A blower door test is a diagnostic test that measures how much outside air leaks into, or conditioned air escapes from, a building through unintended openings in its exterior envelope. The test uses a calibrated fan mounted in an exterior doorway to depressurize the home and measure the airflow moving through the fan. This air is replaced by outside air entering through gaps, cracks, and other leakage paths in the building envelope. The results help determine how airtight the home is, which directly affects energy consumption and indoor comfort during extreme weather conditions.