The airtightness of a dwelling, or its air permeability, is expressed in terms of air leakage in cubic metes per hour per square metre of the dwelling envelope area when the building is subjected to a differential pressure of 50 Pascals (m3/(h.m2)@50Pa).
The dwelling envelope area is defined in this context as the total area of all floors, walls and ceilings bordering the dwelling, including elements adjoining other heated or unheated spaces.
Air leakage is defined as the flow of air through gaps and cracks in the building fabric. Uncontrolled air leakage increases the amount of heat loss as warm air is displaced through the envelope by colder air from outside. Air leakage of warm damp air through the building structure can also lead to condensation within the fabric (interstitial condensation), which reduces insulation performance and causes fabric deterioration.
The air permeability of a building can be determined by means of a pressure test. ATTMA TSI: 2006 discusses the methodology for air pressure testing including the test procedures, requirements and conditions of reporting.
Building Regulations 2008 TGD-L (Dwellings) indicates that reasonable provision for airtightness is to achieve a pressure test result of no worse than 10m3/(h.m2)@50Pa. Current good practice for energy efficient dwellings includes achieving airtightness of 7m3/(h.m2)@50Pa and best practice is 3m3/(h.m2)@50Pa.
The airtightness appropriate for a particular dwelling design will depend upon the Building Energy Rating the builder is aiming to achieve.