Air Tightness: A general overview to testing and design

1.0 Key Principles

1. Testing procedure is set out in CIBSE TM 23 and the ATTMA TS1

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

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

 

2.0 Climatic conditions

As mentioned in Chapter 1, the raised pressure differential of 50 Pascals created during an airtightness 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 leakage tests/air tightness tests 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 leakage tests/air tightness tests.

 

3.0 The Test itself

Guidance on testing buildings for Air leakage /air tightness 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 tightness/air leakage 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 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.

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.
Buildings are air tightness 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 leakage test/air tightness test

If the building is under construction, Air tightness testing/air leakage testing 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 throughtout the air leakage/air tightness test

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

 

4.0 Air Leakage Audits

The air leakage 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 leakage/air tightness tests 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 air leakage.

However, most air leakage routes are difficult or impossible to spot without visual aids during the air leakage /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. The building may be positively pressurised during the air leakage/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.

Another technique, which has certain advantages and disadvantages compared to smoke tracing along with an air leakage/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, airtightness testing/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 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 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 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 tigtness 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.

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 tightness/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 openable 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.

The additional costs of creating an ad­equately airtight building can be negligible, but even where costs are increased, 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. 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.

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 on site 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 leakage 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

 

   • 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

 

• .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 airtightness. 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 leakage of air 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. 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. 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 tightness/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 because of unwanted air infiltration (air leakage) generate huge costs to owners and occupants, in envi­ronmental, financial and health terms.

 

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, 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, 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.

 

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.

 

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. 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 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. Complaints by occupants in leaky buildings are common, and remedial measures are usually difficult and expensive.

 

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 be tested 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 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.

 

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 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 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 testing and quoting air leakage figures at 50 Pascals, inaccuracies are reduced and repeatability is improved.

 

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 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 SEDA 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. 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 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 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.

 

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.

 

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.

 

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.

 

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, 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.

 

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.

 

It is worth making a point of considering each and every specified component with regard not only to its own intrinsic airtightness 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.

 

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.

 

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.

 

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. Source: C. Morgan

 

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 paths behind the boards and into floor, partition wall and ceiling cavities. From the perspective of airtightness, drylining should be avoided unless great care is taken. See right.

 

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. Hollow planks however can leak into cavities and require to be sealed at their ends, this will dramatically improve air tightness

 

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 leakage 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.

 

Design & Detailing for Airtightness - Implementing Airtightness

In addition to the intrinsic lack of airtight­ness, a problem of drylining is that it can create hidden pathways for air, 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 eitherhollow planks are often left ungrouted where they meet the external wall, which could lead to extensive air leakage internally.

 

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 leakage/air tightness failure will surely follow

 

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

 

3.       Ideally at least 2 air leakage tests (air tightness tests) will be undertaken; the first when the building is weather tight, and the second air tightness 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.

 

5.   Remedial air tightness works to existing properties can reap substantial benefits without undue disruption.

 

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.

 

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 leakage/air tightness testing failures.

 

Ideally too, the Designer will understand the issues sufficient to prepare a sound performance specification – giving achievable targets for 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 tightness testing 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.

 

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. 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 air tightness / air leakage testing issues.

 

Contractor

The Main Contractor’s principal responsibility is to deliver the air ­tightness 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. It is always prudent to place someone in the site team who has experience of air leakage/air tightness on their previous projects as their experience  may avert a potential air leakage/air tightness 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 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 per­formance has been achieved by Contractors who employ a dedi­cated individual (or team) to carry responsibility for airtightness/air leakage, to inspect the works and instruct as required.

 

For Contractors, the issues of airtightness/air leakage 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 tightness testing at the first attempt

 

7.7   Inspection

Air leakage 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 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.’

Of the background air leakage subsequently investigated, the principal air leakage routes 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