A blower door test is a systematic procedure used to evaluate the air-tightness of a building, essential for assessing energy efficiency, indoor air quality, and overall building performance. The test involves using a calibrated fan mounted in an exterior door frame to pressurise or depressurise the structure, with a manometer measuring the pressure differential. This process helps identify air leaks, allowing for corrective actions that improve energy efficiency, reduce consumption, and enhance indoor air quality. Additionally, the data collected from the test, often measured in air changes per hour or cubic feet per minute, guides the optimisation of HVAC systems and compliance with building performance standards.