ACH50
ACH50 is the airtightness of a building, measured as the number of complete air changes per hour when the home is depressurized to 50 Pascals during a blower-door test. Lower ACH50 means a tighter envelope. Code-minimum new construction is typically 3-5 ACH50; Passive House requires 0.6 ACH50 (PHIUS) or below; legacy homes can run 10-20 ACH50. Tightening to under 1.5 ACH50 makes mechanical ventilation (HRV/ERV) essential for indoor air quality.