Results of the blower door test provide a standard measure of the leakage of a home, measured in cubic feet per minute (CFM) of airflow. This number is then converted to air changes per hour (ACH) so you can compare the building’s leakage to the building code ACH number needed for the building final inspection.

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What does ACH (50) mean? 

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One measure of a building’s air leakage rate is air changes per hour (ACH), which estimates how many times in one hour the entire volume of air inside the building is exchanged with outside air. To determine a building’s ACH, the blower door performs an airtightness test with the house depressurized (or pressurized) to 50 Pascals. Fifty Pascals is approximately equivalent to a 20 mile-per-hour wind blowing against all surface of the building.

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The leakier the house, the higher the number of air changes per hour, the higher the heating and cooling costs, and the greater the potential for moisture, discomfort, and health problems. Builders of energy-efficient homes generally strive for less than 5 air changes per hour at 50 Pascals pressure (ACH50).

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