Introduction to obtaining air tightness in house extensions
Home energy use is responsible for 28 per cent of UK carbon dioxide emissions which
contribute to climate change. By following the Energy Saving Trust’s best practice standards, new build and
existing housing will be more energy efficient and will reduce these emissions, saving energy, money and the
environment. Air leakage from buildings, both new build and existing, is a major cause of energy loss and
Improving airtightness in dwellings will reduce air leakage – the uncontrolled flow of air through gaps and
cracks in the fabric of dwellings (sometimes referred to as infiltration, exfiltration or draughts). This is not to
be confused with ventilation, the controlled flow of air into and out of the dwelling through purpose-built
ventilators that is required for the comfort and safety of the occupants. Too much air leakage leads to unnecessary
heat loss and discomfort from cold draughts. With more stringent building regulations requiring better energy
efficiency, airtightness is an increasingly important issue. The aim should be to ‘build tight – ventilate right’.
Buildings cannot be too airtight; it is, however, essential to ensure appropriate ventilation.
Air leakage is quantified as air permeability. This is the rate of leakage (m3/h/m2) in or out of the dwelling.
It is measured at a reference pressure difference of 50Pa between the inside and outside of the dwelling. In the UK
the good and best practice standards for air permeability in dwellings are shown in Table 1.
However, in a recent study of 100 new dwellings of all types in England and Wales, none achieved the best
practice performance of 3m3/h/m2. The research, undertaken by the Energy Efficiency Partnership for Homes (EEPfH),
showed that around a third even failed to achieve the worst acceptable permeability (10m3/h/m2) expected in the new
building regulations for England and Wales and Northern Ireland. Less than 20 per cent of these dwellings tested
met the good practice standard of 7m3/h/m2. More information on the results of this study can be found on the
To ensure airtightness, the issue needs to be addressed at design stage. At an early stage the designer should
identify a line through the envelope of the dwelling where the barrier to air leakage will be: this is the
dwelling’s air barrier. Details that are vital to achieving good airtightness need to be identified at this point.
Careful thought must be given to sealing gaps and ensuring the continuity of the air barrier. The next and equally
important step, is to ensure these details are carried over into the construction phase. It is far simpler to
design and build an airtight dwelling than to carry out remedial measures in a draughty home.
This guide explains where air leakage commonly occurs in existing dwellings, and suggests how airtightness can
be significantly improved with careful design and construction. Airtight construction combined with well-designed
ventilativentilation will improve energy efficiency and comfort levels.