Blower door test

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 test must be carried out as soon as all sealing building elements have been installed, but before the cladding over them has been put in place – usually after the windows, the vapour barrier and the interior plaster have been installed. It is advisable to invite the tradesmen concerned to the test. Experience shows that they are perfectly willing to rework leaks immediately after they have been found – sealing materials should be on site then!

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: combined with positive pressure the fog becomes visible on the outer surface where the leaks are.

If the locations of the leaks are supposed to be recorded permanently, infrared thermography is a more expensive, but effective medium. Outdoor air is drawn in at negative pressure and the entry points are captured thermographically. The greater the difference between indoor and outdoor temperature, the more effective this method is.

Blower door test: in this case the device was built into a window because the front door was very likely to leak (source: Schulze Darup)