A blower door test measures a building’s air leakage rate by pressurizing or depressurizing the building with a calibrated fan and measuring airflow in CFM50 — converted to ACH50 (air changes per hour at 50 Pascals). IECC 2021 requires a maximum of 5.0 ACH50 for Zones 5 and 6, verified by a blower door test before the permit is finalized. The test is critical because visual insulation inspection cannot detect air leakage at electrical boxes, plumbing pipes, recessed lights, rim joists, and attic hatches — the locations where most air leakage occurs. A building with correct R-values installed can still fail the blower door test if penetrations are not sealed. RainFire Builders air-seals all penetrations before insulation installation, targeting results well below the code limit. A failed blower door holds the insulation inspection until leaks are located and sealed — building to pass on the first attempt is more efficient.