Air leakage is the uncontrolled exchange of air both into (infiltration) and out of (exfiltration) a building through cracks, gaps and other unintentional openings in the building envelope. It is driven by the same physical processes that drive natural ventilation, namely:
The rate of air leakage is therefore dependent upon the wind speed and direction, the temperature difference between the inside and outside of the building, as well as within the building, and the air permeability of the construction.
Airtightness is a term used to describe the ‘leakiness’ of a building. The airtightness of a building determines the uncontrolled background ventilation or leakage rate of a building which, together with purpose-provided ventilation, makes up the total ventilation rate for the building. In other words:
Uncontrolled Background Ventilation + Purpose Provided Ventilation = Total Ventilation Rate
It is frequently expressed in terms of a whole building leakage rate at an artificially induced pressure, in the UK 50Pa is used (n50), or in terms of an equivalent leakage area. Traditionally, airtightness was expressed in air changes per hour (with units conventionally used being ach or h-1). However, nowadays air permeability is more commonly used (with units m3/(h.m2) representing m3 of air flow per hour, per m2 of envelope area) as it takes into consideration the effects of shape and size. The lower the air permeability of a building, the greater the airtightness.