The blower door test is a method for checking how airtight a building is. For this purpose a fan is installed airtight in the front door, and a pressure differential is created and increased step by step up to 50 pascal (equivalent to a pressure of 5 mm water column).

The values measured are listed and entered into a coordinate system (flowrate/pressure differential). The intersection point at 50 pascal is read off for negative and positive pressure. Usually these values are close to each other, unless there is a check-valve effect from a leak or wind effects are too high. The mean is the measured ACP₅₀ value, i.e. the rate of air change at a pressure differential of 50 pascal.

The leaks can be localized using an anemometer, which measures the speed of incoming air at damage-prone spots at negative pressure. Alternatively a smoke generator in the form of a small tube can be used to make air movements visible. For hard-to-access leaks a fog generator can also be employed: in conjunction with positive pressure the fog becomes visible on the outside surface where the leaks are.