Residential: impact of airtightness on indoor environments

Sealing the envelope

11 November 2015

Chris Scivyer considers the impact of airtightness on indoor environments

Over the past 20 years, there has been a steady drive to make new buildings more airtight and reduce the leakiness of existing structures. The principal motive behind this has always been to improve energy efficiency. By reducing heat loss and cold-air ingress through leaky construction, a building will require less heating, which results in lower energy use and significant savings. This might be considered to be good airtightness, where the occupier sees reduced heating bills coupled with improved indoor comfort levels.

To encourage improvements, airtightness guidance was introduced into the Building Regulations within Approved Document L (AD L), together with air permeability testing to demonstrate compliance. The test results are used in the AD L energy assessment calculations, which consider a building’s thermal insulation and energy use.

It is fair to say that the requirement to test buildings for air permeability has focused minds across the construction industry. As a consequence, we have seen improvements in design and coordination across different trades and in the quality of workmanship on site. To pass the test, it is no longer possible to adopt an ‘out of sight, out of mind’ approach towards gaps in construction. Following trades must not damage finished works and must identify potential leakage paths before hiding them behind non-airtight finishes.

The air permeability test itself is relatively straightforward and usually undertaken when the external envelope is complete, immediately prior to handover. For a dwelling, it involves connecting a fan to a suitable aperture in the building envelope, usually the front door opening (see Figure 1) and subjecting it to a range of pressures.

The fan speed is increased in steps up to a maximum and then decreased in the same manner. Air-volume flow rate through the fan (equal to the air leaking through the building envelope) and the pressure difference across the building envelope are recorded at each fan speed. In calculating air permeability, corrections are made for temperature and barometric pressure.

If the property does not achieve the airtightness level required, an air leakage audit is carried out. This usually involves running the fan so that the property is under pressure and using a smoke pencil to locate leakage paths. Once the leaks are sealed, the property can be retested.

Improved airtightness does not necessarily mean increased costs for construction. Initially, much of the improvement was achieved by getting contractors to build correctly what was already being specified. As the targets become tighter, designers have had to focus more on identifying the line construction that forms the air barrier within the building. This is often marked on drawings as a red line.

The principal aim is to separate conditioned (heated or chilled) spaces from unconditioned spaces. For a dwelling, this usually means separating indoors from outdoors so that the external envelope forms the line of the air barrier. This would comprise the ground floor, external walls and the ceiling to the top floor (assuming the roof space is not occupied).

Where the construction is made up of several layers, as with an external wall, consideration needs to be given to where the air barrier lies. There are two aspects to this: the first considers which material offers the best level of airtightness while being cost effective to install, and the second involves the thermal implications for the building.

The latter is often overlooked, resulting in comfort problems and reduced energy efficiency. An example of this is where leaky masonry construction is finished with internal dry-lining on dabs and the dry-lining is considered to be the line of airtightness. In winter, cold outdoor air can pass through the leaky masonry and any insulation to reach the back of the dry-lining, resulting in the adjacent room feeling cold.

Growing evidence suggests overheating is occurring, with serious health implications in modern UK dwellings

Having identified the material that is to form the air barrier, it is important to minimise any penetrations through the barrier. This includes openings for doors and windows, and service openings for ventilation, pipes and cables. Where these cannot be avoided, the barrier needs to be sealed back to the penetrating construction. A variety of sleeves, tapes and sealants have been developed to simplify this process.

Seals must be robust, because airtightness has to last for the life of the building, not just long enough to pass the test. Although it is not common today, inthe future, building retesting a year or so after construction might be introduced to check airtightness performance as part of the regular maintenance of energy efficient mechanical ventilation and heat recovery (MVHR) systems.


In addition to airtightness and thermal insulation, the level of air permeability also impacts on other indoor environment issues, such as ventilation. From the outset, BRE was acutely aware that airtightness must be considered alongside building ventilation. The phrase ‘build tight, ventilate right’ was coined to emphasise this. A building cannot be too airtight but it can be under-ventilated. Continuing improvements over time mean that designers need to consider the implications of very airtight buildings, which might be called ‘bad airtightness’.

In some cases, buildings are more airtight than they really need to be. This can arise from designers failing to realise the capability of different construction systems and materials, but can happen where contractors will incur a financial penalty if the target is not achieved. To protect themselves, the contractor applies an even tighter figure than specified. Quite often, they achieve the revised target, and the contractor is not penalised but the building is now tighter than the design team intended.

Over the past few years, BRE has noticed an increase in complaints about indoor air quality including smells, mould and condensation in homes built or refurbished to higher levels of airtightness without improving the ventilation provision. Often ventilation and heating has been correctly specified and installed, but no advice is given to the occupiers; in rented properties, tenants often switch off mechanical ventilation systems to reduce energy bills. MVHR systems can resolve the conflict of balancing airtightness with ventilation, as long as they are correctly specified.

Overheating/heat rejection

These issues were covered by my colleague, Dr Michael Swainson in Property journal September/October 2014 (see p32, Comfort zones). Overheating may be defined as the temperature within a building that is too high for comfort. There is a growing body of evidence that suggests overheating is occurring in a range of modern UK dwellings and the health implications may be serious for vulnerable groups in society, such as the elderly.

The key to minimising it is to understand and reduce the excessive heat to a building. This can be from sources such as external solar gains and outside air temperature from the urban heat island effect, micro-climate and climate change, and internal gains from occupants and their activities, electric appliances and domestic hot water services.

Heat rejection becomes an issue in naturally ventilated buildings; thermal mass has been introduced to attenuate the peaks in temperature, because the heat must be rejected each night, and across the hours of peak air temperature outside.

The issue is more acute in dwellings, because they are occupied overnight, when the cool outside air could be used to remove heat that has built up the preceding day. However, it is the ability to effectively reject heat at night while the building is occupied that is now largely missing from many modern UK dwellings. The problem has increased in recent times with requirements for enhanced airtightness, while not fully considering ventilation needs.

Chris Scivyer is Principal Consultant at Building Technology Group, BRE

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