Air tightness testing was introduced into the England and Wales Building Regulations for buildings other than dwellings in 2002, but became a requirement for all new buildings in April last year.

For timber frame dwellings, it could be said the current requirements are relatively straightforward to meet, provided design and workmanship are reasonable. The regulations state that at a pressure difference of 50 Pascals between inside and outside, every square metre of the thermal envelope (external wall, top ceiling and floor), must not leak more than 10m3 of air in an hour. When you think about how many square metres of external envelope there are in a typical house, that’s a lot of air!

As the construction industry becomes more familiar with air tightness detailing, regulations are likely to get tougher. This time the regulations really are just a “starter for 10”. However, do not sit back and relax – spectacular failures are possible. Testing has to be done when the building is complete, which will mean costly and time-consuming remedial measures if the building fails.

Raising the internal air pressure of the building to a constant rate of 50 Pascals, while measuring the speed at which a large fan needs to work to maintain it, allows comparison of one building against another. Natural air pressure is constantly fluctuating with local conditions such as wind speed and temperature, so a 50 Pascals pressure is used to overcome small fluctuations, although anything more than a breeze can make taking measurements impossible. This method is quick, inexpensive, relatively weather independent and can give instant results, whereas measuring natural infiltration of air is none of these.

So why this new legislation now? Global warming, of course – heating air and allowing it to escape from buildings typically uses fossil fuels, produces carbon dioxide and costs us money. If we stop it escaping so fast, the world will thank us. Currently in the UK, air infiltration has a greater role in air movement than planned ventilation. An ideal house would be ventilated by design. In winter, the air warmed by the heating systems flows up and out of cracks and crevices all around the house. It takes with it moisture, which it may deposit in the fabric of the building, reducing the effectiveness of insulation and possibly causing damage. As it is warm air moving away from us, we don’t feel it and we can’t see it - but we do feel the cold air coming in around other gaps to replace it. This cold air needs heating, and so the cycle goes on.

Timber frame has a strong and successful air tightness history. It has proved its worth and we can learn much from current practice in Canada and Scandinavia. The Canadian R2000 timber frame house system prides itself on achieving just one and a half air changes per hour. Canada and Scandinavia both suffer extreme winter conditions and their choice of building systems must therefore address the issue of keeping out a howling gale of minus 30 degrees, while maintaining a comfortable environment inside, without using vast quantities of carbon-based fuel. Those countries which have championed the benefits of air tight buildings for many years also happen to be the countries which have championed timber frame buildings for many years. This is no coincidence.

For developments of more than two dwellings, each different dwelling type must be tested for air tightness to comply with Part L. To meet Approved Document L1A’s CO2 emission limits, or a client’s particular requirements, a designer may specify a building to achieve better air tightness performance than the minimum requirement of 10m3/m2. It is then that particular care must be taken at all stages. The Approved Document details the number of buildings to be tested, but this number can be reduced by adopting ‘Accredited Details’, (a recently published document to be found at www.planningportal.gov.uk/england/professionals/en/1115314255961.html). It provides annotated drawings with the aim of reducing thermal bridging and air leakage.

When a building is at concept stage, consideration should be given to air tightness. A complicated building shape with multiple levels, many corners, lots of external wall and awkward junctions will require much greater care to achieve air tightness than a rectangular one with few openings. Integral garages with rooms over them create a suspended floor at high level, again complicating and increasing junctions, with potential for gaps.

Consideration should be given to air tightness at concept stage

As architectural details progress, the location of ducts should be considered, too. These have the potential to act as chimneys, often from rooms such as kitchens/bathrooms, linking through floor cavities, drawing warm damp air into the roof space. Reducing the number of ducts and junctions reduces potential air leakage routes. Electrical sockets and pipes in the external wall and top floor ceiling have the potential to draw air in around them, unless the air barrier is fully sealed around them. If services are designed to be located on internal walls, the potential for problems is eliminated, except for top floor ceiling penetrations. Air leakage at floor joist junctions with external walls should also be considered. This is a well-known route for air in many forms of construction and is extremely difficult to access after a test failure. Whether floor cassettes are made in a factory, or the joists and deck are assembled on site, care must be taken that joists and blocking are assembled tightly with mastic seals at floor/wall junctions and gaps between components closed.

Accredited Details for Part L indicate using joists and blocking as part of the air barrier line between ground and first floor external walls. It is not yet usual practice in the UK to lap a vapour permeable air barrier with the ground floor wall vapour control layer, extend it out into the external wall cavity at ceiling level, up the edge of the floor in the external wall cavity and then back inside, to lap with the first floor walls at skirting level; but we may be doing this in the future, since the costs are minimal and the air tightness test results are much improved.

Timber Frame: Standard details for house and flats illustrates important design details for making buildings air tight

Typically, the junction where internal walls meet external walls sees the vapour control polythene installed only to the external wall after all the timber frame has been erected. But by installing a vertical strip of polythene into this junction, at the time when the internal walls are erected, it is then possible, at the stage when insulation and vapour control are installed, to lap polythene to polythene, creating a continuous air barrier to the external wall. This is a detail shown in TRADA’s ‘Timber frame: Standard details for houses and flats’, but not yet a requirement for Accredited Details compliance, which relies on the internal wall end stud to contribute to air tightness. Built-in furniture such as services under a kitchen sink or behind a bath panel can be hard to access, making installation difficult and achieving air tightness around service penetrations even more so. But hidden from view does not mean hidden from an air tightness test and these are common places for large air leaks and test failures.

Duplex plasterboard with an integrated vapour control layer eliminates the need for a polythene vapour control layer, but creates many junctions. Services and openings in the external wall envelope also need to be considered. Beads of mastic applied into junctions improve air tightness; proprietary products may also be needed for sealing service penetrations.

The good news is that constructing a building to meet or exceed air tightness test requirements needs relatively low cost materials and just a bit more care. By addressing design issues, briefing your workforce and supervising work in progress you can deliver a better quality, more energy efficient and comfortable building.