ACH50 measures the air changes per hour at 50 pascals of pressure difference, quantifying a building’s air leakage rate through a standardized blower door test. One air change means the entire building volume is replaced with outside air once. Lower ACH50 values indicate tighter construction, with 1.5 ACH50 representing extremely tight homes and 7 ACH50 representing typical older construction.
Testing and Standards
Blower door testing pressurizes or depressurizes a building to 50 Pa while measuring airflow to quantify leakage. Passive House standards require ACH50 of 0.6 or less. Net-zero energy homes typically achieve 2.0 to 3.0 ACH50. Building codes increasingly specify maximum ACH50 requirements, with some states requiring 7.0 ACH50 or better for new construction.
Energy and Performance Impact
Reducing ACH50 from 7.0 to 3.0 decreases heating and cooling energy use by 20-30 percent. Sealing air leaks through weatherstripping, caulking, spray foam, and air barriers improves comfort by eliminating drafts. Lower ACH50 rates also enhance moisture and radon control, improving indoor air quality and structural durability.