What does a blower-door test measure?

A blower door is a calibrated fan mounted in an exterior doorway. It depressurizes or pressurizes the building to a standard test pressure, usually 50 pascals, and the airflow required to hold that pressure is the measurement. The result is reported as CFM50, or as ACH50 once divided by the building's volume: air changes per hour at 50 pascals.

The test measures the airtightness of the envelope under a fixed pressure. It does not measure the air leakage the building experiences in service, which varies with wind, stack effect, and temperature; that rate is estimated from the test result, roughly ACH50 divided by 20, or from a multipoint test across a range of pressures.

Manual J's tightness classes are defined in ACH50, so a blower-door result supplies that input directly. With the fan running, leaks can also be located by hand, smoke, or infrared camera. The test is not part of the Manual J service; we do not offer blower-door testing in-house at this time.

Last reviewed 2026-09-06. Accurate to the best of our knowledge as of that date. Rules change — that's why the date is there.