This glossary has been developed to be as consistent as possible with those defined in Technical Note AIVC 36 Air Infiltration and Ventilation Glossary

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 CO₂emissions 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

• 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 rooflights. This is the experience of countries where envelope airtightness generally is more developed. As a result they achive far better first time air leakage/air tightness results

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 be tested 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 for the overall building.

7.8 Testing and Audit Schedule

In many cases to date, an air leakage 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 leakage test/air tightness test at a time where remedial works are relatively simple to perform. On the other hand, it is important that a Air leakage 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.

Ideally therefore, two Air leakage tests/air tightness tests at least should be carried out. The first Air leakage test/air tightness test 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 leakage test/air tightness test 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 leakage test/air tightness test serves simply to confirm the performance of the building, hopefully at a slightly improved level from the first Air leakage test/air tightness test without the need for costly and complex operations late in the day.

Such a air tightness/air leakage test schedule is nonetheless costly in itself, but for those who have been involved in such testing 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. With a sufficiently good first Air leakage test/air tightness test performance, it may even be possible to dispense with the final air tightness 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 air leakage/air tightness testing. 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 leakage test/air tightness test for a week and carry it out when the envelope is complete and ‘as intended’.

On larger projects, more air leakage/air tightness tests 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 airtightness levels early on.

7.9 Remedial Airtightness Works

With airtightness testing and a general awareness of airtightness 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, 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 tightness/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. 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.

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, 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 tightness test first time

8. Testing Airtightness

Key Principles

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

2. A pressure test/ 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. Pressure tests/ air tightness tests are typically followed by an 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.

8.1 Climatic conditions

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

8.2 The Test itself

Essentially the process is one of 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 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, this will be easily recognisable within the first couple of minutes during the air leakage test / air tightness test

Buildings are 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, air tightness testing/air leakage testing is ideally undertaken out with 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 tightness test.

In existing buildings, 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 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 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 leakage, this is a good way of lowering the risk of a air leakage test / air tightness test 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. 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.

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 tightness testing / air leakage testing may be undertaken at the same time as ‘standard’ ventilation system commissioning and associated studies

8.4 Component Testing

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 pressure 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 pressure 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. 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.

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.

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, 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.

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.

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 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. 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.

b) Blockwork Maximum Air Permeability by Component Test

An alternative to wet plastering the blockwork on the inner leaf is to require a component 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.

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

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 amd improve air tightness in and out of the

building. 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. 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 paths

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) 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

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

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

completely airtight seal at this particularly vulnerable point.

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

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.

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 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

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 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.

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

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 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

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 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), 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. Copper is non-corrosive but can affect polyethylene, whereas stainless steel has no effect on polyethlene and so should be preferred.

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. 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 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 tests is a huge advantage

(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.

(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.

(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 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.

(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 aids efforts to guard against infiltration/air leakage, 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.

(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.

(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.

(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

(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.

(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.

(t) Top Runner Strip Seal

The use of this strip, lapped and sealed with subsequent membranes both sides prevents infiltration into the wall itself from the ventilated eaves area, thus ensuring continuity of the airtight layer.

(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.

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 carefuly considered in detail each occurrence and made adequate provision, to avoid large amounts of ad hoc remedial work.

The roof membrane must be carefully sealed and the perimeter condition considered so that a continuous and positive connection can be made.

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.

(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.

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 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 and thermal insulation related risks.

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.