Abstract: A method and apparatus for testing the air-tightness of an enclosure such as a house or building, includes the increasing of the pressure within the enclosure relatively rapidly to above a predetermined amount with all doors and windows being closed and then measuring the leakage of decay of the increased pressure together with the time in order to arrive at a leakage coefficient of the enclosure. The increase in pressure should be high enough above atmospheric pressure to allow accurate pressure measurement, but not high enough to damage the structure.