Air Tightness

Thermal insulation is pretty much a waste of time unless there is sufficient airtightness in a building - imagine heating a house where effectively all the windows are open the whole time and you will understand why . Even relatively small amounts of air leakage have a significant effect on energy use. This increases as we attempt to increase the insulation of a building. The chart below illustrates the amount of heat lost through fabric and through air leakage (infiltration loss) in a theoretical building built to part L (2006), with air tightness set at the minimal level , and then the amount lost in a super insulated building with the same standard of airtightness.

This issue is finally being recognised by Building Regulations and airtightness testing will probably be the single most significant change to Building Regulations as regards thermal performance, since insulation was introduced in 1976. However the standards set are low, and it remains to be seen whether they are reached and how builders will avoid their obligations.

Unfortunately airtightness is usually confused with inadequate ventilation and so is seen as unhealthy. On the contrary, however, leakage of warm moist air from living areas into the structure of an insulated building is likely to lead to condensation problems which are a threat to the health of the building and its occupants. Airtightness, therefore, is not only good for energy conservation, but, with the correct moisture control and ventilation system, it helps preserve building fabric and the health of occupants.

What we do know to date however is that new buildings are on average no more airtight than the average for the existing building stock – ie they leak like sieves . We also know that buildings in the UK may well be up to 50% more leaky 3 – 4 years after construction, than when first completed. This is not surprising as buildings move and settle and most of the supposed airtightness measures comprise of surface applied mastic. In the Orkneys it was found that timber frame buildings constructed in the early 1990s were 300% more leaky after building settlement . Post construction mastic is no substitute even in the short term for properly designed airtight structures which will stay airtight for the life of the building. The question is whether this can be achieved. Until it is achieved however the theory of energy efficient structures will not be realised in practice.