so passes the air test first time .

 

Ideally therefore, two Air tests/air tightness tests at least should be carried out. The first Air test /air tightness test uk should be undertaken as soon as a meaningfully air- and weathertight envelope has been installed. Ideally, all air barriers are still accessible and any defects can be readily put right. This air test /air tightness test uk plus the audit techniques which are likely to accompany it, may be used to ensure an acceptable airtightness performance and give a good indication of where subsequent works may ad­vantageously targeted.

.

In this way, the second and final air test /air tightness test uk serves simply to confirm the performance of the building, hopefully at a slightly improved level from the first air test /air tightness test uk without the need for costly and complex operations late in the day.

 

Such air tightness uk /air leakage testing uk schedule is nonetheless costly in itself, but for those who have been involved in such air leakage testing uk schedules, experience suggests that this remains the most cost effective way to deal with the issue. Certainly it is worth avoiding excessive remedial works at the eleventh hour, just prior to the air test works. With a sufficiently good first air test /air tightness test uk per­formance, it may even be possible to dispense with the final air test , if this is deemed acceptable to the Design Team Leader or Cli­ent.

 

It is often the case that the envelope is not sufficiently complete on the due date for the air test /air tightness testing uk. This then necessitates a complex process of temporary sealing of the incomplete areas. It is harder than to ascertain the location of the air leakage and allowances are made which may prove misleading. Experience suggests that this is not ideal and it would be better to put off the air test /air tightness test uk for a week and carry it out when the envelope is complete and ‘as intended’ this will drastically improve the chances of a air test pass.

 

 

On larger projects, more air leakage uk /air tests uk may be needed, or more specific tests of individual areas required. Large projects with multiple units of a similar nature may benefit from either pre-installation component testing, or insitu testing of one installed component to establish acceptable air test /airtightness uk levels early on.

 

7.9  Remedial Airtightness Works

With airtightness testing and a general awareness of airtightness uk issues developing around new build situations, the principal area of concern, as with energy efficiency in general is the existing building stock. In terms of airtightness uk, the UK building stock is considerably worse than comparable northern latitude countries  and there is a good deal of room for improvement, if these improvements do not occur then a large percentage of companies will fail their air test /air leakage tests.

 

Either as a standalone measure or as part of a package of en­ergy efficiency measures generally, there is scope for remedial works to most of the existing UK building stock. Relatively simple measures may in many cases be sufficient, using a wide range of sealants to control air leakage uk. However, it is important that such measures are combined with attention to the ventilation require­ments of buildings where, to date, insufficient ventilation has been ‘augmented’ by infiltration and exfiltration which, if reduced, could lead to other problems and a subsequent air test failure.

 

As with thermal insulation, there is an extent to which controlling some of the air leakage merely diverts the flow of air, inward or outward, to another defect or gap, (this will still result in an air test failure) but there is such scope for improvement that even fairly basic efforts are likely to reap sub­stantial environmental, financial and comfort benefits for owners and occupiers alike.

 

There are many examples of remedial works described in the various publications noted in the references. Some of the more successful measures included carefully sealed secondary glaz­ing installed where old windows had to be kept for conservation purposes, draughtproofing of doors and entranceways generally, and installation of lobbies in well trafficked reception areas, at­tention to draughtproofing of existing windows and targeted use of flexible sealants to ill fitting components and joints between different construction types, will drastically improve your air leakage rate and will should enable  you to pass your air test first time

 

9.       Testing Airtightness

 

Key Principles

 

5.       Air test procedure is set out in CIBSE TM 23 and in BS EN 13829: 2001.

 

6.       An air test / air tightness test uk involves sealing all ‘normal’ gaps such as vents and pressurising or depressuris­ing the building. The level of fanpower required to maintain the pressure differential indicates the ‘leakiness’ or ‘permeability’ of the building.

 

7.       Air  tests uk/ air tightness tests are typically followed by an audit (using smoke pencils, for example) to expose and make visible the various air leakage uk routes during the air test .

 

8.       Where projects comprise large quantities of a single component, component testing in the labora­tory may be appropriate as well as on site element air testing .

 

8.1 Climatic conditions

As mentioned in Chapter 1, the raised pressure differential of 50 Pascals created during an air test /air leakage test uk is quite small. Whilst this is adequate to overcome most of the common pressure dif­ferential anomalies, such a small differential is vulnerable to larg­er pressure differences created by climatic conditions.

 

Air tests uk/ air tightness tests uk require calm days – i.e. a reading on the Beau­fort Scale of 3 or less (3.4 to 5.4 metres per second wind speed at 10 m above ground) This corresponds to a gentle breeze with leaves and small twigs in constant motion. In winter conditions and on exposed sites therefore testing may not be possible, al­though it is often possible to make allowances, so long as these are carefully recorded, during the air test .

 

8.3   The Air Test itself

Essentially the process is one of pressurising the inside of the whole building (during the air test )  and of measuring the rate at which air needs to be blown or sucked to maintain that pressure differential; a building suffering large amounts of air leakage will equalise readily and require a greater measurable effort to maintain the 50 Pascal differential, while a air tight building will easily contain the enforced differential and require little additional input during the air test , this will be easily recognisable within the first couple of minutes during the air test / air tightness test uk

 

The pressure difference is induced by one or more calibrated fans that are normally mounted within a suitable doorway. An adjustable door panel system, sealed around the edges is used which can also be connected to large external fans via collaps­ible ductwork if required. The rate of the fan, or the volume flow of air through the fan can be understood as the rate of air entering / escaping throughout the remainder of the building envelope, this is recorded throughout the air test .

 

Buildings are air tested in such a way as to recreate ‘normal’ condi­tions. Doors and windows are closed, trickle ventilators closed, extract fans and such like are closed but not sealed. Internal doors are wedged open.

 

If the building is under construction the  air test /air leakage testing uk is ideally undertaken out of working hours, but sometimes this is not practical so some scheduling of work needs to be thought through in advance. With all external doors and windows sealed shut, some work becomes impossible (such as work with solvents requiring ventilation) and internal trades are normally ‘sealed in’ for a short time, where they can carry on undisturbed during air test .

In existing buildings, air tests uk/air tightness tests are normally carried out when the building is unoccupied if possible because of the disruption.

 

8.3 Air Leakage Audits

The air test /air tightness test uk quantifies the rate of air leakage uk through the envelope as a whole, but it cannot locate the air leakage uk paths. Where remedial work is required therefore, air test /air tightness tests uk are followed by a range of auditing techniques designed to identify the specific places where air is leaking.

 

In many cases a simple visual inspection may be sufficient – es­pecially if undertaken by someone with experience of the likely locations of leakage, this is a good way of lowering the risk of a air test / air tightness test uk failure

 

However, most leakage routes are difficult or impossible to spot without visual aids. One common technique is to use smoke trac­ers – smoke pencils or smoke machines. These render the air leakage paths visible in certain situations during the air test . The building may be positively pressurised and the air leaks witnessed externally, or, more usually, negatively pressurised while a smoke pencil is drawn over likely gaps and defects which become visible as the smoke is sucked inwards during the air test .

 

Another technique, which has certain advantages and disadvan­tages compared to smoke tracing, is the use of an infrared cam­era by undertaking a thermographic survey, Used either externally or internally, these ther­mographic cameras register the radiant heat levels of surfaces and so are able to ‘see’ for example, where cold air is cooling the fabric around a gap internally, or conversely where warm air is escaping and heating the colder materials on the external face.

 

To work effectively, there needs to be a recognisable difference between the internal and external ambient temperature, so be­fore any heating has been installed and on a warm summer’s day thermography / thermographic surveys may not be effective. Similarly on warm and sunny days, sunshine on external surfaces can distort the true situation so it is better on such days to wait until early evening. Conversely, rain on external surfaces can be equally distorting of the true thermal situation. However, these cameras are useful in identifying problems at high level or difficult to reach areas, and are also very helpful in identifying other construction defects such as poorly installed (or non-existent!) insulation within the fabric.

 

On larger commercial buildings, air tests uk/ air tightness testing may be un­dertaken at the same time as ‘standard’ ventilation system com­missioning and associated studies

 

8.4 Component Testing

A distinct aspect of overall air tightness testing is the individual component air test . This may be undertaken quite separately, in the laboratory or by the manufacturer of a particular component. Such air tests uk may be deemed necessary on a large project where large areas of one particular type of component, for example curtain walling, are to be specified.

 

Insitu element air testing involves isolating the area within a tem­porary sealed compartment, which is then pressurised, and the air leakage related to the area of interest assessed. In this way sample areas of a building may be air tested using smaller fans as required.

 

9. The Details

Caveat

It is important to emphasise the scope and purpose of the following drawings and specifications.

They are included solely to show practitioners the sort of altera­tions that can be made in order to enable buildings to be much more airtight in general.

 

Their purpose is not to offer approved details in any sense, but to illustrate the difference between details and specifications which do not address airtightness issues, and those that do. It is the dif­ferences between the originals and alternatives which is intended to be illustrative, not necessarily the alternatives themselves.

 

The original details have been taken from conventional details and specifications we believe to be broadly representative of their construction types. We hope the principles shown, and the specific references made will assist designers in making similar changes in their own work, but it goes without saying that air test /air testing cannot take responsibility for any work undertaken as a result of the use of these details.

 

Specifically, these details are not intended to show best practice in any sense, nor are they even intended to be up to date. We have striven in the preparation of these details and specifications to keep as close to the original as possible. We have done this in order to show that some quite fundamental alterations – in terms of airtightness - may be made with the minimum of visual or func­tional impact on the original. Where these original details and specifications do not meet current standards or aspirations, the alternatives given are likely to be similarly wanting. To re-iterate, the purpose is not to produce approved details, but to illustrate the process of improvement – in terms of airtightness only – that may be made.

 

Consideration of priorities in airtightness design and specifica­tion is potentially misleading since, in effect, all gaps, cracks or tears let in air and the sealing of one simply redirects infiltration to somewhere else, this becomes all the more apparent during the air test . Like thermal insulation, what is important is the level of continuity generally, not any particular detail on its own. Nonetheless some prioritisation has been attempted in order to help Designers to prioritise their own efforts since not all measures may be necessary.

 


9.1 Steel Frame + Concrete Block Cavity Wall

 

Original Specification

 

Discussion

Because of the largely wet trades involved, one might imagine a masonry construction is  inher­ently more airtight than the dry fixed timber frame and curtain walling construction types. However, insofar as concrete inevitably shrinks as it dries, as mortar beds and perpends are often poorly filled, and due to the differential movement between masonry and the steel frame, the myriad pathways that open up can make masonry buildings extremely susceptible to infiltration, this can lead to an air test failure.

 

To make things worse, construction such as this does not easily lend itself to a simple, single airtight layer which can be applied separately and therefore the need for vigilance, and some care and attention to a number of small but potentially time consuming sealing jobs is high, however these must be undertaken if you are to pass an air test on the first attempt.

 

It would be possible to form an airtight layer inter­nally through the use of an applied membrane and the adoption of a service void. This would have the advantage of allowing for changes in the service or fit-out provision without the risk of damage of compromise of the airtight mem­brane,(this will lead to a air test failure) and for those inclined to this solution.

 

A parge coat and service void could have a similar effect, but the use of plaster internally is a common and effective technique for creating an airtight layer and is preferable in this instance as it is closer to the original detail and will improve the overall air test results

 

HIGH PRIORITY

·         Wet Plaster Finish or Wet plaster coat costs more but provides a better finish overall, as well as significantly improved airtightness across the masonry leaf. Plaster should be extended to all wall areas and not left off in areas which will not be seen,( such as suspended ceilings.

 

·         Membranes Lapped & Sealed2 lines of tape and a positive mechanical fixing by batten ensure laps are sealed for the long term

 

·         Mastic to Skirtings, Linings etc.

 

·         Critical in this detail since the plaster cannot form a continuous layer at these junctions

 

·         Sealed Cavity Closer: Gaps around openings are common so care is needed here to prevent infiltration around the frame and into the cavity

 

·         Vapour Barrier Seal at Eaves: Important here since no effective seal is noted on the original which could lead to excessive airflow at this vulnerable point.

 

 

Costs

The most significant cost implication is associated with the addition of the wet plaster coat to the inner leaf of blockwork. This results in approximately a 60% increase in cost, although the quality of the blockwork is not as critical. This item is also significant in that is changes the ‘look’ of the detail but is probably the highest priority.

 

Otherwise, most of the costs are associated with the additional time, effort and care implicated within the specification and details. Of these, the most significant is the additional labour and materials required for the joining of the vapour barrier in the roof, and sealing it around the perimeter. This work almost certainly more than doubles the cost of the vapour barrier in the original detail, but again, represents a critical factor in reducing air leakage and saves the cost of multiple air test failures.

 

A number of the measures described represent no more than a re-iteration of good practice, such as the sealing of perpends, lapping and sealing of membranes, draught stripping of windows and so on. These may assumed to incur no cost implication, but perhaps one of attention to details (this usually results in first time air test passes) on site.

 

The mastic sealant to skirtings, cills and the like would add about 50% to the costs of these items, though these items represent only a small fraction of the overall costs.

Taping of the insulation boards would depend largely on the board type, but might realistically attract only a marginal cost increase, as would the use of com­pressible foam around the steelwork.

 

MEDIUM PRIORITY

·         Concrete Slab Floors:Concrete slabs form an airtight layer but joints with penetrations such as perimeter blockwork, insulation or structural columns must be sealed.

 

·         Cill to Window Sealing: Double sealed detail which increases the chance of securing an airtight seal at this often overlooked junction

 

·         Compressible Foam between Steel and Blockwork: Potential solution to the inevitable gap which will form here, also sealable with mastic oninside face only.

 

LOW PRIORITY

·         Perpends Fully Filled: Not critical if a wet plaster finish is applied internally, but high priority if they are not.

 

·         T&G and Taped Insulation:Not technically part of the airtight layer, but gaps here simply increase the likelihood of infiltration (air leakage) and are relatively easily sealed.

 

·         Expanding Foam to Gap at Eaves: Not part of the airtight layer but by seal­ing a large gap in the fabric, this reduces the wind pressure driven airflow within the cavity thus reducing the risk of infiltration (air leakage) indirectly.

 


9.2 Index

a) Perpends fully filled

A common problem with blockwork and brickwork buildings is that perpends are not completely

filled and this leads to air flow (air leakage) through the wall this becomes apparent during the air test . To an extent this measure is superceded by both points (aa) and (d), but it is still worth making the point in order to draw attention to this workmanship issue in general, it’s the attention to detail that ensures you pass the air test first time

 

b) Blockwork Maximum Air Permeability by Component Test

An alternative to wet plastering the blockwork on the inner leaf is to require a component air test of the blockwork to satisfy a maximum air permeability of, say, 5m3/hr/m2 or less. On larger

projects, or where wet plastering is unlikely to be effective or desirable, this is one method of

ensuring a reasonable degree of airtightness from the blockwork leaf. These conditions may also be used for the outer leaf but is not as important because it is the inner leaf which is providing the main air barrier for the air test .

 

c) Membrane Lapped and Sealed

Typically membranes are lapped and stapled or tacked, but in order to create airtight layers, it is

important that these laps are rigorously sealed. Best practice in this regard - beyond the correct

use of Manufacturers’ overlap dimensions, proprietary tapes and other accessories - is to run a

layer of double sided tape between the membranes at the overlap and run a tape over the

leading edge of the outer sheet. In addition, since many tapes tend not to last too well, it is

advisable to ensure that laps are made directly over supported areas (i.e. with studs or dwangs

directly behind) and are held down positively with battens fixed through forming a mechanically

tight, as well as an adhesive seal, this will provide an especially strong air tightness seal and will improve the chances of an air test pass

 

d) T&G or Shiplap and Taped Non-Mineral Fibre Insulation

Mineral fibre is permeable to air movement and cannot be counted upon to help in reducing air

leak­age. Extruded polystyrene and other closed cell plastic insulation materials do not suffer from

this and so have the potential to reduce air leakage and improve air tightness in and out of the

building(improving the chance of passing the air test ). However, they are only likely to do so if they are effectively joined at their edges, at corners, openings and around wall ties etc. For this reason, it is likely that t&g or shiplap edge boards (which are available from a number of Manufacturers) will offer better connections, and these can be further augmented by the use of a sealant tape externally.

 

e) Wet Plaster Finish Internally

An alternative to arranging component tests for the blockwork. A simple block finish with 2 coats

of paint which in terms of airtightness is an improvement on a uncoated block wall but is not

sufficient to consider the blockwork airtight in the least. Wet plastering of the blockwork is more

expensive but ensures an airtight masonry leaf, this will improve the chances of a air test . The plaster should extend to all areas of the wall, regardless of whether they will be hidden by suspended ceilings or raised floors. It should extend right to the floor and to undersides of steel beams etc. and where broken by service boxes etc. should be conscientiously filled and sealed.

An alternative which would have a similar effect would be to use a parge coat over the blockwork,

before application of a service void and separate finish layer. The simple wet plaster finish is

closer to the original.

 

f) Mastic Both Edges to Skirtings, Reveal Linings etc.

Where the corner junction behind has been carefully sealed then this measure may not be

required, but in the examples shown on this construction, this particular detail is critical since it

forms an integral part of the airtight layer, particularly where the plaster has to be discontinuous.

 

g) Concrete Slab Floors

Unchanged from the original detail, this is simply to note that concrete slabs form an airtight

barrier and may therefore be considered good practice in this regard. However, no note is made

of the need for care to be taken where the slab meets elements of structure which pass through,

steel columns, for example. At these junctions, a compressible foam strip may be laid around the

steel prior to pour­ing the concrete if practicable, or a mastic sealant may be used subsequent to

the pour to seal the inevitable shrinkage cracks which will form and become air leakage uk paths and will increase the chances of passing the air test

 


h) Insulated and Robust Cavity Closer

A robust and insulated cavity closer enables the cavity to be effectively closed, the gap to be

bridged with insulation without risk of moisture flow between inner and outer and the window to

be securely fixed at the head and jambs if required. The gap between window frames and the

main wall is a no­torious place for infiltration (air leakage uk) and so increases the chances of a air test failure so it is important that this junction is carefully sealed. The flanges of the cavity barrier should be closed against the blockwork faces with a continuous mastic bead between on each flange so that airflow into the cavity from outside or in is prevented, thus resulting in better air tightness uk

 

i) Proprietary Cill with Foam Sealant Internally and Mastic Sealant Externally

In addition to the mechanical fixing of the window frame through the cill piece, it is important that

this fixing is made through a compressible foam strip which is then sealed against air leakage uk

from outside with a mastic type sealant. This gives the Contractors two opportunities to ensure a

completely airtight seal at this particularly vulnerable point, prior to the air test .

 

k) Compressible Foam Strip beneath Steel Beam to Blockwork Top

For reasons of both initial shrinkage and subsequent structural movement, it is to be expected

that a direct connection between a steel beam (or column) and a block wall will open up over time

to form a potential route for infiltration (air leakage). One way to try and reduce this inevitable gap

is to build the blockwork against a compressible foam strip which immediately expands to fill the

gap between and remains flexible thus continuing to fill the gap even after shrinkage and

movement. Since compressible foam strips are not intrinsically airtight, mastic sealant should be

used in addition to form a neat internal joint which should further seal the connection, thus

drastically improving air tightness uk and the chances of passing your air test first time.

 

l) Vapour Barrier Detail at Eaves

Here the vapour barrier is positively sealed to the steel perimeter beam to properly seal the

ceiling vapour - and air - barrier along its edge. Assuming that the steel beam is without

penetrations (a specification note has been added to ensure that this is checked) then as long as

the plaster seal to the underside of the beam is adequate, an airtight layer has been formed

which may be discontinuous in materials but continuous in terms of airtightness uk, this building method should help you pass the air test at the fist attempt.

 

m) Expanding Foam to Large Gap at Eaves

Whilst not strictly part of the airtight layer, this measure reduces the potential wind pressures on

the cavity which in turn reduces the risk of air infiltration through the airtight layer itself. Note also the introduc­tion of a ply layer above to support the insulation (nothing is noted as doing so in the

original detail) but significantly against which the foam can create a firm seal which should

drastically improve air tightness uk and a air test pass

 

n) Mastic Sealant to Joints

Additional notes to seal connections between dissimilar materials which are likely to provide

routeways for airflow (air leakage) unless conscientiously sealed.

 

o) Draughtstripping to Windows and Doors

Most commercially available joinery, metal or plastic windows and doors will be adequately

draughtstripped but it is important to explicitly ensure that this is the case, and that seals

(preferably tubular rubber / epdm type) are accessible and can be easily replaced should they

begin to fail to adequately seal when closed.

 

9.3 Timber Frame with Con­crete Block Outer Leaf

Original Specification

Discussion

Despite the inherently dry fixed nature of timber frame construction, it offers good opportunities to ensure airtightness uk because of the existing convention of using vapour control layers internal to the insulation and breather membranes externally. This gives the Designer two layers with which to work to form a robust airtight envelope overall, and without introducing any significant or new component. The outer layer of blockwork (or brick, or dry cladding of any type) need not perform any major role in the airtightness strategy, and should not affect the air test result.

 

Although there are a large number of small adjust­ments to conventional practice outlined, none of these, except perhaps the addition of the service void and backing board involve any major shift in construc­tion process. Experience suggests that such changes are readily made and subsumed within the standard details and specification clauses of the practice.

 

More tricky is the need to convey the need for greater effort, co-ordination, care and vigilance to Contractors for whom there is little to be gained from the good practice noted, and quite a lot to be lost in terms of potentially time consuming additional tasks. In the short term it is important to emphasise the additional co-ordination and tasks to Contractors at the time of tendering so that these are not overlooked and the extra effort can be adequately assessed.

 

HIGH PRIORITY

a) Continuity of Layer / Co-ordination of Trades

General measures to ensure tradesmen are aware of the need for air tightness that all involved are conscientious and rigorous, and that someone is responsible for co-ordination between trades prior to the air test

 

b) Service Void

Use of a service void means most if not all penetrations through the vapour control and airtight layer can be avoided.

 

c) Joist Hangers

Use of Joist hangers avoids the common problems of air infiltration where joists are built into the inner leaf

 

d) Membrane to Floor Perimeter Beams

Slightly awkward solution for solving the problems of discontinuity at this area which is nearly impossible to solve otherwise.

 

e) Flexible Foam around Joinery

Gaps around openings are common and neat, effective solutions can be difficult, careful use of flexible foam enables effec­tive and durable seals to be formed.

 

f) Continuous Layer Over Partitions

High priority because of the high potential exfiltration rates and condensation risks at this point

 

g) Backing Boards

Use of backing boards makes installation of the membrane easy and thus less prone to poor workmanship and subsequent air test failure.

 

MEDIUM PRIORITY

a) Membranes Lapped & Sealed

2 lines of tape and a positive mechanical fixing by batten ensure laps are sealed for the long term

 


Costs

Not surprisingly, the addition of the service voids adds considerably to the costs of both the walls and ceilings. Of course, such costs say nothing of the increased ease of services installation, nor of the long term benefits of a much greater access for upgrading and alterations.

Nonetheless, the addition of the OSB and battens forming the service void in the walls adds approxi­mately 35% to the cost of the external wall. Mechani­cally fixing the vapour barrier to the floor and taping would add approximately 4% to the overall wall cost in addition.

Adding the service void to the ceiling would represent an approximate 130% increase in cost over just the 2 layers of plasterboard. But again, services instal­lation would be easier.

 

The additional work associated with the breather membrane would incur a similar additional cost, but may not be a priority if the internal vapour barrier is well installed.

The mastic sealing of the skirting boards would increase the cost of their installation by about 50%, although these represent only small costs overall, the use of polythene strips at the floor and eaves, and the use of foam around the windows would attract only a marginal cost increase.

The use of flexible insulation need not attract any increase in cost if a common, economical type was chosen. Remember all of the above can be far more economical that not passing the air test and therefore resulting in costly LED’s

 

MEDIUM PRIORITY

a) Joinery Draughtstripping

Tubular seals are probably the best option.it is important that they can be easily ac­cessed for maintenance and replacement.

 

b) Continuity at Openings

Continuity between the framing sealant (m) and the membrane can be tricky and care is needed to ensure a good, durable seal.

 

c) Seal Loft Hatches

Unsealed loft hatches may contribute to air leakage, so worth some care.

 

d) Plasterboard Penetrations

If the airtight layers are sound then this should not matter, but still worth attention.

 

e) Flexible Not Rigid Insulation

Flexible Insulation provides a better fill between studs, rafters etc.

 

LOW PRIORITY

a) Continuity Behind Lintols

An extra strip of membrane to form a con­tinuous layer when the main one is lapped over the cavity barrier, also fill behind lintol.

 

b) Mastic to Skirting’s, Linings, Cornices

Not necessary if the airtight layer is sound

 

c) Air Barrier to Ceiling

High Priority in separating floors

 

d) Laying Tape to Plasterboard Junctions

 

e) Wall Tie Fixings

 

f) Top Runner Strip Seal

 

g) Airtight Service Boxes

 

h) Corrosion Resistant Fixings

 


9.3 Index

e) Wall Tie Fixings to Timber Frame

The breather membrane is not the main air barrier, but it is nonetheless a useful ally in reducing air leakage uk through the construction generally. Ensure that wall tie fixings do not lead to damage to the membrane (as this will lead to large amounts of air leakage uk, and a subsequent air test failure) ,  ideally, by taping over the area of membrane at which the tie is fixed.

 

(b) Use of Corrosion Resistant Staples or Fixings

Non-corrosion resistant fixings to external breather membrane can corrode to a point where they fail, allowing the membrane to come loose, often creating a small hole in the membrane and reducing the effectiveness of the membrane as an airtight layer, this will allow for air leakage uk and a probable air test failure. Copper is non-corrosive but can affect polyethyl­ene, whereas stainless steel has no effect on polyethlene and so should be preferred.

 

(c) Membranes to be Lapped and Sealed

Typically both internal and external membranes are lapped and stapled or tacked, but in order to create airtight layers, it is important that these laps are rigorously sealed, this will ensure a air test pass. Best practice in this regard - beyond the correct use of Manufacturers’ overlap dimensions, proprietary tapes and other acces­sories - is to run a layer of double sided tape between the membranes at the overlap and run a tape over the leading edge of the outer sheet. In addition, since many tapes tend not to last too well, it is advisable to ensure that laps are made directly over supported areas (i.e. with studs directly behind) and are held down positively with battens fixed through forming a mechanically tight, as well as an adhesive seal. This requires consideration of lap positions early on if extra framing or subsequent battening is needed.

 

(d) Ensure Continuity of Membrane / Co-ordination of Trades 

Whilst this is easy to achieve across large, flat areas, it is more difficult at the many awkward angles, junctions, corners and so on a typical site. There is no specific guidance except to ensure that those responsible for installation of the membrane are rigorous and conscientious in their attention to all of the inevitable nooks and crannies, and that the person responsible for co-ordination is equally attentive, particularly when the junctions are between separate forms of joint and separate trades. Having someone experienced on previous projects that required air tightness uk tests is a huge advantage and dramatically improves the chance of a air test pass

 

(e) Ensure Membrane is taken into Opening Reveals, Taped and Sealed and Made Continuous with Opening Seals

it is typical at openings in timber frame buildings to allow the membrane to run across the opening initially, then form a star cut into the opening, folding over the sections of membrane and trimming as necessary. In these cases, there are inevitable gaps in the airtight layer at the corners of the opening, and it is important to ensure that these are made good before subsequent installation of joinery etc and the following air test .

 

(f) Fix Airtight Membranes to Firm Backing Boards

In conventional timber frame construction, vapour barriers are fixed across studwork, usually after the installation of insulation and prior to the fixing of the internal lining. Equally external breather membranes are sometimes installed across gaps between rafters or studs. In both cases membranes are susceptible to pressures from both sides, leading to the membrane breaking free of its fixing and creating holes in the airtight layer. Ideally, membranes should be fixed against a firm backing board by way of protection against damage of this nature, this in turn will improve the chances of passing the initial air test .

 

(g) Service Void 

The principal advantage of a service void is related to functionality and maintenance over time, but a secondary advantage which relates directly to airtightness uk is that since all services may be incorpo­rated within, that is, on the inside of the vapour control layer, there is no need to penetrate the layer at each and every service installation, thus significantly cutting down on the myriad potential gaps that are typically formed and either left, as this will surely lead to a air test failure or made good which is time consuming and costly.

 

(h) Laying Tape at Plasterboard Junctions

Using laying tape at junctions makes the formation of an airtight junction both conscious and relatively easy, even allowing for subsequent shrinkage and cracking of the skim layer.

 

(i) Airtight Service Boxes 

Developed in Canada where airtight construction is more advanced, these service boxes are fitted with gaskets and a flange surround allowing for an airtight seal at all openings in the lining.

 

 (j) Mastic Both Edges to Skirtings, Reveal Linings, Cornices etc.

Where the corner junction behind has been carefully sealed then this measure may not be required. In addition to the nail or screw fixing, a mastic seal both edges aid’s efforts to guard against infiltration/air leakage (which will increase the chances of passing a air test ) but it makes removal and alterations more difficult.

 

(k) Ensure Continuity of Membrane behind and around Lintols 

It is likely that to achieve this requires two separate measures. First the breather membrane needs to be continuous and extend into the opening, thus a second strip should be affixed to the wall and lapped and sealed to the main membrane which must lap over the lintol or cavity barrier etc. Second, it is likely that gaps could form between the top, outer edge of the joinery and the lower, inner edge of the lintol, leading to a cavity behind the lintol. This cavity should be filled with expanding foam or mineral wool and if possible the gap filled, probably with a mastic sealant, this will drastically improve the chances of passing your air test at the first attempt

 

(l) Flexible Foam Sealant around Joinery Insertions

Gaps around openings are one of the most common of infiltration paths. They range from 0 to 20mm, which is too large to be filled by mastic. Compressible flexible foams are ideal for this application. En­sure that the airtight membrane meets the seal on both sides to maintain the airtight layer overall, and subsequently pass the air test

 

(m) Draughtstripping of Openings in Joinery

Draughtstripping of joinery comes in many forms. It appears that synthetic rubber or elastomeric tubular seals work well, creating good seals with minimal compression, depending on the size of the gap. It is important that seals are unaffected by paintwork and subsequent decoration, or are easily acces­sible and removable. This is important so that seals can be replaced as they start to fail to maintain the airtight layer.

 

(n) Seal all Penetrations in Plasterboard / Internal Lining 

Even with the use of airtight outlet boxes there will be inevitable penetrations such as ceiling pendants, pull cords, recessed fittings etc. which must be made good manually, typically with mastic, otherwise you may end up failing your air test

 

(o) Seal Loft Hatches

Generally, this involves a continuous bead of mastic to the underside flange, and, depending on the design, the use of compressed and flexible foam, or mineral fibre etc. above. Please note in our experience this is a common area for air leakage, and a major cause of air test failures

 

(p) Use of Joist Hangars as Opposed to Built-in Joists

The original specification here is already good practice, that is, the use of joist hangars which sidestep the problems of joist movement and shrinkage allowing infiltration and airflow within the floor voids, another major cause of air test failures

 

(q) Membrane Strip to Inner Face of Floor perimeter Beams 

100 gauge polythene or similar fixed to the inner face of the perimeter beams early on in the framing process can lapped and sealed to the internal vapour control layer typically installed a good deal later, so that a continuous internal vapour control and airtight layer may be effectively created.

 

(r) Continuity of Membrane to Ceiling over Partition Walls 

ideally this would comprise a continuous membrane affixed before the partitions are installed. However it is more likely that partitions are installed before, therefore such a layer would require strips to be fixed to the partition top runners to be later lapped and sealed to the ceiling vapour control layer.

 

(s) Flexible, Rather than Rigid Insulation 

Rigid insulation between joists, studs or trusses generally has to be cut to fit and this is never 100% accurate, leading to myriad gaps and routes for airflow. Flexible insulation avoids this problem and improves the chances of passing your air test at the first attempt.

 

(t) Top Runner Strip Seal

The use of this strip, lapped and sealed with subsequent membranes both sides prevents air infiltration into the wall itself from the ventilated eaves area, thus ensuring continuity of the airtight layer, which should help you to achieve an air test

 

(t) Air Barrier to Ceilings 

In ceilings within dwellings of the same occupancy, this is unlikely to be useful, but in separating floors, it is extremely important that an air barrier is included in the floor and ceiling make-up. Noted here by way of a reminder.

 

9.4 Steel Frame + Glazed

Façade

 

Discussion

It is important to be confident that the curtain walling manufacturer, supplier and installers all share an ex­plicit commitment to producing an airtight wall overall, as it will be very difficult for the Main Contractor to ensure a continuous airtight fabric if this element is not firmly ‘tied down’ before the start on site, this is one of the main causes for air test failure

 

The focus of concern then falls to all the various cor­ers and perimeters where the system meets other construction elements and here both Designer and Contractor need to have carefully considered in detail each occurrence and made adequate provision, to avoid large amounts of ad hoc remedial work, during the air test .

 

The roof membrane must be carefully sealed and the perimeter condition considered so that a continuous and positive connection can be made. Note this is another major cause of air test failure

 

HIGH PRIORITY

(a) Curtain Walling Performance Spec.

Since this represents the largest area exposed to wind it is important that the performance specification is adequate and that the components are conscientiously installed

 

(b) Mastic Perimeter Seals

With the main curtain walling components installed and airtight, the next most signifi­cant air leakage route is likely to be the pe­rimeter seals. Both mastic and membrane seals are valuable in this regard. Note this is another major cause of air test failure

 

(c) Membrane Perimeter Seals

With the main curtain walling components installed and airtight, the next most signifi­cant air leakage route is likely to be the pe­rimeter seals. Both mastic and membrane seals are valuable in this regard. Note as above this is another major cause of air test failure

 

 

(e) Roof Membrane Sealing

Any leakage in the roof membrane or at the roof / wall junction could be serious in terms of both energy waste and risk of moisture related damage to the roof build-up, so this detail is important. By properly preparing for your air test this will alleviate any of these future problems

 

MEDIUM PRIORITY

(h) Plates Added to Beam

Because of the difficulty in forming an adequate seal to protruding beams, this is likely to be a major source of air leakage in the long term so designed, rather than ad hoc site measures to reduce air infiltration are important.

 

(f) T&G and Taped Insulation

Potentially a minor issue, but given higher priority becasue it is relatively easy to solve and reduce airtightness/air test  uk failure and thermal insula­tion related risks.


 

Costs

It is difficult to ascertain any meaningful cost implica­tions with this detail because of the variety of curtain walling systems available.

 

The measures outlined are fairly standard in most installations and should in all cases represent no more than a re-iteration of good or best practice. However, they could attract an additional cost where one particular system did not address airtightness and the subsequent air test in one way or another.

Measures such as the additional efforts associated with air barriers at the separating floor, eaves and flor / wall junction might attract additional costs over that aspect of the original detail by approximately 30% largely because of the additional labour and attention required. However these costs are still cheaper than suffering costly LED’s for not passing the air test

 

LOW PRIORITY

(g) Membrane Seal between Floors

The existing detail should provide a rea­sonable degree of airtightness, but this measure will make the task conscious and affect a greater degree of separation.

 

(d) Foam Filler

Should not be required if the measures in (b) and (c) are completed, but an additional measure that also has value in provid­ing a backing to a continuous mastic seal internally.


 

9.4 Index

(a) Airtight Performance Specification for Curtain Walling

The de facto standard for curtain walling air permeability that most curtain walling manufacturers comply with is the CWCT (Centre for window and cladding technology) ‘Standard and Guide to Good Practice for Curtain Walling’. This specifies a maximum air permeability of 1.5 m3 / hour/m2 @600pascals for an area of fixed glazing, and 2m3/hr/linear metre of joint for opening panels. This is the same as the British Standard BS EN 12152:2002, category A4. However, the BS has a further category, AE that achieves 1.5m3 /hour /m2 at a pressure differential of more than 600pascals. Specification of this ‘exceptional category may be possible but it may mean a reduction in choice as this is a more stringent level of air testing . The rule is: If wind load up to 2400kn then curtain walling to be tested to 600 pascals. If wind load greater than 2400kn then test to wind load/4, e.g if 4000kn, test curtain wall to 1000 pascals.

Maximising airtightness can be done by having vulcanised welded joints to gaskets within the curtain wall frame, instead of usual mitred ones. This should ensure that the unit its self is airtight, although it is an expensive option, it will help you to pass the air test

 

(b) Mastic Bedded Fixings

Where membranes and components are connected, it is often possible for thin - and often more or less invisible gaps to be left between the joint. A continuous mastic seal used along the line of any such mechanical fixing ensures that any minor cracks like this are completely sealed.

 

(c) Additional Membrane Seal at Junction

Some Manufacturers (eg Schuco) supply as part of their system an EPDM perimeter gasket seal that should be tied into vertical DPM. Angle at jambs and loose dpm to wrap ensure good seal with EPDM. This is a particularly good way to ensure airtightness and the chances of a air test pass. At these critical junctions because it requires a conscious task (sealing the membrane) to ensure all ‘loose ends’ are firmly fixed, as opposed to leaving the airtightness to be achieved through the use of applied sealants.

 

(d) Foam Filler to Internal Joint

Assuming that the seal mentioned above is installed correctly this should not be required, but such a seal acts as an additional check against air leakage uk and could be used as a backing strip against which to seal a continuous mastic seal internally.

 

(e) Membranes to be Lapped and Sealed

Best practice in this regard - beyond the correct use of Manufacturers’ overlap dimensions, proprietary tapes and other accessories - is to run a layer of double sided tape between the membranes at the overlap and run a tape over the leading edge of the outer sheet. In addition, since many tapes tend not to last too well, it is advisable to ensure that laps are made directly over supported areas (i.e. with solid materials directly behind) and are held down positively with battens fixed through, or some other ‘positive’ connection forming a mechanically tight, as well as an adhesive seal. This may require consideration of lap positions early on.

 

(f) T&G Jointed and Taped Rigid Insulation

Butt jointed insulation, even if installed firmly may be subject to movement during the course of con­struction and over time, and is unlikely to offer a continuous insulation layer in the long term. Using t&g slabs overcomes some of this problem and taping the slabs ensures that air leakage paths cannot form between the minute, but inevitable cracks between the units, which can later lead to air test failure

 

(g) Additional Airtight Membrane

It is likely that the fireproof stopping will not be able to create an adequately airtight seal and so this measure ensures the task is performed consciously. Using a simple polythene membrane and forming positive connections to the underside of the slab and the top of the curtain walling ensures an airtight seal between floors, and at the vulnerable connection of curtain walling to spandrel panels. One of the main causes of air test failures


 

 (h) Localised Welding of Plates of Beam

It is practically very difficult to form an airtight seal perpendicular to an ‘I’ beam or similar, expanding foam tends to be used because no ‘built’ connection appears workable, nor cost effective. Such ad hoc seals are unlikely to last in the long term.

Ideally plates should be welded to the beam such that there is no air route along the length of the beam (a plate welded perpendicular to the web and extending between the two flanges) and such that airtight seals are easily formed around the beam as it passes the airtight layer. Side plates fixed between flanges form a sort of localised rectangular section which is more easily sealed. This makes the task more readily achieved on site, and more durable in the long term


 

9.5 Refurbishment of Masonry Building

Discussion

If the existing masonry fabric of a refurbished building is in good condition, it is potentially simple to render it relatively airtight if the details proposed - particularly the use of service voids - are followed. All the work can be carried out internally and is simple to install and check and will drastically improve the chances of a air test pass

In addition there is no cavity in this form of construction and this means there are fewer opportunities for undetected airways.

 

It goes without saying that any cracks or damage to the existing fabric should be made good before installation of the internal frame, otherwise this may help lead to a air test failure

If there is enough space, it might be best to retain all existing lath and plaster on ceilings and walls, ensure that it is effectively sealed, and work inwards from there. Experience suggests that lath and plaster itself is fairly airtight and removing it merely creates more waste. One potential disadvantage is that in keeping the existing plaster, it may not be possible to access the gaps behind which may run into floor voids and partitions creating air leakage uk paths throughout the building.

 

A number of reviewers of this Guide commented that it is more common to maintain a cavity between the existing wall and any new-build internal leaf. The alternative proposed keeps to the same format as the original, but the advantages of the use of a cavity are well understood.

 

HIGH PRIORITY

(a) Membranes Lapped & Sealed

With only one membrane to ensure airtight­ness it is crucial that laps and junctions are conscientiously sealed.

 

(b) Service Void

Use of a service void means most if not all penetrations through the vapour control and airtight layer can be avoided.

 

(c) Joinery Edge Sealing Batten

If the membrane generally is well sealed, the only other major area for air infiltration uk is the openings and the gap between the frame and masonry. If the windows can be effectively sealed by (medium priority e) then this measure is not necessary.

 

(d) Joinery Draughtstripping

A particular issue with sash and case windows. It is important that seals can be easily accessed for maintenance and replacement.

 

MEDIUM PRIORITY

(e) Flexible Foam around Joinery

Gaps around openings are common and neat, effective solutions can be difficult, careful use of flexible foam enables effec­tive and durable seals to be formed. If this can be effectively achieved with the sash and case window then (f) is not necessary.

 

(f) Continuity at Openings

Continuity between the framing sealant (m) and the membrane can be tricky and care is needed to ensure a good, durable seal.

 

(g) Backing Boards

Use of backing boards makes installation of the membrane easy and thus less prone to poor workmanship and subsequent failure.


 

Costs

The retention of the ceiling lath and plaster saves approximately 24% of the costs of that element, while the addition of the service void and vapour check represents a 18% cost increase, thus, without the addition of the breather membrane over the ceiling joists (a medium priority measure) there is a cost saving to complement the increase in ease and cost of services installation.

The breather membrane represents a 13% increase in cost and therefore tips the balance of the ceiling cost overall. However this is still cheaper than repercussions due to failing your air test

 

The addition of the OSB backing board and service void to the walls constitutes around a 46% increase in cost of the wall, the majority of which (33%) is made up by the OSB, so perhaps a cheaper, yet firm back­ing board might alleviate the cost burden. The addition of the OSB layer to form a service void beneath the floor boards would add approximately one third to the cost of the original detail. Again this is still cheaper than repercussions due to failing your air test

 

 

Double mastic sealing of the skirting boards adds approximately 50% to their installation cost, although their overall costs are small in the overall picture.The sealing of the vapour control layer above and below the intermediate floor should not attract any additional cost if assumed to be part of a standard, if careful installation. Measures to help seal around the window would add marginally to a standard installation cost.

 

MEDIUM PRIORITY

(a) Batten Seal at Corners

A version of (b) but worth particular men­tion as these junctions are particularly important to seal well.

 

(b) Keep Existing Lath and Plaster

Really a version of (e) except in this case we suggest retaining the existing firm base of lath and plaster against which to affix the vapour control layer.

 

(c) Plasterboard Penetrations

If the airtight layers are sound then this should not matter, but still worth attention.

 

(d) Membrane Over Roof Insulation

Protects installed insulation from disrup­tion and provides a secondary layer at this important area.

 

LOW PRIORITY

(a) Silicone to Joinery Externally

Should be standard practice, but forms useful role in airtightness as well as weath­erproofing.

 

(b) Mastic to Skirtings, Linings, Cornices

Not necessary if the airtight layer is sound but worth attention in these examples.

 


9.6 Index

(a) Breather Membrane over Insulation 

In well ventilated loft areas, loose insulation may become dislodged by air movement. This precau­tionary measure ensures that the initial fully fitting installation of batts against joists etc is maintained over time, reduces dirt and debris entering and provides an additional airtight layer (which is useful since the loft is ventilated) whilst allowing for vapour egress into the ventilated space.

 

(b) Membranes to be Lapped and Sealed

In order to create airtight layers, it is important that laps are rigorously sealed. Best practice in this regard - beyond the correct use of Manufacturers’ overlap dimensions, proprietary tapes and other accessories - is to run a layer of double sided tape between the membranes at the overlap and run a tape over the leading edge of the outer sheet. In addition, since many tapes tend not to last too well, it is advisable to ensure that laps are made directly over supported areas (i.e. with studs or dwangs directly behind) and are held down positively with battens fixed through forming a mechanically tight, as well as an adhesive seal. This requires consideration of lap positions early on if extra framing or subsequent battening is needed.

 

(c) Vapour Control Layer over Existing Lath and Plaster 

Rather than remove the existing lath and plaster ceiling, this detail saves a little money, time and resources by reusing the existing ceiling as a backing to the installation of the vapour control layer (refer also (e)) Plaster need not be repaired if damage is localised and does not threaten the integrity of the vapour control layer.

 

(d) Service Void 

The principal advantage of a service void is related to functionality and maintenance of services over time, but a secondary advantage which relates directly to airtightness is that since all services may be incorporated within, that is, on the inside of the vapour control layer, there is no need to penetrate the layer at each and every service installation, thus significantly cutting down on the myriad potential gaps that are typically formed and either left, or made good which is time consuming and costly, and could lead to a air test failure

 

(e) Fix Airtight Membranes to Firm Backing Boards 

In many situations membranes required for vapour control and airtightness are installed unsupported and are thus susceptible to pressures from both sides, leading to the membrane breaking free of its fixing and creating holes in the airtight layer. Ideally, membranes should be fixed against a firm back­ing board by way of protection against damage of this nature.

 

(f) Mastic Both Edges to Skirtings, Reveal Linings etc. 

Where the corner junction behind has been carefully sealed then this measure may not be required, In the examples shown on this construction, this particular detail is not critical but is nonetheless valu­able in helping to ensure a good seal at all points.

 

(g) Airtight Layer Taken Behind Batten at Corners 

As noted in (b) above, the best airtight seal is a positive and mechanical one such as shown here whereby at corners and edges, a membrane is not only lapped and taped against the adjoining sur­face, but held firm by a batten fixed through. This overcomes any potential adhesive failures or tears in staples or tacks etc. In the ceiling junction where the plasterboard layer must be continuous for reasons of fire spread prevention, it is also advisable to install laying tape at the junction between the plasterboard and the wall to ensure an airtight seal here also, thus improving the chances of a air test pass

 

(h) Seal all Penetrations in Plasterboard / Internal Lining

Even with the use of airtight outlet boxes there will be inevitable penetrations such as ceiling pen­dants, pull cords, recessed fittings etc. which must be made good manually, typically with mastic, and in this case, with a suitably fireproof mastic to maintain the fire barrier.


 

(i) Ensure Membrane is taken into Opening Reveals, Taped and Sealed and Made Continuous with Opening Seals, it is typical at openings to allow the membrane to run across the opening initially, then form a star cut into the opening, folding over the sections of membrane and trimming as necessary. In these cases, there are inevitable gaps in the airtight layer at the corners of the opening, and it is important to ensure that these are made good before subsequent installation of joinery etc.

 

(j) Sealing Batten 

This detail may be considered as an alternative, or ideally as an additional measure with (k). Since it is possible that replacement sash and case windows cannot be easily sealed around their perimeter (they are often ‘open’ around the outer edge) it may be necessary to use this detail which creates the airtight seal on the inside of the frame rather than ‘in line’ with the frame as noted below.

 

(k) Flexible Foam Sealant around Joinery Insertions

Gaps around openings are one of the most common of air infiltration uk paths. They range from 0 to 20mm, which is too large to be filled by mastic. Compressible flexible foams are ideal for this application. En­sure that the airtight membrane meets the seal on both sides to maintain the airtight layer overall and imrove the chances of a air test pass.

 

(l) Draughtstripping of Openings in Joinery 

Draughtstripping of joinery comes in many forms. It appears that synthetic rubber or elastomeric tubular seals work well, creating good seals with minimal compression. It is important that seals are unaffected by paintwork and subsequent decoration, or are easily accessible and removable. This is important so that seals can be replaced as they start to fail to maintain the airtight layer. Brush seals are likely to be used in sash and case windows.

 

(m) Silicone Sealant to External Window Surround 

Some form of neat and potentially paintable edge seal will be required externally

.

(n) Breather Membrane Instead of Netlon 

Notwithstanding the air barrier placed above, mineral wool is permeable to air movement and so replacing the netlon with a vapour permeable but airtight breather membrane reduces air movement in the insulation, improving insulation levels and reducing the risk of air leakage uk and air test failures generally.


 

9.7 Concrete Frame and

Panel

Discussion

Concrete panel construction represents a potentially good airtight form of construction. This is because the panels themselves are essentially airtight and being large, have fewer gaps which must be sealed. Being fairly predictable in terms of thermal and structural movement they are easy to seal well, and the only areas of concern then are the service penetrations and junctions with openings. With care and attention in these areas, a very good overall airtight external envelope is easily within reach. Having said that, in some early examples of this build­ing type, the sealants between panels have failed, highlighting the vulnerability of the system to such air test failure and the importance of correct specification and application.

 

A number of systems are available but the principles outlined for the improvement of the system chosen are widely applicable. Where two leafs of concrete panel are used, it is unlikely that the outer layer will be used as a rain ­screen layer, but this is sometimes done, and in these cases the airtightness of the internal layer of panels becomes critical, and may be augmented by the ap­plication of a vapour control and airtight membrane on the inner face of the insulation, applied to the panels before the insulation is installed. Guidance on the application of this membrane, and on poten­tially more airtight forms of insulation may be found

In Sweden, concrete panels are sometimes sealed to each other using polyurethane foam which is claimed to increase the airtightness levels and subsequent air test passes , but there does not appear to be any evidence of this form of sealant in the UK.

 

HIGH PRIORITY

(a) Integral Beam and Internal Panel

Important because this reduces the number of joints and simplifies construction.

 

(b) Sealing of all Penetrations

Care and attention to detail at all services and other penetrations is vital, most pres­sure tested panel buildings suffer air leakage uk at these locations, if these are not sealed it will result in a air test failure

 

(d) Sealing around Windows

The other major source of air leakage uk in con­crete panel buildings apart from (b) above, care and attention to detail along all joints needed.

 

(c) Screed Edge Strip and Seal

Ensures that air does not leak between floors around the perimeter and at other floor penetrations and breaks in the screed, another major cause of air test failure

 

MEDIUM PRIORITY

(a) Accessible Draughtstripping

It is important that the draughtstripping is accessible since it is likely that it will not last as long as the windows themselves and require replacement.

 

(b) Membrane around Windows

Required for vapour and air leakage con­trol, this also required attention and inspec­tion and can be seen as complementary to the mastic / silicon sealants

 

(c) Double Silicon Seal to External Panels

Double silicon sealant lines in the external panels is normally standard practice, and is typically good enough to ensure that the outer panels provide an effective airtight seal throughout, and help towards passing your air test

 


Costs

The alternative specification highlights best practice installation and should not incur any additional costs. The design of the panel construction system itself would dictate any cost difference.

 

LOW PRIORITY

(h) Membrane Under Roof Insulation

May not be required if the screed below is fully sealed against vapour and air flow, but given the typical number of penetrations in a commercial roof screed, the addition of a dedicated membrane may be considered advisable



1.0 Key Principles

1. The air test procedure is set out in CIBSE TM 23 and the ATTMA TS1

2. A air test involves sealing all ‘normal’ gaps such as vents and pressurising or depressurising the building. The level of fanpower required to maintain the pressure differential indicates the ‘leakiness’ or ‘permeability’ of the building.

3. Air test are typically followed by an air test audit (using smoke pencils, for example) to expose and make visible the various air test leakage routes.

4. Where projects comprise large quantities of a single component, component air testing in the laboratory may be appropriate as well as on site element air testing   .

                                  

2.0 Climatic conditions

As mentioned in Chapter 1, the raised pressure differential of 50 Pascals created during an air test is quite small. Whilst this is adequate to overcome most of the common pressure differential anomalies, such a small differential is vulnerable to larger pressure differences created by climatic conditions.

Air tests uk require calm days – i.e. a reading on the Beaufort Scale of 3 or less (3.4 to 5.4 metres per second wind speed at 10 m above ground) This corresponds to a gentle breeze with leaves and small twigs in constant motion. In winter conditions, and on exposed sites therefore an air test may not be possible, although it is often possible to make allowances, so long as these are carefully recorded during the air tests uk .

 

3.0 The Test itself

Guidance on testing buildings for an air test is contained in CIBSE Technical Memorandum TM23 Testing Buildings for Airtightness and in BS EN 13829: 2001. All accredited air tightness testers test to the guidelines contained in the BS EN and within the ATTMA TS1

 

Essentially the air test process is one of pressurising or (less commonly) de-pressurising the inside of the whole building, and of measuring the rate at which air needs to be blown or sucked to maintain that pressure differential; a building suffering from high amounts of air leakage uk during the air test will equalise readily and require a greater measurable effort to maintain the 50 Pascal differential, while a air tight building will easily contain the enforced differential and require little additional input during the air test .

The pressure difference is induced by one or more calibrated fans that are normally mounted within a suitable doorway. An adjustable door panel system, sealed around the edges is used which can also be connected to large external fans via collapsible ductwork if required. The rate of the fan or the volume flow of air through the fan during the air test can be understood as the rate of air entering / escaping throughout the remainder of the building envelope.
Buildings are air tested/air leakage tested in such a way as to recreate ‘normal’ conditions. Doors and windows are closed; trickle ventilators closed, extract fans and such like are closed but not sealed. Internal doors are wedged open. All of this must be actioned prior to the air test /air tightness test
uk

If the building is under construction the air test /air leakage testing uk can be  undertaken during  working hours, but sometimes this is not practical so some scheduling of work needs to be thought through in advance. With all external doors and windows sealed shut, some work becomes impossible (such as work with solvents requiring ventilation) and internal trades are normally ‘sealed in’ for a short time, where they can carry on undisturbed throughout the air test /air tightness test uk

In existing buildings the air tests uk /air tightness tests uk are normally carried out when the building is unoccupied if possible because of the disruption.

 

4.0 Air Leakage Audits

The air test /air tightness test quantifies the rate of air leakage  through the envelope as a whole, but it cannot locate the air leakage paths. Where remedial work is required therefore, air test /air tightness tests are followed by a range of auditing techniques designed to identify the specific places where air leakage is apparent  through the building envelope.

In many cases a simple visual inspection may be sufficient – especially if undertaken by someone with experience of the likely locations of air leakage.

However, most air leakage routes are difficult or impossible to spot without visual aids during the air test /air tightness test. One common technique is to use smoke tracers – smoke pencils or smoke machines. These render the air paths visible in certain situations during the air test . The building may be positively pressurised during the air test /air tightness test and the leaks witnessed externally, or, more usually, negatively pressurised while a smoke pencil is drawn over likely gaps and defects which become visible as the smoke is sucked inwards, during the air test

Another technique, which has certain advantages and disadvantages compared to smoke tracing along with an air test /air tightness test, is the use of an infrared camera during a Thermographic test. Used either externally or internally, these thermographic cameras register the radiant heat levels of surfaces and so are able to ‘see’ for example, where cold air is cooling the fabric around a gap internally, or conversely where warm air is escaping and heating the colder materials on the external face.

To work effectively, there needs to be a recognisable difference between the internal and external ambient temperature, so before any heating has been installed and on a warm summer’s day thermography testing may not be effective. Similarly on warm and sunny days, sunshine on external surfaces can distort the true situation so it is better on such days to wait until early evening. Conversely, rain on external surfaces can be equally distorting of the true thermal situation. However the  Thermographic cameras are useful in identifying problems at high level or difficult to reach areas, and are also very helpful in identifying other construction defects such as poorly installed (or non-existent!) insulation within the fabric.

On larger commercial buildings, the air test /air leakage testing may be undertaken at the same time as ‘standard’ ventilation system commissioning and associated studies, but these are not discussed as part of this guide.

5.0 Component Testing

A distinct aspect of overall air test /air tightness testing is the individual component test. This may be undertaken quite separately, in the laboratory or by the manufacturer of a particular component. Such air testing /air tightness sampling tests may be deemed necessary on a large project where large areas of one particular type of component, for example curtain walling, are to be specified, Insitu element air testing involves isolating the area within a temporary sealed compartment, which is then pressurized during the air test , and the air leakage related to the area of interest assessed. In this way sample areas of a building may be air test /air tightness tested using smaller fans as required.

5.1 Concrete frame and panel 

 

1 Introduction

As thermal insulation levels have risen in the last few years the proportion of energy lost to draughts has increased to the extent that now in some cases around half of all heat losses are due to air leakage across the building fabric. Given that approximately half of all energy used in the UK is in buildings, it is not hard to see that draughts account for a staggering amount of energy - and therefore cost – wastage, this where the air test comes into its own

The situation is such that further increasing thermal insulation levels would be largely unproductive unless air tightness is con­scientiously addressed. Air leakage has been shown to reduce the effectiveness of thermal insulation by up to 70% and so it is clear that if energy efficiency is to be improved in buildings, the next efforts will have to focus on air test /air leakage testing.

Many people make the mistake of thinking that an airtight building is necessarily a ‘stuffy’ building. This is not the case. All buildings have to be ventilated for health and comfort and airtight buildings are no different. An adequate ventilation system (which may well include open able windows as well as fans etc.) has to be planned for every building. The difference will be that a great deal of un­planned air leakage needs to be stemmed, this can be ascertained during the air test

The additional costs of creating an ad­equately airtight building can be negligible, but even where costs are increased e.g. for the air test , these can be more than offset by a reduction in the capital cost of heating and ventilation equipment, not to mention the long term savings in energy.

Given that the vast majority of building stock is existing, a great deal of attention will need to be given, in the foreseeable future, to remedial works to existing buildings, all existing air leakage paths can quickly be found during a air test . This guide specifically in­cludes examples of good and best practice remedial work in terms of air tightness and shows that such works can offer substantial benefits without undue disruption or cost, an air test will have a low impact on site works

Air Pressure Testing hopes to provide practical guidance on how to save energy and costs and protect building fabric. On the basis that prevention is cheaper and easier than cure, one purpose of this guide is to enable Designers to design inherently more robust and durable solutions which avoid costly and time consuming remedial works after the air test after a potential air leakage failure

The general guidance here is firmly focused on the idea of practical design and detailing, and should be read in conjunction with other guidance on sustainable design, energy efficiency and air tight­ness where necessary to provide an overall design framework. The details provided have been fully costed, tested and subjected to a Defects Liability insurance assessment. They are offered as viable alternatives to standard details, and illustrate the possibili­ties that exist. It simply remains for you, the reader, to apply them appropriately in the context of your next project prior to your air test/air tightness test

 Aims of this Guide       

 

• To highlight benefits of air tightness which include both energy and cost efficiency, improved comfort and reduced risks of damage to building fabric

 

• To improve awareness of the need for air tightness in con­struction at design stage

 

   • To promote detailing and specification solutions which cre­ate airtight and efficient buildings thus reducing the need for remedial works after air leakage failure- ‘prevention rather than cure’

 

• To show that new build and remedial air tightness are achiev­able without undue cost penalties to construction works due to multiple air leakage failures, you should be passing after the first air test

 

• .and in this way to help to ‘mainstream’ the good and best practice outlined in the document

 

Target audience

 

This Guide will help all those who wish to improve the airtightness/air leakage rate and energy efficiency of buildings through their construction, e.g:

 

• clients –building owners and users, principal and specialist contractors, interior designers architects and technicians, structural engineers, building service engineers, building surveyors, quantity surveyors/ cost consultants, maintenance and facilities managers, project managers , planning officers and building control officers, funding bodies and their professional advisors, government and non-governmental agencies

 

How to use this Guide

 

This Guide is divided into six sections. The first two sections provide an overview of the issues surrounding air tightness. Sections Three, Four and Five describe the requirements for the design process, the procure­ment and the air leakage/air tightness testing involved in designing for airtight buildings.

 

Section Six provides a number of representative details which have been optimised in terms of preparing for the air test . These are compared with standard details for a variety of construction types, and costed. This section will be primarily of interest to the design team. It should be read in conjunction with sections Three, Four and Five in particular, as all details must be placed in a suitable context.

 

6.0  The Context

 

Key Principles

 

1. Most UK construction is ‘leaky’ and wastes energy and money. Building airtight buildings can save energy and money, improve comfort and reduce the risk of damage to building fabric.

2. Airtight building will NOT mean ‘stuffy’ buildings. Good ventilation is vital for health and comfort - it is the UNPLANNED air leakage that we are aiming to stem.

3. Legislation is slowly catching up with best practice in Scotland, the UK and elsewhere and we can expect a greater emphasis on airtightness in all types of construction in due course.

4. Good and Best Practice Targets of air tightness/air leakage have been set for many types of buildings and are easily achiev­able.

6.1 Infiltration, Ventilation and Airtightness

 

Air infiltration (air leakage) is the uncontrolled flow of air through gaps in the fabric of buildings; this is all the more apparent during the air tes uk. It is driven by wind pressure and temperature differences and as a result is variable, responding in particular to changes in the weather. Infiltration (air leakage) levels are strongly affected by both design decisions and construction quality.

 

Ventilation, on the other hand, is the intended and controlled in­gress and egress of air through buildings, delivering fresh air, and exhausting stale air in combination with the designed heating sys­tem and humidity control, and the fabric of the building itself.

 

Whilst some unwanted air infiltration (air leakage)  will at times aid comfort lev­els, it is not reliable and moreover brings with it a range of signifi­cant disadvantages such as high levels of heat loss, reduction in performance of the installed thermal insulation, poor comfort, poor controllability and risks to the longevity of the building fabric it­self. It cannot be considered an acceptable alternative to designed ventilation. Air Infiltration (air leakage) needs to be reduced as much as possible if we are to create efficient, controllable, comfortable, healthy and durable buildings. This can be achieved by delivering airtight buildings that pass the air test /air leakage tests first time.

 

Air tightness is a term used to describe the ‘leakiness’ of the build­ing fabric. An airtight building will resist most unwanted air infiltra­tion (air leakage) while satisfying its fresh air requirements through a control­led ventilation strategy. Most existing buildings, even those built recently, are far from being airtight and may fail an air test and because of unwanted air infiltration (air leakage) generate huge costs to owners and occupants, in envi­ronmental, financial and health terms. One way of overcoming this making sure the building passes the air test/air leakage

 

It is important to emphasise the distinction between infiltration (air leakage) and ventilation, because while the primary purpose of this document is to show how buildings can be designed and constructed to be airtight and so pass the air test /air leakage test first time, it is equally important to stress that good levels of ventila­tion and a clear ventilation strategy will be required in every case. As the saying goes: ‘build tight, ventilate right.’

 

6.2  Why Build Airtight?

 

Legislation

At a rather prosaic level, the issue is important because it is now part of the Building Regulations in England and Wales concerning non-domestic new buildings over 500 sqm in area, all will now require an air test /air leakage, and is likely to affect a wider range of buildings soon. Whilst the initial targets set for airtightness of buildings are easy to achieve, it is equally likely that once in place, those targets will be ratcheted up to create ever more airtight and efficient buildings in the UK, in line with many of our European neighbours, at present many EU countries have a much higher first air test pass rate.

 

Energy and Cost Saving

 

Typically, the largest heat losses in most buildings are related to levels of thermal insulation, followed by those related to infiltration (air leakage), followed by those related to inefficient plant. Quite rightly therefore, most efforts to save energy and costs have until recently been di­rected at increasing thermal insulation levels. But as these levels have risen, so the relative contribution of infiltration (air leakage) has increased to the point where it can represent around half of all heat loss in a build­ing. In highly insulated buildings, the percentage may be higher.

This is reflected in the fact that total space heating costs in an airtight building may be as much as 40% less than in a leaky one

We are at the stage where it is likely that any further increase in thermal insulation levels would be ineffective until levels of air tight­ness in construction have improved considerably, this is where the air test in invaluable

 

Space Heating System Reduction

Clearly there is potential to reduce the capacity of space heating systems sized to cope with current levels of heat loss if those levels can be reduced by a half or more. Ensuring you achieve a low pass rate during the air test . In addition, airtight buildings are more predictable in terms of environmental control and the capital cost savings of installing smaller heating plant may be augmented by reduced plant room sizes in certain cases and particularly by reduced running costs in the longer term. As well as reducing the need for heating plant, airtight buildings of­fer much greater potential to respond positively to the local external climate through passive, or climate responsive design strategies such as natural ventilation, day lighting, the use of thermal mass and passive solar gain. Energy savings, capital and running costs, along with CO2 emissions can thus be further reduced.

 

Comfort and Control

 

As noted above, airtight buildings are not as affected by variations in external conditions. This makes them easier to control from an Engineer or Designer’s point of view, but it also makes them more comfortable from the point of view of the occupant.

In buildings with high levels of infiltration/air leakage those occupants near draughty windows, for example, will suffer the cold, particularly on windy days, whereas those elsewhere may well suffer from too much heat locally as the system tries to raise the temperature overall. Those who try to achieve comfortable levels through the use of the provided ventilation controls will find these to be rela­tively ineffective, whereas in more airtight buildings greater levels of control and comfort generally are achievable and local control and variation by occupants can have a more direct effect. In one example of an existing superstore, the ambient temperature in the store was raised by 5oC after the store had been sealed after the air test . Complaints by occupants in leaky buildings are common, and remedial measures are usually difficult and expensive, an air test can finds all air leakage paths quickly and effectively

 

 

Deterioration of Fabric

 

Leaky buildings allow cold air in through the construction causing discomfort, they also allow warm (and often moist) air out, causing heat loss. This warm and often moist air can find itself in colder parts of the outer construction where it can cool, and the moisture in the air can condense, leading to a buildup of moisture. This in turn can lead to:

 

decay of organic materials such as timber frames

 

saturation of insulating materials thus reducing their insulative effect (which increases heat loss further)

 

corrosion of metal components

 

frost damage where moisture has accumulated on the cold side of the insulation.

 

6.3  Legislation

 

In England and Wales the relevant regulation on air tightness is contained within Approved Document L1 for dwellings and L2 for non-domestic buildings (2006). There is general encouragement to consider air tightness issues, with a target air permeability for all buildings of 10 m3/hr/m2 envelope area at 50 Pa. In L2, build­ings with a floor area of greater than 500 m2 are required to have a air test if approved details are not used. Further tightening of the regulations are due in 2010.

 

Proposals for changes to the Energy standards were issued to public consultation in March 2006, including guidance that air tightness testing would be required if the calculation of energy performance included air permeability rates lower than 10m3/m2h at 50 Pa.

 

6.4  Measurement

 

A range of units for measuring air tightness/air leakage have been used in the past and this can complicate matters. However, one method only – “air permeability” - is the measure used in European Stan­dards, the new editions of the various UK Building regulations and in CIBSE’s TM23 Air Testing methodology and has been used throughout this document. The Air Permeability is defined as the volume flow in cubic metres of air per hour per square metre of the total building surface area (including the floor) at 50 Pascals pressure differential, expressed in m3/hr/m2 @ 50 Pa.

 

The main difference between the air permeability and previous practice in the UK is the inclusion of the non-exposed ground floor in the calculation of the ‘total surface area’ of the building. The difference between the new measurements and older ones tend only to be marked therefore where there are large volumes and ground floor areas. These new rules must be taken into account during the air test

 

Of the range of measurements used previously, the “Average Air Leakage Rate (or Index)” is similar to the “Air Permeability” except that non-exposed floors are excluded from the measure­ment. Another common expression is the “Air Changes per Hour at 50 Pascals (ACH @ 50 Pa). This is a useful measurement in particular because, when divided by twenty, it gives an approxi­mate value of the natural infiltration rate of the building at normal atmospheric pressure, which can then be used to help size heating and ventilating plant etc.

 

Yet another measurement is the “Equivalent Air Leakage Area” (ELA) at 50, 10 and/or 4 Pascals. This figure gives a representation of the sum of all of the individual cracks, gaps and openings as a single orifice and helps to visualise the scale of the air leakage problem. The main problem of changing the measurement technique is the ability to compare data

The standard pressure differential used is 50 Pascals. This is not in fact a very large pressure differential and corresponds to the pressure exerted by a column of water 5mm high. Compared to the fact that buildings can withstand wind induced pressures of at least 500 Pascals, this seems insignificant, but it is larger than wind induced pressure on a calm day, and by air testing and quoting air leakage figures at 50 Pascals, inaccuracies are reduced and repeatability is improved using this air test method.

 

6.5 Targets

As noted above, the only ‘official’ guidance in the UK applies in England and Wales and relates to non-domestic buildings over 500 sq.m in area. As can be seen from the table below, the target of 10 m3/hr/m2 at 50 Pa. is relatively easily achieved compared to the good and best practice noted in the 2000 document by CIBSE, TM23. This sets out the air tightness/air leakage testing methodology which is the de-facto methodology now followed for a air test in the UK.

 

A number of air tightness experts believe the stated targets are in­adequate when compared with the overwhelming need to address carbon emission reductions, and the potential to do so through air tightness measures. For example, the house illustrated to the right was built in 1992 for the same cost as nearby houses and improved upon the standards noted above by two thirds.

 

Building Type Air Permeability (m3/hr/m2 at 50 Pa)

Good Practice/ Best Practice

Dwellings 10.0/ 5.0

Dwellings (with balanced mech. vent.) 5.0/ 3.0

Offices (naturally ventilated) 7.0/ 3.5

Offices (with balanced mech. vent.) 3.5/ 2.0

Superstores 3.0/ 1.5

Offices (low energy) 3.5/ 2.0

Industrial 10.0/ 2.0

Museum and Archival Storage 1.7/ 1.25

Cold Storage 0.8/ 0.4

 

7.0 Designing for Airtightness

 

7.1 Key Principles

 

1. A Performance Specification is a crucial document for establishing the appropriate targets for airtightness, along with the methodology for achieving it, and the roles and responsibilities of those involved.

 

2. Conceiving of a building in zones and air barriers will help all involved to visualise the task.

 

3. Air barriers must be impermeable, continuous, durable and accessible. They should be supported by positive mechanical seals where possible.

 

4. The simplest solutions will be the most buildable and durable.

 

5. A culture of airtight construction does not yet prevail and until it does, it may be necessary to follow up targets with specific details and specifications, along with guidance on the process of implementing the necessary level of co-ordination and attention to detail.

 

Unlike design for deconstruction (the subject of the first in this series of Guides) and the forthcoming guide on chemi­cal-free design, the design of airtight buildings cannot be left to the specification and details, at least, not until the industry as a whole recognises the need and has sufficiently widespread ex­perience, unfortunately this alone would surely end in an air test failure. For the next few years, it will be necessary not only to provide careful details and performance specification, but also to develop thorough inspection and testing regimes, hence the need for Chapters 7.4 and 7.5 of this guide.

 

7.2  Performance Specification

The Performance specification may be the only document need­ed by the Architect / Designer / Client if the building is to be pro­cured through Design and Build or similar route. However, it is more likely to be part of a suite of documents including detailed drawings.

The performance specification allows appropriate targets to be set for the project, along with a description of how the process is to be conducted, in terms of scheduling, audits and air testing, and potentially remedial works. Given the increasing use of special­ist subcontractors, particularly in larger projects, it is also critical that the performance specification sets out both the responsibil­ity for, and constructive guidance regarding the co-ordination of trades with respect to the final air permeability of the completed envelope.

 

Zones and Barriers

Once appropriate targets have been set for the project, the next task is to identify zones which require greater or lesser airtight­ness uk levels. Ideally, these zones need to be identified on a draw­ing which also identifies the specific air barriers in red.

 


For example an industrial unit with office space is divided into five separate zones, and air barriers are identified as required. Such a drawing, however diagrammatic initially, helps to conceive of the subsequent specification and detailing needs, giving an overview of the problem.

Heated zones need to be kept separate from unheated zones such as roof voids, delivery bays etc. whilst service shafts may require particular attention. Boiler rooms with large flues and in­take vents may need to be separated prior to the air test

 

Entrances are often significant sources of draughts. Lobbies with doors set apart by around 4m, so that one door closes before the second is opened, can be effective, whereas in highly trafficked areas revolving doors are likely to be preferable. Tall buildings, with atria, stairways and service shafts all of which rise through the building can be prone to ‘stack effect’ air movement whereby warm air rises, dragging in cooler air from outside at the lower levels creating more acute air leakage problems. A number of tactics may be employed to reduce the effects, but in any event issues of airtightness are likely to be highlighted in these cases prior to the air test

 

7.3 Design

 

With the zones and air barriers located, it is necessary to design the air barriers themselves.

To be effective, the air barrier must:

 

• be made of suitably air impermeable materials;

 

• be continuous around the envelope or zone

 

• have sufficient strength to withstand any pressures created by wind, stack effect or air control systems

 

• be easily installed

 

• be durable

 

• be accessible for maintenance / replacement if appropriate

 

The last of these is important since there is evidence that the air­tightness of some constructions will tend to decrease over time and in particular the first period after completion.

 

There are a number of strategic measures which can be employed to simplify the business of designing an airtight building. Since service penetrations in and out of a building provide a major source of air leaks, one strategy is to collect all such penetrations into one accessible area, this will drastically improve the air test results

 

In construction types such as steel and timber frame, it is usually wise to employ a specific membrane or layer as the air barrier, rather than rely on sealant between, for example, the sheathing boards. Such a membrane can usually double up as the vapour barrier if used internally and gives the Designer the opportunity to consider and address airtightness explicitly, rather than as a function of other elements. Bear in mind that most membranes are flimsy and will need support in all areas, although there is minimal air pressure during the air test it can still move unsupported membranes this can result in an air test failure.

 

Another strategy is to employ service voids. Creating a service void internally allows for alteration and maintenance of services and fin­ishes without recourse to penetrations through the air barrier. This allows for long term good performance in contrast to membranes which are liable to penetration at all service points, necessitating careful sealing of each and every penetration, in the short term this will help to reduce the air leakage uk  rate and vastly improve your chances of an air test pass, not only initially, but over the years of alterations and maintenance to come.

 

Generally, it is better to conceive of the joints in airtight layers as ‘positively’ connected, anticipating differential movement and de­cay of adhesive or chemical bonds. For example, where different components of a curtain walling system are liable to differential movement, it is clear that a joint whereby the two components are held together with a positive mechanical connection across a compressed gasket is likely to remain airtight longer that a simple butt joint with a mastic sealant between, this attention to detail will improve your air leakage uk rate and the chances of an initial air test pass.

 

Finally it is clear that complex solutions to airtightness are likely to be more prone to poor execution and potentially to greater vul­nerability to differential movement, failure of sealants, dislocation of components and so on. It is important therefore to aim for the simplest solutions to providing a robust airtight layer, using the fewest separate materials, junctions and penetrations, and the easiest installation and maintenance, this will improve the buildablity and improve the chances of a air test /leakage  pass.

 

It is worth making a point of considering each and every specified component with regard not only to its own intrinsic airtightness uk characteristics, but with regard to the connections between it and adjacent components. It is important to provide explicit details and guidance at specific, and particularly tricky detail areas. On design and build contracts it may be necessary to allow for some form of review of proposed solutions and procedures, to try and out design any problematic/complicated junction which will dramatically improve the chance of an air test pass

 

The following provide a few examples whereby airtightness can be simplified at the earliest design stages.

However good the workmanship, blockwork on its own can never be considered airtight. Once plastered, on the other hand, it may be considered extremely airtight, with concern only for those edges and corners where cracking or gaps can appear. This may be contrasted with the more common practice of drylining block walls with plasterboard on battens or dabs, either way when either is built correctly they can form a excellent air test barrier.

 

Design & Detailing for Airtightness

 

Services Zones or Rooms enable a range of services to be collected together before exiting the building, allowing most of the penetrations in the external fabric to be grouped and sealed effectively prior to the air test .

 

Service voids enable cables and pipe­work to be installed and altered without needing to penetrate the air barrier. Note however that if they are not run in con­duit, protection may be needed against subsequent fixings, if not undertaken prior to the air test this will result in an air test failure

 

Positive physical connections are to be preferred over any other joint such as one relying on adhesives. In the timber frame example shown the air barrier membrane is shown lapped and sealed with mastic over a firm background (boards with stud behind) and with a positive mechanical fix - a batten - fixed over the top and through to the stud.

 


Trinsically non-airtight block wall behind, this form of construction typically gives rise to a wide range of air leakage uk paths behind the boards and into floor, partition wall and ceiling cavities. From the perspective of airtightness, dry lining should be avoided unless great care is taken, otherwise it will result in a air test failure

 

Similarly, timber floors are difficult to seal well without a good deal of care. On the continent - and to an increasing extent in the UK at large - concrete floor systems are being used for both ground and first floors (often for other reasons such as acoustics, fire and the desire for underfloor heating) and these are easier to make adequately airtight prior to the air test . Hollow planks however can leak into cavities and require to be sealed at their ends, this will dramatically improve air tightness and the chances of an air test pass

 

One important and often quoted example is the timber first floor connection with a block wall inner leaf. Who is responsible for ensuring absolute airtightness when the timber joists rest on the wall and are infilled between with block and mortar? Presumably the bricklayer, but is it then his fault if the timber is installed at the wrong moisture level and subsequently twists and warps, leaving cracks around every joint? Is it really feasible to attempt to tape or mastic seal around them all, and what if the underside of the ceiling is to be exposed? Far better perhaps, to do away with the joist-onto-wall detail al­together and replace with joist hangers. Increasingly, the de­signer should be seeking solutions which are intrinsically airtight because of the design, rather than continuing as before while ac­cepting an increased use of duck tape and mastic on site! Whilst these may get you through the initial airtightness tests/air tests, they are short term solutions and not likely to lead to the anticipated energy savings for the Client in the long term.

 

A good review of the various materials and components which al­low the Designer to create an air barrier may be found in the BRE Report BR448: Airtightness in Commercial and Public Buildings

 

7.4 Detailed Specification

Beyond the performance specification illustrated earlier, it is im­portant that the issue of air tightness/air tightness testing becomes embedded within the standard specification vocabulary.

Where an equal or approved alternative may be allowed, it is critical that an airtightness performance specification is part and parcel of that equality of performance. For example, it may no longer remain satisfactory merely to specify a membrane, but in addition to specify the fairly precise nature of the sealing, over­lapping and potentially the subsequent layers as well. Simply of­fering a performance specification and ensuring the responsibil­ity resides with the Contractor is all very well, but it is important too to offer solutions that will enable a satisfactory outcome to be achieved and subsequently improve air tightness uk and the chances of an air test pass.

 

Design & Detailing for Airtightness - Implementing Airtightness

In addition to the intrinsic lack of airtight­ness uk, a problem of drylining is that it can create hidden pathways for air and (unfortunately raise the chances of an air test failure), as above, into the void above suspended ceilings and elsewhere throughout the building

 

Timber joists built into a block wall - a poor detail for airtightness. Far better to use joist hangars and avoid the problem. Source. Concrete planks are not free of problems either hollow planks are often left ungrouted where they meet the external wall, which could lead to extensive air leakage internally and subsequently an air test failure.

 

7.5  Implementing Airtightness

 

Key Principles

 

9.       The Contractor or Project Manager must be made responsible for achieving the air tightness levels set. In particular, this will involve careful co-ordination between trades; if this doesn’t happen then an air test /air tightness failure will surely follow

 

10.   Inspection remains an integral part of achieving air tightness and passing the air test .

 

11.   Ideally at least 2 air tests (air tightness tests) will be undertaken; the first when the building is weather tight, and the second air test a couple of weeks or so before handover.

 

12.   Experience suggest that making one person (or team) responsible for air tightness is the most ef­fective way to tackle the issue, this will drastically improve the chances of a air test pass.

 

5.   Remedial air tightness works to existing properties can reap substantial benefits without undue disruption and improve the chances of an air test pass.

 

It is not yet generally possible within the UK to specify that a building shall be airtight and leave it to the Architect or Contrac­tor to sort out. There is not yet a culture of airtight construction, except perhaps, amongst those who construct superstores, these companies (as a whole) pass far more air tests on there fist attempt.

 

The responsibility of the Designer in regards to the airtightness cannot be overestimated, for if airtight buildings are to become main stream, as they are else­where in the world, the techniques must be above all simple and buildable, with most if not all of the ‘tricky’ areas designed out from the start. In this way, such techniques can become ‘second nature’ to Contractors and there is less reliance on potentially adversarial inspection and air test/air tightness testing failures.

 

Ideally too, the Designer will understand the issues sufficient to prepare a sound performance specification – giving achievable targets for air tests/airtightness as well as a clear description of respon­sibilities and procedures, and a clear and practical set of overall and detail drawings, along with a detailed specification.

 

In the meantime, and even with good documents, there is likely to be a need for effort and vigilance by both the Design Team and the Main Contractor or Project Manager on site. This chapter briefly describes this effort, while the next describes in more de­tail the actual air test procedures and auditing techniques used.

 

7.5  Plan of Work

The RIBA Plan of Work provides a framework for the entire de­sign and construction process. The table on the next page allo­cates specific tasks relating to airtightness to each Work Stage to enable a schedule of tasks and responsibilities for the Design team to be prepared according to each project, this will drastically improve the chances of a air test pass.

 

 

7.6  Roles and Responsibilities on Site

 

Designer / Design Team

The responsibilities of the Design Team are detailed on the follow­ing page, showing all stages including site works and beyond. Buildings usually comprise a number of different components, creating a myriad of routes through which air can escape if not carefully sealed at each and every junction. The Designer’s role is to simplify these details to reduce difficulties on site.


It is critical that the purpose of pursuing air tightness is explained so that all concerned understand why they are being asked to attend to these issues. The initial briefing of key personnel at mobilisation stage – whether or not this involves the air tightness specialist – is also critical in determining the approach to con­ducting the works, inspection, air tightness testing and auditing etc, prior to the air test . which will need to be dovetailed into the many other concerns on site.

 

On large projects it may be useful for one member of the Design Team to take special responsibility for the air tightness / air test issues.

 

Contractor

The Main Contractor’s principal responsibility is to deliver the air ­tightness/air test performance overall and the most likely task on any but the smallest jobs will be that of co-ordination between the sub­contractors. The Main Contractor must be clear that he carries responsibility for the overall air tightness and in turn must ensure that all subcontractors are clear about the extent of their respon­sibilities. This is important since there may be some deviation to conventional practice in order for air tightness to be achieved and a air test pass. It is always prudent to place someone in the site team who has experience of air leakage/air tests on their previous projects as their experience  may avert a potential air leakage/air test failure

 

RIBA Work Stage Design Team Tasks

 

A          Appraisal Establish appropriate air permeability rate

 

B          Feasibility / Briefing Note Microclimate

Test existing buildings / building to be refurbished

Identify procedure for review and air testing

 

C          Outline proposals Consider a/t issues in relation to decisions about form of construction

Identify zones and layers

 

D          Detailed Proposals Identify requirement of additional consultants / design by specialists

 

E          Final Proposals Ensure co-ordination between DT to ensure a/t envelope & penetrations

Detailed application of airtight materials, junctions, service penetrations

 

F          Production Info Select sub-contractors for specialist works (incl. testing)

Careful specification of components, membranes, materials

Emphasise methods for airtightness on documentation

Careful specification of components, membranes, materials

Emphasise responsibilities in specification for dealing with ‘loose ends’ between sub-contractor interfaces

 

G          Tender Docum’n Define Contractors’ responsibilities for co-ordinating work sequences

 

H          Tender Action Ensure selected tenders include adequate airtightness procedures

 

J           Mobilisation Brief all involved in areas critical to air infiltration before work starts

Preparation of samples, training, testing and QA procedures

 

K-L       Site Works Co-ordinate inspection with Building Control if required

Ensure inspection of areas to be covered

Ensure audits and testing schedule is adhered to

Ensure design changes do not compromise airtightness performance

 

M         Post Completion Obtain feedback from concerning comfort and energy consumption

Carry out remedial work as required at end of DLP.

 

As with the Design Team, experience suggests that the best (air test ) per­formance has been achieved by Contractors who employ a dedi­cated individual (or team) to carry responsibility for airtightness and the air test , to inspect the works and instruct as required.

 

For Contractors, the issues of airtightness/air leakage uk and the passing of the air test are intimately linked to issues of good or bad workmanship in general and this can make the issue both more sensitive, but also more difficult to control. Even simple buildings are immensely complex and so the most important aspect of all is the creation of an overall culture of care­ful, tidy, accurate and airtight construction, something which can­not be simply forced through with a performance specification.

 

It is easier to specify and draw an airtight detail than to build it, and so the emphasis on inspection and Contractor responsibility has not developed from a prejudice against Contractors, but from a realistic appreciation that this issue cannot be entirely resolved ‘on paper.’ It is genuinely about a culture shift (at least for many in the industry) and this is where the real challenge lies. Once this shift is adopted there will be a far higher percentage of companies passing their air leakage/air tests uk at the first attempt

 

7.7   Inspection

Air leakage uk found in dwellings according to studies undertaken by BRE. The studies offer a range of conclusions, the most significant of which is that the greatest volume of air leakage uk is occurring in areas out with the ‘normal’ consideration of ventilation, through the myriad of cracks and openings all over the building which is described as ‘background air leakage uk, a common cause of air test failures’

Of the background air leakage subsequently investigated, the principal air leakage routes the greatest cause of air test failures  were noted as being:

 

• Plasterboard dry lining on dabs or battens, often linked to routes behind skirtings etc.

 

• Cracks and joints in the main structure; open perpends, shrink­age & settlement cracks

 

• Joists penetrating external walls, esp. inner leaf of cavity walls

 

• Timber floors, under skirtings and between boards

 

• Internal stud walls, at junctions with timber floors and ceilings

 

• Service entries and ducts

 

• Areas of unplastered masonry walls; intermediate floors, be­hind baths, inside service ducts

.


It is perhaps worth mentioning that the BRE results were based on buildings using dry lining on masonry walls and timber floors. Had the masonry walls been plastered, if concrete floors had been used, and if basic airtightness measures were taken, it is likely that the principal problems would occur around service penetrations, and, to a lesser extent, around windows, doors and roof lights. This is the experience of countries where envelope airtightness generally is more developed. As a result they achieve far better first time air leakage/air test results and subsequent air test passes

 

The following table lists many of the most common infiltration problem areas. On larger projects, common problems include:

 

·         Incomplete bulkheads at eaves;

 

·         Gaps where block work abuts to steel columns or beams

 

·         Uncapped cavity walls, at eaves (right) and mid-points where cavity walls change to composite panels

 

·         Gaps along the underside of corrugated roof linings 

 

·         Gaps between block work and steel, and uncapped cavity wall at join with composite panels Common Locations for Inspection (Applicable to all types of Construction)

 

Foundation / Ground Floor

·         Check wall and floor dpcs form an adequate air tightness layer, is a separate layer needed?

 

·         Check gaps at perimeter insulation strips

 

·         Check potential movement gaps between loadbearing structure such as columns and adjacent non- loadbearing slab

 

First and Intermediate Floor Levels

·         Concrete floors: Check joint between the floor and plasterboard to walls

 

·         Check gaps between concrete planks, or beam & blocks are sealed at the wall

 

·         Check voids under floor finishes and service run penetrations

 

·         Timber floors: Check a membrane seal has been incorporated if required

 

·         Check any membrane used is supported between joists

 

Eaves and Verge

·         Check continuity of airtight layer between wall and roof / ceiling

 

Ceiling level beneath the roof

·         Check for separation between deliberate roof ventilation and the conditioned zone

 

·         Check for service penetrations and hatches which pass across the airtight layer

 

Boundaries between different wall envelope systems

·         Check all systems have a dedicated airtightness layer assigned, and that these can be constructed to be continuous across dissimilar elements

 

Windows and Doors

·         Check that the frame to wall junction is properly sealed and continuous with the wall airtight layer, particularly at cills

 

·         Check the windows and doors have appropriate weather seals between the opening unit and the frame

 

Services penetrations

·         Check for proper seals at service entry points, and at points of entry into conditioned zones. These may also require fire protection

 

Main Entrances

·         Check that the whole entrance area is separated from the conditioned zone by an inner airtight layer

 

Lift Shafts, Service Cores, Delivery Areas / Car Park

·         Check these have been separated from conditioned zones with air barriers and draughtproofed access doors

 

Where profile fillers are used poor workmanship is common

 

• Perforated (acoustic) roofs, where the unsealed mineral fibre acoustic layer bridges the eaves of the building, constituting a major leakage point

 

• Gaps where plasterboard or wall linings are incomplete, com­monly above suspended ceilings and to the underside of beams

 

• Incomplete door and window reveals

 

• Services Penetrations into the building, and between zones inside the building

 

Another common issue is porous blockwork, particularly when internal walls are drylined rather than plastered or painted. Where this is likely to be unavoidable, it may be worth requiring blockwork to have an initial air test for air permeability(air leakage), and to have an AP value (by an accredited lab) that is no more than 50% of the target Air Perme­ability/air leakage  uk for the overall building.

 

7.8  Air Testing and Audit Schedule

In many cases to date, an air test /air tightness test has been carried out a week or so before practical completion. If the result is poor – a high rate of air leakage – then a great deal of work suddenly needs to be done, often to areas which have been covered up and the whole business can be both costly and time consuming, just at the point where in many contracts there is already considerable pressure on Contractors.

 

Far better therefore to schedule the air test /air tightness test uk at a time where remedial works are relatively simple to perform. On the other hand, it is important that a air test /air tightness test is undertaken close to han­dover so that the Client and Design Team can be sure that the completed building accords with the performance specification, and so passes the air test first time .

 

Ideally therefore, two Air tests/air tightness tests at least should be carried out. The first Air test /air tightness test uk should be undertaken as soon as a meaningfully air- and weathertight envelope has been installed. Ideally, all air barriers are still accessible and any defects can be readily put right. This air test /air tightness test uk plus the audit techniques which are likely to accompany it, may be used to ensure an acceptable airtightness performance and give a good indication of where subsequent works may ad­vantageously targeted.

.

In this way, the second and final air test /air tightness test uk serves simply to confirm the performance of the building, hopefully at a slightly improved level from the first air test /air tightness test uk without the need for costly and complex operations late in the day.

 

Such air tightness uk /air leakage testing uk schedule is nonetheless costly in itself, but for those who have been involved in such air leakage testing uk schedules, experience suggests that this remains the most cost effective way to deal with the issue. Certainly it is worth avoiding excessive remedial works at the eleventh hour, just prior to the air test works. With a sufficiently good first air test /air tightness test uk per­formance, it may even be possible to dispense with the final air test , if this is deemed acceptable to the Design Team Leader or Cli­ent.

 

It is often the case that the envelope is not sufficiently complete on the due date for the air test /air tightness testing uk. This then necessitates a complex process of temporary sealing of the incomplete areas. It is harder than to ascertain the location of the air leakage and allowances are made which may prove misleading. Experience suggests that this is not ideal and it would be better to put off the air test /air tightness test uk for a week and carry it out when the envelope is complete and ‘as intended’ this will drastically improve the chances of a air test pass.

 

 

On larger projects, more air leakage uk /air tests uk may be needed, or more specific tests of individual areas required. Large projects with multiple units of a similar nature may benefit from either pre-installation component testing, or insitu testing of one installed component to establish acceptable air test /airtightness uk levels early on.

 

7.9  Remedial Airtightness Works

With airtightness testing and a general awareness of airtightness uk issues developing around new build situations, the principal area of concern, as with energy efficiency in general is the existing building stock. In terms of airtightness uk, the UK building stock is considerably worse than comparable northern latitude countries  and there is a good deal of room for improvement, if these improvements do not occur then a large percentage of companies will fail their air test /air leakage tests.

 

Either as a standalone measure or as part of a package of en­ergy efficiency measures generally, there is scope for remedial works to most of the existing UK building stock. Relatively simple measures may in many cases be sufficient, using a wide range of sealants to control air leakage uk. However, it is important that such measures are combined with attention to the ventilation require­ments of buildings where, to date, insufficient ventilation has been ‘augmented’ by infiltration and exfiltration which, if reduced, could lead to other problems and a subsequent air test failure.

 

As with thermal insulation, there is an extent to which controlling some of the air leakage merely diverts the flow of air, inward or outward, to another defect or gap, (this will still result in an air test failure) but there is such scope for improvement that even fairly basic efforts are likely to reap sub­stantial environmental, financial and comfort benefits for owners and occupiers alike.

 

There are many examples of remedial works described in the various publications noted in the references. Some of the more successful measures included carefully sealed secondary glaz­ing installed where old windows had to be kept for conservation purposes, draughtproofing of doors and entranceways generally, and installation of lobbies in well trafficked reception areas, at­tention to draughtproofing of existing windows and targeted use of flexible sealants to ill fitting components and joints between different construction types, will drastically improve your air leakage rate and will should enable  you to pass your air test first time

 

10.   Testing Airtightness

 

Key Principles

 

9.       Air test procedure is set out in CIBSE TM 23 and in BS EN 13829: 2001.

 

10.   An air test / air tightness test uk involves sealing all ‘normal’ gaps such as vents and pressurising or depressuris­ing the building. The level of fanpower required to maintain the pressure differential indicates the ‘leakiness’ or ‘permeability’ of the building.

 

11.   Air  tests uk/ air tightness tests are typically followed by an audit (using smoke pencils, for example) to expose and make visible the various air leakage uk routes during the air test .

 

12.   Where projects comprise large quantities of a single component, component testing in the labora­tory may be appropriate as well as on site element air testing .

 

8.1 Climatic conditions

As mentioned in Chapter 1, the raised pressure differential of 50 Pascals created during an air test /air leakage test uk is quite small. Whilst this is adequate to overcome most of the common pressure dif­ferential anomalies, such a small differential is vulnerable to larg­er pressure differences created by climatic conditions.

 

Air tests uk/ air tightness tests uk require calm days – i.e. a reading on the Beau­fort Scale of 3 or less (3.4 to 5.4 metres per second wind speed at 10 m above ground) This corresponds to a gentle breeze with leaves and small twigs in constant motion. In winter conditions and on exposed sites therefore testing may not be possible, al­though it is often possible to make allowances, so long as these are carefully recorded, during the air test .

 

8.4   The Air Test itself

Essentially the process is one of pressurising the inside of the whole building (during the air test )  and of measuring the rate at which air needs to be blown or sucked to maintain that pressure differential; a building suffering large amounts of air leakage will equalise readily and require a greater measurable effort to maintain the 50 Pascal differential, while a air tight building will easily contain the enforced differential and require little additional input during the air test , this will be easily recognisable within the first couple of minutes during the air test / air tightness test uk

 

The pressure difference is induced by one or more calibrated fans that are normally mounted within a suitable doorway. An adjustable door panel system, sealed around the edges is used which can also be connected to large external fans via collaps­ible ductwork if required. The rate of the fan, or the volume flow of air through the fan can be understood as the rate of air entering / escaping throughout the remainder of the building envelope, this is recorded throughout the air test .

 

Buildings are air tested in such a way as to recreate ‘normal’ condi­tions. Doors and windows are closed, trickle ventilators closed, extract fans and such like are closed but not sealed. Internal doors are wedged open.

 

If the building is under construction the  air test /air leakage testing uk is ideally undertaken out of working hours, but sometimes this is not practical so some scheduling of work needs to be thought through in advance. With all external doors and windows sealed shut, some work becomes impossible (such as work with solvents requiring ventilation) and internal trades are normally ‘sealed in’ for a short time, where they can carry on undisturbed during air test .

In existing buildings, air tests uk/air tightness tests are normally carried out when the building is unoccupied if possible because of the disruption.

 

8.3 Air Leakage Audits

The air test /air tightness test uk quantifies the rate of air leakage uk through the envelope as a whole, but it cannot locate the air leakage uk paths. Where remedial work is required therefore, air test /air tightness tests uk are followed by a range of auditing techniques designed to identify the specific places where air is leaking.

 

In many cases a simple visual inspection may be sufficient – es­pecially if undertaken by someone with experience of the likely locations of leakage, this is a good way of lowering the risk of a air test / air tightness test uk failure

 

However, most leakage routes are difficult or impossible to spot without visual aids. One common technique is to use smoke trac­ers – smoke pencils or smoke machines. These render the air leakage paths visible in certain situations during the air test . The building may be positively pressurised and the air leaks witnessed externally, or, more usually, negatively pressurised while a smoke pencil is drawn over likely gaps and defects which become visible as the smoke is sucked inwards during the air test .

 

Another technique, which has certain advantages and disadvan­tages compared to smoke tracing, is the use of an infrared cam­era by undertaking a thermographic survey, Used either externally or internally, these ther­mographic cameras register the radiant heat levels of surfaces and so are able to ‘see’ for example, where cold air is cooling the fabric around a gap internally, or conversely where warm air is escaping and heating the colder materials on the external face.

 

To work effectively, there needs to be a recognisable difference between the internal and external ambient temperature, so be­fore any heating has been installed and on a warm summer’s day thermography / thermographic surveys may not be effective. Similarly on warm and sunny days, sunshine on external surfaces can distort the true situation so it is better on such days to wait until early evening. Conversely, rain on external surfaces can be equally distorting of the true thermal situation. However, these cameras are useful in identifying problems at high level or difficult to reach areas, and are also very helpful in identifying other construction defects such as poorly installed (or non-existent!) insulation within the fabric.

 

On larger commercial buildings, air tests uk/ air tightness testing may be un­dertaken at the same time as ‘standard’ ventilation system com­missioning and associated studies

 

8.4 Component Testing

A distinct aspect of overall air tightness testing is the individual component air test . This may be undertaken quite separately, in the laboratory or by the manufacturer of a particular component. Such air tests uk may be deemed necessary on a large project where large areas of one particular type of component, for example curtain walling, are to be specified.

 

Insitu element air testing involves isolating the area within a tem­porary sealed compartment, which is then pressurised, and the air leakage related to the area of interest assessed. In this way sample areas of a building may be air tested using smaller fans as required.

 

9. The Details

Caveat

It is important to emphasise the scope and purpose of the following drawings and specifications.

They are included solely to show practitioners the sort of altera­tions that can be made in order to enable buildings to be much more airtight in general.

 

Their purpose is not to offer approved details in any sense, but to illustrate the difference between details and specifications which do not address airtightness issues, and those that do. It is the dif­ferences between the originals and alternatives which is intended to be illustrative, not necessarily the alternatives themselves.

 

The original details have been taken from conventional details and specifications we believe to be broadly representative of their construction types. We hope the principles shown, and the specific references made will assist designers in making similar changes in their own work, but it goes without saying that air test /air testing cannot take responsibility for any work undertaken as a result of the use of these details.

 

Specifically, these details are not intended to show best practice in any sense, nor are they even intended to be up to date. We have striven in the preparation of these details and specifications to keep as close to the original as possible. We have done this in order to show that some quite fundamental alterations – in terms of airtightness - may be made with the minimum of visual or func­tional impact on the original. Where these original details and specifications do not meet current standards or aspirations, the alternatives given are likely to be similarly wanting. To re-iterate, the purpose is not to produce approved details, but to illustrate the process of improvement – in terms of airtightness only – that may be made.

 

Consideration of priorities in airtightness design and specifica­tion is potentially misleading since, in effect, all gaps, cracks or tears let in air and the sealing of one simply redirects infiltration to somewhere else, this becomes all the more apparent during the air test . Like thermal insulation, what is important is the level of continuity generally, not any particular detail on its own. Nonetheless some prioritisation has been attempted in order to help Designers to prioritise their own efforts since not all measures may be necessary.

 


9.1 Steel Frame + Concrete Block Cavity Wall

 

Original Specification

 

Discussion

Because of the largely wet trades involved, one might imagine a masonry construction is  inher­ently more airtight than the dry fixed timber frame and curtain walling construction types. However, insofar as concrete inevitably shrinks as it dries, as mortar beds and perpends are often poorly filled, and due to the differential movement between masonry and the steel frame, the myriad pathways that open up can make masonry buildings extremely susceptible to infiltration, this can lead to an air test failure.

 

To make things worse, construction such as this does not easily lend itself to a simple, single airtight layer which can be applied separately and therefore the need for vigilance, and some care and attention to a number of small but potentially time consuming sealing jobs is high, however these must be undertaken if you are to pass an air test on the first attempt.

 

It would be possible to form an airtight layer inter­nally through the use of an applied membrane and the adoption of a service void. This would have the advantage of allowing for changes in the service or fit-out provision without the risk of damage of compromise of the airtight mem­brane,(this will lead to a air test failure) and for those inclined to this solution.

 

A parge coat and service void could have a similar effect, but the use of plaster internally is a common and effective technique for creating an airtight layer and is preferable in this instance as it is closer to the original detail and will improve the overall air test results

 

HIGH PRIORITY

·         Wet Plaster Finish or Wet plaster coat costs more but provides a better finish overall, as well as significantly improved airtightness across the masonry leaf. Plaster should be extended to all wall areas and not left off in areas which will not be seen,( such as suspended ceilings.

 

·         Membranes Lapped & Sealed2 lines of tape and a positive mechanical fixing by batten ensure laps are sealed for the long term

 

·         Mastic to Skirtings, Linings etc.

 

·         Critical in this detail since the plaster cannot form a continuous layer at these junctions

 

·         Sealed Cavity Closer: Gaps around openings are common so care is needed here to prevent infiltration around the frame and into the cavity

 

·         Vapour Barrier Seal at Eaves: Important here since no effective seal is noted on the original which could lead to excessive airflow at this vulnerable point.

 

 

Costs

The most significant cost implication is associated with the addition of the wet plaster coat to the inner leaf of blockwork. This results in approximately a 60% increase in cost, although the quality of the blockwork is not as critical. This item is also significant in that is changes the ‘look’ of the detail but is probably the highest priority.

 

Otherwise, most of the costs are associated with the additional time, effort and care implicated within the specification and details. Of these, the most significant is the additional labour and materials required for the joining of the vapour barrier in the roof, and sealing it around the perimeter. This work almost certainly more than doubles the cost of the vapour barrier in the original detail, but again, represents a critical factor in reducing air leakage and saves the cost of multiple air test failures.

 

A number of the measures described represent no more than a re-iteration of good practice, such as the sealing of perpends, lapping and sealing of membranes, draught stripping of windows and so on. These may assumed to incur no cost implication, but perhaps one of attention to details (this usually results in first time air test passes) on site.

 

The mastic sealant to skirtings, cills and the like would add about 50% to the costs of these items, though these items represent only a small fraction of the overall costs.

Taping of the insulation boards would depend largely on the board type, but might realistically attract only a marginal cost increase, as would the use of com­pressible foam around the steelwork.

 

MEDIUM PRIORITY

·         Concrete Slab Floors:Concrete slabs form an airtight layer but joints with penetrations such as perimeter blockwork, insulation or structural columns must be sealed.

 

·         Cill to Window Sealing: Double sealed detail which increases the chance of securing an airtight seal at this often overlooked junction

 

·         Compressible Foam between Steel and Blockwork: Potential solution to the inevitable gap which will form here, also sealable with mastic oninside face only.

 

LOW PRIORITY

·         Perpends Fully Filled: Not critical if a wet plaster finish is applied internally, but high priority if they are not.

 

·         T&G and Taped Insulation:Not technically part of the airtight layer, but gaps here simply increase the likelihood of infiltration (air leakage) and are relatively easily sealed.

 

·         Expanding Foam to Gap at Eaves: Not part of the airtight layer but by seal­ing a large gap in the fabric, this reduces the wind pressure driven airflow within the cavity thus reducing the risk of infiltration (air leakage) indirectly.

 


9.2 Index

a) Perpends fully filled

A common problem with blockwork and brickwork buildings is that perpends are not completely

filled and this leads to air flow (air leakage) through the wall this becomes apparent during the air test . To an extent this measure is superceded by both points (aa) and (d), but it is still worth making the point in order to draw attention to this workmanship issue in general, it’s the attention to detail that ensures you pass the air test first time

 

b) Blockwork Maximum Air Permeability by Component Test

An alternative to wet plastering the blockwork on the inner leaf is to require a component air test of the blockwork to satisfy a maximum air permeability of, say, 5m3/hr/m2 or less. On larger

projects, or where wet plastering is unlikely to be effective or desirable, this is one method of

ensuring a reasonable degree of airtightness from the blockwork leaf. These conditions may also be used for the outer leaf but is not as important because it is the inner leaf which is providing the main air barrier for the air test .

 

c) Membrane Lapped and Sealed

Typically membranes are lapped and stapled or tacked, but in order to create airtight layers, it is

important that these laps are rigorously sealed. Best practice in this regard - beyond the correct

use of Manufacturers’ overlap dimensions, proprietary tapes and other accessories - is to run a

layer of double sided tape between the membranes at the overlap and run a tape over the

leading edge of the outer sheet. In addition, since many tapes tend not to last too well, it is

advisable to ensure that laps are made directly over supported areas (i.e. with studs or dwangs

directly behind) and are held down positively with battens fixed through forming a mechanically

tight, as well as an adhesive seal, this will provide an especially strong air tightness seal and will improve the chances of an air test pass

 

d) T&G or Shiplap and Taped Non-Mineral Fibre Insulation

Mineral fibre is permeable to air movement and cannot be counted upon to help in reducing air

leak­age. Extruded polystyrene and other closed cell plastic insulation materials do not suffer from

this and so have the potential to reduce air leakage and improve air tightness in and out of the

building(improving the chance of passing the air test ). However, they are only likely to do so if they are effectively joined at their edges, at corners, openings and around wall ties etc. For this reason, it is likely that t&g or shiplap edge boards (which are available from a number of Manufacturers) will offer better connections, and these can be further augmented by the use of a sealant tape externally.

 

e) Wet Plaster Finish Internally

An alternative to arranging component tests for the blockwork. A simple block finish with 2 coats

of paint which in terms of airtightness is an improvement on a uncoated block wall but is not

sufficient to consider the blockwork airtight in the least. Wet plastering of the blockwork is more

expensive but ensures an airtight masonry leaf, this will improve the chances of a air test . The plaster should extend to all areas of the wall, regardless of whether they will be hidden by suspended ceilings or raised floors. It should extend right to the floor and to undersides of steel beams etc. and where broken by service boxes etc. should be conscientiously filled and sealed.

An alternative which would have a similar effect would be to use a parge coat over the blockwork,

before application of a service void and separate finish layer. The simple wet plaster finish is

closer to the original.

 

f) Mastic Both Edges to Skirtings, Reveal Linings etc.

Where the corner junction behind has been carefully sealed then this measure may not be

required, but in the examples shown on this construction, this particular detail is critical since it

forms an integral part of the airtight layer, particularly where the plaster has to be discontinuous.

 

g) Concrete Slab Floors

Unchanged from the original detail, this is simply to note that concrete slabs form an airtight

barrier and may therefore be considered good practice in this regard. However, no note is made

of the need for care to be taken where the slab meets elements of structure which pass through,

steel columns, for example. At these junctions, a compressible foam strip may be laid around the

steel prior to pour­ing the concrete if practicable, or a mastic sealant may be used subsequent to

the pour to seal the inevitable shrinkage cracks which will form and become air leakage uk paths and will increase the chances of passing the air test

 


h) Insulated and Robust Cavity Closer

A robust and insulated cavity closer enables the cavity to be effectively closed, the gap to be

bridged with insulation without risk of moisture flow between inner and outer and the window to

be securely fixed at the head and jambs if required. The gap between window frames and the

main wall is a no­torious place for infiltration (air leakage uk) and so increases the chances of a air test failure so it is important that this junction is carefully sealed. The flanges of the cavity barrier should be closed against the blockwork faces with a continuous mastic bead between on each flange so that airflow into the cavity from outside or in is prevented, thus resulting in better air tightness uk

 

i) Proprietary Cill with Foam Sealant Internally and Mastic Sealant Externally

In addition to the mechanical fixing of the window frame through the cill piece, it is important that

this fixing is made through a compressible foam strip which is then sealed against air leakage uk

from outside with a mastic type sealant. This gives the Contractors two opportunities to ensure a

completely airtight seal at this particularly vulnerable point, prior to the air test .

 

k) Compressible Foam Strip beneath Steel Beam to Blockwork Top

For reasons of both initial shrinkage and subsequent structural movement, it is to be expected

that a direct connection between a steel beam (or column) and a block wall will open up over time

to form a potential route for infiltration (air leakage). One way to try and reduce this inevitable gap

is to build the blockwork against a compressible foam strip which immediately expands to fill the

gap between and remains flexible thus continuing to fill the gap even after shrinkage and

movement. Since compressible foam strips are not intrinsically airtight, mastic sealant should be

used in addition to form a neat internal joint which should further seal the connection, thus

drastically improving air tightness uk and the chances of passing your air test first time.

 

l) Vapour Barrier Detail at Eaves

Here the vapour barrier is positively sealed to the steel perimeter beam to properly seal the

ceiling vapour - and air - barrier along its edge. Assuming that the steel beam is without

penetrations (a specification note has been added to ensure that this is checked) then as long as

the plaster seal to the underside of the beam is adequate, an airtight layer has been formed

which may be discontinuous in materials but continuous in terms of airtightness uk, this building method should help you pass the air test at the fist attempt.

 

m) Expanding Foam to Large Gap at Eaves

Whilst not strictly part of the airtight layer, this measure reduces the potential wind pressures on

the cavity which in turn reduces the risk of air infiltration through the airtight layer itself. Note also the introduc­tion of a ply layer above to support the insulation (nothing is noted as doing so in the

original detail) but significantly against which the foam can create a firm seal which should

drastically improve air tightness uk and a air test pass

 

n) Mastic Sealant to Joints

Additional notes to seal connections between dissimilar materials which are likely to provide

routeways for airflow (air leakage) unless conscientiously sealed.

 

o) Draughtstripping to Windows and Doors

Most commercially available joinery, metal or plastic windows and doors will be adequately

draughtstripped but it is important to explicitly ensure that this is the case, and that seals

(preferably tubular rubber / epdm type) are accessible and can be easily replaced should they

begin to fail to adequately seal when closed.

 

9.3 Timber Frame with Con­crete Block Outer Leaf

Original Specification

Discussion

Despite the inherently dry fixed nature of timber frame construction, it offers good opportunities to ensure airtightness uk because of the existing convention of using vapour control layers internal to the insulation and breather membranes externally. This gives the Designer two layers with which to work to form a robust airtight envelope overall, and without introducing any significant or new component. The outer layer of blockwork (or brick, or dry cladding of any type) need not perform any major role in the airtightness strategy, and should not affect the air test result.

 

Although there are a large number of small adjust­ments to conventional practice outlined, none of these, except perhaps the addition of the service void and backing board involve any major shift in construc­tion process. Experience suggests that such changes are readily made and subsumed within the standard details and specification clauses of the practice.

 

More tricky is the need to convey the need for greater effort, co-ordination, care and vigilance to Contractors for whom there is little to be gained from the good practice noted, and quite a lot to be lost in terms of potentially time consuming additional tasks. In the short term it is important to emphasise the additional co-ordination and tasks to Contractors at the time of tendering so that these are not overlooked and the extra effort can be adequately assessed.

 

HIGH PRIORITY

a) Continuity of Layer / Co-ordination of Trades

General measures to ensure tradesmen are aware of the need for air tightness that all involved are conscientious and rigorous, and that someone is responsible for co-ordination between trades prior to the air test

 

b) Service Void

Use of a service void means most if not all penetrations through the vapour control and airtight layer can be avoided.

 

c) Joist Hangers

Use of Joist hangers avoids the common problems of air infiltration where joists are built into the inner leaf

 

d) Membrane to Floor Perimeter Beams

Slightly awkward solution for solving the problems of discontinuity at this area which is nearly impossible to solve otherwise.

 

e) Flexible Foam around Joinery

Gaps around openings are common and neat, effective solutions can be difficult, careful use of flexible foam enables effec­tive and durable seals to be formed.

 

f) Continuous Layer Over Partitions

High priority because of the high potential exfiltration rates and condensation risks at this point

 

g) Backing Boards

Use of backing boards makes installation of the membrane easy and thus less prone to poor workmanship and subsequent air test failure.

 

MEDIUM PRIORITY

a) Membranes Lapped & Sealed

2 lines of tape and a positive mechanical fixing by batten ensure laps are sealed for the long term

 


Costs

Not surprisingly, the addition of the service voids adds considerably to the costs of both the walls and ceilings. Of course, such costs say nothing of the increased ease of services installation, nor of the long term benefits of a much greater access for upgrading and alterations.

Nonetheless, the addition of the OSB and battens forming the service void in the walls adds approxi­mately 35% to the cost of the external wall. Mechani­cally fixing the vapour barrier to the floor and taping would add approximately 4% to the overall wall cost in addition.

Adding the service void to the ceiling would represent an approximate 130% increase in cost over just the 2 layers of plasterboard. But again, services instal­lation would be easier.

 

The additional work associated with the breather membrane would incur a similar additional cost, but may not be a priority if the internal vapour barrier is well installed.

The mastic sealing of the skirting boards would increase the cost of their installation by about 50%, although these represent only small costs overall, the use of polythene strips at the floor and eaves, and the use of foam around the windows would attract only a marginal cost increase.

The use of flexible insulation need not attract any increase in cost if a common, economical type was chosen. Remember all of the above can be far more economical that not passing the air test and therefore resulting in costly LED’s

 

MEDIUM PRIORITY

a) Joinery Draughtstripping

Tubular seals are probably the best option.it is important that they can be easily ac­cessed for maintenance and replacement.

 

b) Continuity at Openings

Continuity between the framing sealant (m) and the membrane can be tricky and care is needed to ensure a good, durable seal.

 

c) Seal Loft Hatches

Unsealed loft hatches may contribute to air leakage, so worth some care.

 

d) Plasterboard Penetrations

If the airtight layers are sound then this should not matter, but still worth attention.

 

e) Flexible Not Rigid Insulation

Flexible Insulation provides a better fill between studs, rafters etc.

 

LOW PRIORITY

a) Continuity Behind Lintols

An extra strip of membrane to form a con­tinuous layer when the main one is lapped over the cavity barrier, also fill behind lintol.

 

b) Mastic to Skirting’s, Linings, Cornices

Not necessary if the airtight layer is sound

 

c) Air Barrier to Ceiling

High Priority in separating floors

 

d) Laying Tape to Plasterboard Junctions

 

e) Wall Tie Fixings

 

f) Top Runner Strip Seal

 

g) Airtight Service Boxes

 

h) Corrosion Resistant Fixings

 


9.3 Index

e) Wall Tie Fixings to Timber Frame

The breather membrane is not the main air barrier, but it is nonetheless a useful ally in reducing air leakage uk through the construction generally. Ensure that wall tie fixings do not lead to damage to the membrane (as this will lead to large amounts of air leakage uk, and a subsequent air test failure) ,  ideally, by taping over the area of membrane at which the tie is fixed.

 

(b) Use of Corrosion Resistant Staples or Fixings

Non-corrosion resistant fixings to external breather membrane can corrode to a point where they fail, allowing the membrane to come loose, often creating a small hole in the membrane and reducing the effectiveness of the membrane as an airtight layer, this will allow for air leakage uk and a probable air test failure. Copper is non-corrosive but can affect polyethyl­ene, whereas stainless steel has no effect on polyethlene and so should be preferred.

 

(c) Membranes to be Lapped and Sealed

Typically both internal and external membranes are lapped and stapled or tacked, but in order to create airtight layers, it is important that these laps are rigorously sealed, this will ensure a air test pass. Best practice in this regard - beyond the correct use of Manufacturers’ overlap dimensions, proprietary tapes and other acces­sories - is to run a layer of double sided tape between the membranes at the overlap and run a tape over the leading edge of the outer sheet. In addition, since many tapes tend not to last too well, it is advisable to ensure that laps are made directly over supported areas (i.e. with studs directly behind) and are held down positively with battens fixed through forming a mechanically tight, as well as an adhesive seal. This requires consideration of lap positions early on if extra framing or subsequent battening is needed.

 

(d) Ensure Continuity of Membrane / Co-ordination of Trades 

Whilst this is easy to achieve across large, flat areas, it is more difficult at the many awkward angles, junctions, corners and so on a typical site. There is no specific guidance except to ensure that those responsible for installation of the membrane are rigorous and conscientious in their attention to all of the inevitable nooks and crannies, and that the person responsible for co-ordination is equally attentive, particularly when the junctions are between separate forms of joint and separate trades. Having someone experienced on previous projects that required air tightness uk tests is a huge advantage and dramatically improves the chance of a air test pass

 

(e) Ensure Membrane is taken into Opening Reveals, Taped and Sealed and Made Continuous with Opening Seals

it is typical at openings in timber frame buildings to allow the membrane to run across the opening initially, then form a star cut into the opening, folding over the sections of membrane and trimming as necessary. In these cases, there are inevitable gaps in the airtight layer at the corners of the opening, and it is important to ensure that these are made good before subsequent installation of joinery etc and the following air test .

 

(f) Fix Airtight Membranes to Firm Backing Boards

In conventional timber frame construction, vapour barriers are fixed across studwork, usually after the installation of insulation and prior to the fixing of the internal lining. Equally external breather membranes are sometimes installed across gaps between rafters or studs. In both cases membranes are susceptible to pressures from both sides, leading to the membrane breaking free of its fixing and creating holes in the airtight layer. Ideally, membranes should be fixed against a firm backing board by way of protection against damage of this nature, this in turn will improve the chances of passing the initial air test .

 

(g) Service Void  

The principal advantage of a service void is related to functionality and maintenance over time, but a secondary advantage which relates directly to airtightness uk is that since all services may be incorpo­rated within, that is, on the inside of the vapour control layer, there is no need to penetrate the layer at each and every service installation, thus significantly cutting down on the myriad potential gaps that are typically formed and either left, as this will surely lead to a air test failure or made good which is time consuming and costly.

 

(h) Laying Tape at Plasterboard Junctions

Using laying tape at junctions makes the formation of an airtight junction both conscious and relatively easy, even allowing for subsequent shrinkage and cracking of the skim layer.

 

(i) Airtight Service Boxes 

Developed in Canada where airtight construction is more advanced, these service boxes are fitted with gaskets and a flange surround allowing for an airtight seal at all openings in the lining.

 

 (j) Mastic Both Edges to Skirtings, Reveal Linings, Cornices etc.

Where the corner junction behind has been carefully sealed then this measure may not be required. In addition to the nail or screw fixing, a mastic seal both edges aid’s efforts to guard against infiltration/air leakage (which will increase the chances of passing a air test ) but it makes removal and alterations more difficult.

 

(k) Ensure Continuity of Membrane behind and around Lintols 

It is likely that to achieve this requires two separate measures. First the breather membrane needs to be continuous and extend into the opening, thus a second strip should be affixed to the wall and lapped and sealed to the main membrane which must lap over the lintol or cavity barrier etc. Second, it is likely that gaps could form between the top, outer edge of the joinery and the lower, inner edge of the lintol, leading to a cavity behind the lintol. This cavity should be filled with expanding foam or mineral wool and if possible the gap filled, probably with a mastic sealant, this will drastically improve the chances of passing your air test at the first attempt

 

(l) Flexible Foam Sealant around Joinery Insertions

Gaps around openings are one of the most common of infiltration paths. They range from 0 to 20mm, which is too large to be filled by mastic. Compressible flexible foams are ideal for this application. En­sure that the airtight membrane meets the seal on both sides to maintain the airtight layer overall, and subsequently pass the air test

 

(m) Draughtstripping of Openings in Joinery

Draughtstripping of joinery comes in many forms. It appears that synthetic rubber or elastomeric tubular seals work well, creating good seals with minimal compression, depending on the size of the gap. It is important that seals are unaffected by paintwork and subsequent decoration, or are easily acces­sible and removable. This is important so that seals can be replaced as they start to fail to maintain the airtight layer.

 

(n) Seal all Penetrations in Plasterboard / Internal Lining 

Even with the use of airtight outlet boxes there will be inevitable penetrations such as ceiling pendants, pull cords, recessed fittings etc. which must be made good manually, typically with mastic, otherwise you may end up failing your air test

 

(o) Seal Loft Hatches

Generally, this involves a continuous bead of mastic to the underside flange, and, depending on the design, the use of compressed and flexible foam, or mineral fibre etc. above. Please note in our experience this is a common area for air leakage, and a major cause of air test failures

 

(p) Use of Joist Hangars as Opposed to Built-in Joists

The original specification here is already good practice, that is, the use of joist hangars which sidestep the problems of joist movement and shrinkage allowing infiltration and airflow within the floor voids, another major cause of air test failures

 

(q) Membrane Strip to Inner Face of Floor perimeter Beams 

100 gauge polythene or similar fixed to the inner face of the perimeter beams early on in the framing process can lapped and sealed to the internal vapour control layer typically installed a good deal later, so that a continuous internal vapour control and airtight layer may be effectively created.

 

(r) Continuity of Membrane to Ceiling over Partition Walls 

ideally this would comprise a continuous membrane affixed before the partitions are installed. However it is more likely that partitions are installed before, therefore such a layer would require strips to be fixed to the partition top runners to be later lapped and sealed to the ceiling vapour control layer.

 

(s) Flexible, Rather than Rigid Insulation 

Rigid insulation between joists, studs or trusses generally has to be cut to fit and this is never 100% accurate, leading to myriad gaps and routes for airflow. Flexible insulation avoids this problem and improves the chances of passing your air test at the first attempt.

 

(t) Top Runner Strip Seal

The use of this strip, lapped and sealed with subsequent membranes both sides prevents air infiltration into the wall itself from the ventilated eaves area, thus ensuring continuity of the airtight layer, which should help you to achieve an air test

 

(t) Air Barrier to Ceilings 

In ceilings within dwellings of the same occupancy, this is unlikely to be useful, but in separating floors, it is extremely important that an air barrier is included in the floor and ceiling make-up. Noted here by way of a reminder.

 

9.4 Steel Frame + Glazed

Façade

 

Discussion

It is important to be confident that the curtain walling manufacturer, supplier and installers all share an ex­plicit commitment to producing an airtight wall overall, as it will be very difficult for the Main Contractor to ensure a continuous airtight fabric if this element is not firmly ‘tied down’ before the start on site, this is one of the main causes for air test failure

 

The focus of concern then falls to all the various cor­ers and perimeters where the system meets other construction elements and here both Designer and Contractor need to have carefully considered in detail each occurrence and made adequate provision, to avoid large amounts of ad hoc remedial work, during the air test .

 

The roof membrane must be carefully sealed and the perimeter condition considered so that a continuous and positive connection can be made. Note this is another major cause of air test failure

 

HIGH PRIORITY

(a) Curtain Walling Performance Spec.

Since this represents the largest area exposed to wind it is important that the performance specification is adequate and that the components are conscientiously installed

 

(b) Mastic Perimeter Seals

With the main curtain walling components installed and airtight, the next most signifi­cant air leakage route is likely to be the pe­rimeter seals. Both mastic and membrane seals are valuable in this regard. Note this is another major cause of air test failure

 

(c) Membrane Perimeter Seals

With the main curtain walling components installed and airtight, the next most signifi­cant air leakage route is likely to be the pe­rimeter seals. Both mastic and membrane seals are valuable in this regard. Note as above this is another major cause of air test failure

 

 

(e) Roof Membrane Sealing

Any leakage in the roof membrane or at the roof / wall junction could be serious in terms of both energy waste and risk of moisture related damage to the roof build-up, so this detail is important. By properly preparing for your air test this will alleviate any of these future problems

 

MEDIUM PRIORITY

(h) Plates Added to Beam

Because of the difficulty in forming an adequate seal to protruding beams, this is likely to be a major source of air leakage in the long term so designed, rather than ad hoc site measures to reduce air infiltration are important.

 

(f) T&G and Taped Insulation

Potentially a minor issue, but given higher priority becasue it is relatively easy to solve and reduce airtightness/air test  uk failure and thermal insula­tion related risks.


 

Costs

It is difficult to ascertain any meaningful cost implica­tions with this detail because of the variety of curtain walling systems available.

 

The measures outlined are fairly standard in most installations and should in all cases represent no more than a re-iteration of good or best practice. However, they could attract an additional cost where one particular system did not address airtightness and the subsequent air test in one way or another.

Measures such as the additional efforts associated with air barriers at the separating floor, eaves and flor / wall junction might attract additional costs over that aspect of the original detail by approximately 30% largely because of the additional labour and attention required. However these costs are still cheaper than suffering costly LED’s for not passing the air test

 

LOW PRIORITY

(g) Membrane Seal between Floors

The existing detail should provide a rea­sonable degree of airtightness, but this measure will make the task conscious and affect a greater degree of separation.

 

(d) Foam Filler

Should not be required if the measures in (b) and (c) are completed, but an additional measure that also has value in provid­ing a backing to a continuous mastic seal internally.


 

9.4 Index

(a) Airtight Performance Specification for Curtain Walling

The de facto standard for curtain walling air permeability that most curtain walling manufacturers comply with is the CWCT (Centre for window and cladding technology) ‘Standard and Guide to Good Practice for Curtain Walling’. This specifies a maximum air permeability of 1.5 m3 / hour/m2 @600pascals for an area of fixed glazing, and 2m3/hr/linear metre of joint for opening panels. This is the same as the British Standard BS EN 12152:2002, category A4. However, the BS has a further category, AE that achieves 1.5m3 /hour /m2 at a pressure differential of more than 600pascals. Specification of this ‘exceptional category may be possible but it may mean a reduction in choice as this is a more stringent level of air testing . The rule is: If wind load up to 2400kn then curtain walling to be tested to 600 pascals. If wind load greater than 2400kn then test to wind load/4, e.g if 4000kn, test curtain wall to 1000 pascals.

Maximising airtightness can be done by having vulcanised welded joints to gaskets within the curtain wall frame, instead of usual mitred ones. This should ensure that the unit its self is airtight, although it is an expensive option, it will help you to pass the air test

 

(b) Mastic Bedded Fixings

Where membranes and components are connected, it is often possible for thin - and often more or less invisible gaps to be left between the joint. A continuous mastic seal used along the line of any such mechanical fixing ensures that any minor cracks like this are completely sealed.

 

(c) Additional Membrane Seal at Junction

Some Manufacturers (eg Schuco) supply as part of their system an EPDM perimeter gasket seal that should be tied into vertical DPM. Angle at jambs and loose dpm to wrap ensure good seal with EPDM. This is a particularly good way to ensure airtightness and the chances of a air test pass. At these critical junctions because it requires a conscious task (sealing the membrane) to ensure all ‘loose ends’ are firmly fixed, as opposed to leaving the airtightness to be achieved through the use of applied sealants.

 

(d) Foam Filler to Internal Joint

Assuming that the seal mentioned above is installed correctly this should not be required, but such a seal acts as an additional check against air leakage uk and could be used as a backing strip against which to seal a continuous mastic seal internally.

 

(e) Membranes to be Lapped and Sealed

Best practice in this regard - beyond the correct use of Manufacturers’ overlap dimensions, proprietary tapes and other accessories - is to run a layer of double sided tape between the membranes at the overlap and run a tape over the leading edge of the outer sheet. In addition, since many tapes tend not to last too well, it is advisable to ensure that laps are made directly over supported areas (i.e. with solid materials directly behind) and are held down positively with battens fixed through, or some other ‘positive’ connection forming a mechanically tight, as well as an adhesive seal. This may require consideration of lap positions early on.

 

(f) T&G Jointed and Taped Rigid Insulation

Butt jointed insulation, even if installed firmly may be subject to movement during the course of con­struction and over time, and is unlikely to offer a continuous insulation layer in the long term. Using t&g slabs overcomes some of this problem and taping the slabs ensures that air leakage paths cannot form between the minute, but inevitable cracks between the units, which can later lead to air test failure

 

(g) Additional Airtight Membrane

It is likely that the fireproof stopping will not be able to create an adequately airtight seal and so this measure ensures the task is performed consciously. Using a simple polythene membrane and forming positive connections to the underside of the slab and the top of the curtain walling ensures an airtight seal between floors, and at the vulnerable connection of curtain walling to spandrel panels. One of the main causes of air test failures


 

 (h) Localised Welding of Plates of Beam

It is practically very difficult to form an airtight seal perpendicular to an ‘I’ beam or similar, expanding foam tends to be used because no ‘built’ connection appears workable, nor cost effective. Such ad hoc seals are unlikely to last in the long term.

Ideally plates should be welded to the beam such that there is no air route along the length of the beam (a plate welded perpendicular to the web and extending between the two flanges) and such that airtight seals are easily formed around the beam as it passes the airtight layer. Side plates fixed between flanges form a sort of localised rectangular section which is more easily sealed. This makes the task more readily achieved on site, and more durable in the long term


 

9.5 Refurbishment of Masonry Building

Discussion

If the existing masonry fabric of a refurbished building is in good condition, it is potentially simple to render it relatively airtight if the details proposed - particularly the use of service voids - are followed. All the work can be carried out internally and is simple to install and check and will drastically improve the chances of a air test pass

In addition there is no cavity in this form of construction and this means there are fewer opportunities for undetected airways.

 

It goes without saying that any cracks or damage to the existing fabric should be made good before installation of the internal frame, otherwise this may help lead to a air test failure

If there is enough space, it might be best to retain all existing lath and plaster on ceilings and walls, ensure that it is effectively sealed, and work inwards from there. Experience suggests that lath and plaster itself is fairly airtight and removing it merely creates more waste. One potential disadvantage is that in keeping the existing plaster, it may not be possible to access the gaps behind which may run into floor voids and partitions creating air leakage uk paths throughout the building.

 

A number of reviewers of this Guide commented that it is more common to maintain a cavity between the existing wall and any new-build internal leaf. The alternative proposed keeps to the same format as the original, but the advantages of the use of a cavity are well understood.

 

HIGH PRIORITY

(a) Membranes Lapped & Sealed

With only one membrane to ensure airtight­ness it is crucial that laps and junctions are conscientiously sealed.

 

(b) Service Void

Use of a service void means most if not all penetrations through the vapour control and airtight layer can be avoided.

 

(c) Joinery Edge Sealing Batten

If the membrane generally is well sealed, the only other major area for air infiltration uk is the openings and the gap between the frame and masonry. If the windows can be effectively sealed by (medium priority e) then this measure is not necessary.

 

(d) Joinery Draughtstripping

A particular issue with sash and case windows. It is important that seals can be easily accessed for maintenance and replacement.

 

MEDIUM PRIORITY

(e) Flexible Foam around Joinery

Gaps around openings are common and neat, effective solutions can be difficult, careful use of flexible foam enables effec­tive and durable seals to be formed. If this can be effectively achieved with the sash and case window then (f) is not necessary.

 

(f) Continuity at Openings

Continuity between the framing sealant (m) and the membrane can be tricky and care is needed to ensure a good, durable seal.

 

(g) Backing Boards

Use of backing boards makes installation of the membrane easy and thus less prone to poor workmanship and subsequent failure.


 

Costs

The retention of the ceiling lath and plaster saves approximately 24% of the costs of that element, while the addition of the service void and vapour check represents a 18% cost increase, thus, without the addition of the breather membrane over the ceiling joists (a medium priority measure) there is a cost saving to complement the increase in ease and cost of services installation.

The breather membrane represents a 13% increase in cost and therefore tips the balance of the ceiling cost overall. However this is still cheaper than repercussions due to failing your air test

 

The addition of the OSB backing board and service void to the walls constitutes around a 46% increase in cost of the wall, the majority of which (33%) is made up by the OSB, so perhaps a cheaper, yet firm back­ing board might alleviate the cost burden. The addition of the OSB layer to form a service void beneath the floor boards would add approximately one third to the cost of the original detail. Again this is still cheaper than repercussions due to failing your air test

 

 

Double mastic sealing of the skirting boards adds approximately 50% to their installation cost, although their overall costs are small in the overall picture.The sealing of the vapour control layer above and below the intermediate floor should not attract any additional cost if assumed to be part of a standard, if careful installation. Measures to help seal around the window would add marginally to a standard installation cost.

 

MEDIUM PRIORITY

(a) Batten Seal at Corners

A version of (b) but worth particular men­tion as these junctions are particularly important to seal well.

 

(b) Keep Existing Lath and Plaster

Really a version of (e) except in this case we suggest retaining the existing firm base of lath and plaster against which to affix the vapour control layer.

 

(c) Plasterboard Penetrations

If the airtight layers are sound then this should not matter, but still worth attention.

 

(d) Membrane Over Roof Insulation

Protects installed insulation from disrup­tion and provides a secondary layer at this important area.

 

LOW PRIORITY

(a) Silicone to Joinery Externally

Should be standard practice, but forms useful role in airtightness as well as weath­erproofing.

 

(b) Mastic to Skirtings, Linings, Cornices

Not necessary if the airtight layer is sound but worth attention in these examples.

 


9.6 Index

(a) Breather Membrane over Insulation 

In well ventilated loft areas, loose insulation may become dislodged by air movement. This precau­tionary measure ensures that the initial fully fitting installation of batts against joists etc is maintained over time, reduces dirt and debris entering and provides an additional airtight layer (which is useful since the loft is ventilated) whilst allowing for vapour egress into the ventilated space.

 

(b) Membranes to be Lapped and Sealed

In order to create airtight layers, it is important that laps are rigorously sealed. Best practice in this regard - beyond the correct use of Manufacturers’ overlap dimensions, proprietary tapes and other accessories - is to run a layer of double sided tape between the membranes at the overlap and run a tape over the leading edge of the outer sheet. In addition, since many tapes tend not to last too well, it is advisable to ensure that laps are made directly over supported areas (i.e. with studs or dwangs directly behind) and are held down positively with battens fixed through forming a mechanically tight, as well as an adhesive seal. This requires consideration of lap positions early on if extra framing or subsequent battening is needed.

 

(c) Vapour Control Layer over Existing Lath and Plaster 

Rather than remove the existing lath and plaster ceiling, this detail saves a little money, time and resources by reusing the existing ceiling as a backing to the installation of the vapour control layer (refer also (e)) Plaster need not be repaired if damage is localised and does not threaten the integrity of the vapour control layer.

 

(d) Service Void 

The principal advantage of a service void is related to functionality and maintenance of services over time, but a secondary advantage which relates directly to airtightness is that since all services may be incorporated within, that is, on the inside of the vapour control layer, there is no need to penetrate the layer at each and every service installation, thus significantly cutting down on the myriad potential gaps that are typically formed and either left, or made good which is time consuming and costly, and could lead to a air test failure

 

(e) Fix Airtight Membranes to Firm Backing Boards 

In many situations membranes required for vapour control and airtightness are installed unsupported and are thus susceptible to pressures from both sides, leading to the membrane breaking free of its fixing and creating holes in the airtight layer. Ideally, membranes should be fixed against a firm back­ing board by way of protection against damage of this nature.

 

(f) Mastic Both Edges to Skirtings, Reveal Linings etc. 

Where the corner junction behind has been carefully sealed then this measure may not be required, In the examples shown on this construction, this particular detail is not critical but is nonetheless valu­able in helping to ensure a good seal at all points.

 

(g) Airtight Layer Taken Behind Batten at Corners 

As noted in (b) above, the best airtight seal is a positive and mechanical one such as shown here whereby at corners and edges, a membrane is not only lapped and taped against the adjoining sur­face, but held firm by a batten fixed through. This overcomes any potential adhesive failures or tears in staples or tacks etc. In the ceiling junction where the plasterboard layer must be continuous for reasons of fire spread prevention, it is also advisable to install laying tape at the junction between the plasterboard and the wall to ensure an airtight seal here also, thus improving the chances of a air test pass

 

(h) Seal all Penetrations in Plasterboard / Internal Lining

Even with the use of airtight outlet boxes there will be inevitable penetrations such as ceiling pen­dants, pull cords, recessed fittings etc. which must be made good manually, typically with mastic, and in this case, with a suitably fireproof mastic to maintain the fire barrier.


 

(i) Ensure Membrane is taken into Opening Reveals, Taped and Sealed and Made Continuous with Opening Seals, it is typical at openings to allow the membrane to run across the opening initially, then form a star cut into the opening, folding over the sections of membrane and trimming as necessary. In these cases, there are inevitable gaps in the airtight layer at the corners of the opening, and it is important to ensure that these are made good before subsequent installation of joinery etc.

 

(j) Sealing Batten 

This detail may be considered as an alternative, or ideally as an additional measure with (k). Since it is possible that replacement sash and case windows cannot be easily sealed around their perimeter (they are often ‘open’ around the outer edge) it may be necessary to use this detail which creates the airtight seal on the inside of the frame rather than ‘in line’ with the frame as noted below.

 

(k) Flexible Foam Sealant around Joinery Insertions

Gaps around openings are one of the most common of air infiltration uk paths. They range from 0 to 20mm, which is too large to be filled by mastic. Compressible flexible foams are ideal for this application. En­sure that the airtight membrane meets the seal on both sides to maintain the airtight layer overall and imrove the chances of a air test pass.

 

(l) Draughtstripping of Openings in Joinery 

Draughtstripping of joinery comes in many forms. It appears that synthetic rubber or elastomeric tubular seals work well, creating good seals with minimal compression. It is important that seals are unaffected by paintwork and subsequent decoration, or are easily accessible and removable. This is important so that seals can be replaced as they start to fail to maintain the airtight layer. Brush seals are likely to be used in sash and case windows.

 

(m) Silicone Sealant to External Window Surround 

Some form of neat and potentially paintable edge seal will be required externally

.

(n) Breather Membrane Instead of Netlon 

Notwithstanding the air barrier placed above, mineral wool is permeable to air movement and so replacing the netlon with a vapour permeable but airtight breather membrane reduces air movement in the insulation, improving insulation levels and reducing the risk of air leakage uk and air test failures generally.


 

9.7 Concrete Frame and

Panel

Discussion

Concrete panel construction represents a potentially good airtight form of construction. This is because the panels themselves are essentially airtight and being large, have fewer gaps which must be sealed. Being fairly predictable in terms of thermal and structural movement they are easy to seal well, and the only areas of concern then are the service penetrations and junctions with openings. With care and attention in these areas, a very good overall airtight external envelope is easily within reach. Having said that, in some early examples of this build­ing type, the sealants between panels have failed, highlighting the vulnerability of the system to such air test failure and the importance of correct specification and application.

 

A number of systems are available but the principles outlined for the improvement of the system chosen are widely applicable. Where two leafs of concrete panel are used, it is unlikely that the outer layer will be used as a rain ­screen layer, but this is sometimes done, and in these cases the airtightness of the internal layer of panels becomes critical, and may be augmented by the ap­plication of a vapour control and airtight membrane on the inner face of the insulation, applied to the panels before the insulation is installed. Guidance on the application of this membrane, and on poten­tially more airtight forms of insulation may be found

In Sweden, concrete panels are sometimes sealed to each other using polyurethane foam which is claimed to increase the airtightness levels and subsequent air test passes , but there does not appear to be any evidence of this form of sealant in the UK.

 

HIGH PRIORITY

(a) Integral Beam and Internal Panel

Important because this reduces the number of joints and simplifies construction.

 

(b) Sealing of all Penetrations

Care and attention to detail at all services and other penetrations is vital, most pres­sure tested panel buildings suffer air leakage uk at these locations, if these are not sealed it will result in a air test failure

 

(d) Sealing around Windows

The other major source of air leakage uk in con­crete panel buildings apart from (b) above, care and attention to detail along all joints needed.

 

(c) Screed Edge Strip and Seal

Ensures that air does not leak between floors around the perimeter and at other floor penetrations and breaks in the screed, another major cause of air test failure

 

MEDIUM PRIORITY

(a) Accessible Draughtstripping

It is important that the draughtstripping is accessible since it is likely that it will not last as long as the windows themselves and require replacement.

 

(b) Membrane around Windows

Required for vapour and air leakage con­trol, this also required attention and inspec­tion and can be seen as complementary to the mastic / silicon sealants

 

(c) Double Silicon Seal to External Panels

Double silicon sealant lines in the external panels is normally standard practice, and is typically good enough to ensure that the outer panels provide an effective airtight seal throughout, and help towards passing your air test

 


Costs

The alternative specification highlights best practice installation and should not incur any additional costs. The design of the panel construction system itself would dictate any cost difference.

 

LOW PRIORITY

(h) Membrane Under Roof Insulation

May not be required if the screed below is fully sealed against vapour and air flow, but given the typical number of penetrations in a commercial roof screed, the addition of a dedicated membrane may be considered advisable



 

10   Useful Names

Air barrier
An air barrier comprises materials and/or components, which are air impervious or virtually so, separating conditioned spaces (heated), from unconditioned spaces (unheated) e.g. the outer envelope of the building.

Air change rate
The rate at which outside air enters a space divided by the volume of that space. This is expressed as ach (air changes per hour).

Air curtain
A stream of high velocity, temperature-controlled air which is directed across an opening. It enables control of conditions in a space, which has an open entrance.

Air exfiltration
The uncontrolled outward leakage of indoor air through cracks, discontinuities and other unintentional openings in the building envelope.

Air infiltration
The uncontrolled inward leakage of outdoor air through cracks, discontinuities and other unintentional openings in the building envelope.

Air leakage audit
The inspection of materials and components, between conditioned and unconditioned spaces to try to establish where major discontinuities in an air barrier system might exist.

Air leakage index
The leakage of air (m3.h-1) in or out of a building space, per unit area (m2) of envelope (excluding ground floor area ,except for non-ground supported lower floors) at a reference pressure of 50 Pa between inside and outside the building.

Air permeability
The leakage of air (m3.h-1) in or out of a building space, per unit area (m2) of envelope (including ground floor area) at a reference pressure of 50 Pa between inside and outside the building.

Air leakage rate
The leakage of air (m3.h-1) in or out of a building space, per unit volume (m3) at a reference pressure of 50 Pa between inside and outside the building.

Air leakage path
A route by which air enters or leaves a building or flows through a component.

Airtightness
A term describing the leakiness of a building.
The smaller the leakage for a given pressure difference across a building, the tighter the building envelope.

Airtightness layer
A layer built in to the external envelope to minimise air infiltration/exfiltration. It may consist of a wide range of materials (for example,sealants, gaskets, glazing or membranes) and should be continuous to be effective.

Breather membrane
A water-resistant sheet which allows transmission of water vapour, but which provides resistance to airflow.

Conditioned zone
The occupied zone in a building requiring heating or cooling and normally bounded by an airtightness layer.

Draught
Excessive air movement within the conditioned zone, which may cause discomfort.

Draughtproofing
Filling gaps between opening parts of components and their frames.

Envelope area
The boundary or barrier (m2) separating the interior volume of the building from the outside environment. This includes the area of the external walls, roof and depending upon the air leakage parameter specified the area of the ground supported floor.

Fan pressurisation test/Air Leakage test/Air Tightness Test
A method of testing air leakage of a building. It allows airflow and pressure difference across the envelope to be measured and an estimate of leakage to be obtained.

Infiltration rate
The rate at which outside air infiltrates a building or a room under natural meteorological conditions (normally expressed in air changes per hour or litres per second)

Infrared camera
A camera sensitive to the infrared part of the spectrum, which can be used to ‘see’ locally cooled areas on the internal surfaces or heated areas on internal and external surfaces of the envelope of a building.

Minimum ventilation requirement
The minimum quantity of outdoor or conditioned air which must enter a building to maintain an acceptable indoor air environment for occupants.

Natural ventilation
The movement (caused by wind and outside temperature) of outdoor air into a room or space through intentionally provided openings, such as windows and doors and non-powered ventilators.

Smoke test
A building (or parts of it) is filled with smoke using smoke machines and then pressurised to force the smoke through gaps in the building envelope.

Smoke tube/pencil
A hand held device which produces smoke in small quantities for more specific identification of leakage paths within a building under pressurisation or depressurisation, or under natural infiltration.

Stack effect
Air movement through a building caused by differences in the density of air due to temperature differences between the air inside and outside of the building.

Thermography
The use of thermographic cameras sensitive to infrared radiation to identify thermal weak spots in the envelope of the building and to help identify air leakage paths through gaps and cracks in the building.

Vapour control layer
A layer impervious to water vapour and usually enclosing an occupied space.

Ventilation
Supplying or removing air, by natural or mechanical means, to or from a space.