A blower door is a diagnostic tool used to measure the air tightness of buildings. It consists of a powerful fan mounted in a frame that fits into an exterior door opening. When the fan is activated, it either pressurizes or depressurizes the building, allowing professionals to identify air leaks and assess energy efficiency. By creating a controlled environment, a blower door test helps detect unwanted drafts and areas where air escapes or enters. This information is crucial for improving insulation, reducing energy costs, and enhancing indoor comfort. Proper sealing can lead to significant energy savings over time.