Testing for Airtightness with a Blower Door

The Blower Door is an amazing tool. A home energy auditor or home energy rater can use it to do performance testing by quantifying the infiltration rate of a house. They can also do diagnostic testing with it, by running the Blower Door and walking around the house to find leaks.

Basic Operation. The basic principle of its operation is simple. A powerful fan set temporarily in a doorway creates a pressure difference between the house and the outdoors, usually by depressurizing the house. All the air that the fan blows out of the house is replaced by air coming in through all the leaks. For every cubic foot of air that blows out through the fan, a cubic foot leaks in.

So, all we need to do is measure the amount of air moving through the fan, and, voila, we know how much the house leaks at that pressure.  Airflow is measured in cubic feet per minute, and the pressure normally used for this test is 50 Pascals. Hence, the number that everyone who does this testing in the US uses is cfm50.