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What is a blower door test?

The Blower Door Test uses a fan on an exterior door to depressurize the house and then measures the air infiltration into the house. The measurement returned is in CFM or cubic feet of air per minute. This can then be converted to ACH which is air changes per hour. This number explains how many times in an hour does the house has completely new air. A great ACH rating is around .2. A bad ACH rating on a home that has high air leakage could be as high as 2.