A Blower Door Test is a diagnostic tool used to measure the air tightness of a building. It involves installing a powerful fan in an exterior door frame, which either blows air into the building or pulls air out. This process helps identify air leaks, allowing homeowners and builders to understand how much outside air enters the building and how much conditioned air escapes. The results of the test are expressed in terms of air changes per hour (ACH) at a specific pressure. This information is crucial for improving energy efficiency, enhancing indoor air quality, and ensuring compliance with building codes related to energy efficiency and ventilation.