Blower Door Test Explained | CFM50, ACH50 & Home Air Leakage

What does a blower door test actually tell you about a home? In this quick building-science guide, learn how blower door testing measures home air leakage, what CFM50 and ACH50 mean, and why these measurements matter during an energy audit. A blower door uses a calibrated fan and pressure-measuring equipment to create a controlled pressure difference between the home and outdoors. The airflow required to maintain the test pressure provides a quantitative measurement of building-envelope leakage. You’ll learn: ✅ What a blower door test measures ✅ What CFM50 means ✅ What ACH50 means ✅ How CFM50 relates to building air leakage ✅ How ACH50 accounts for building volume ✅ Why air leakage affects energy efficiency and comfort ✅ How blower door testing supports air-sealing work ✅ Why post-work testing can help verify improvement ACH50 Formula: ACH50 = (CFM50 × 60) ÷ Building Volume Blower door testing is an important diagnostic tool for energy auditors, HERS raters, weatherization professionals, HVAC contractors, builders, and home-performance professionals. When combined with thermal imaging, building-envelope observations, HVAC and duct information, energy-use data, photographs, and other appropriate diagnostics, blower door results can contribute to a much more complete understanding of building performance. With BPMS™ — Building Performance Modeling Systems™, diagnostic measurements can become part of a connected building-performance workflow alongside field documentation, thermal imaging, building information, recommendations, WorkScopes, and professional reporting. Measure. Understand. Improve. Verify. Better Data. Better Decisions. Better Buildings. Learn more about BPMS™ at BPMSCloud.com.

Originally uploaded October 7, 2026