The leakage of air from a building is measured through a blower fan door system which, along with other UKAS-calibrated equipment, gives the air permeability of the building.
The testing involves connecting the blower fan door to a suitable door in the building envelope and pressurising the building over a range of pressure differences.
The fan speed is increased in steps up to a maximum and then decreased in steps. Air volume flow rate through the fan and the pressure difference across the building envelope are recorded at each fan speed.
In calculating air permeability, corrections are made for external air temperature and barometric pressure.