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 activated, the fan creates a pressure difference, allowing technicians to identify air leaks and assess the overall energy efficiency of the structure. By using a blower door, professionals can pinpoint areas where air is escaping or entering, which can lead to increased energy costs and discomfort. This tool is essential for energy audits and is often used in conjunction with other assessments, such as thermal imaging, to improve building performance.