What does a good score on a blower door test mean?

What a good score on a blower door test means depends on context of new versus existing homes and climate zone, but generally lower numbers mean tighter and better. Blower door results are reported as CFM50 cubic feet per minute at 50 pascals, ACH50 air changes per hour at 50 pascals, and sometimes CFM per square foot. For new construction, International Energy Conservation Code requires less than 5 ACH50 in climate zones 1 and 2 and less than 3 ACH50 in zones 3 to 8. A good score for new home would be 2 to 3 ACH50, which meets code and indicates careful air sealing. Excellent or high performance homes like Passive House aim for less than 0.6 ACH50, which is extremely tight and requires mechanical ventilation. For existing older homes, a good score after air sealing might be 5 to 7 ACH50, while average older leaky home might be 10 to 15 ACH50. Lower score means less energy waste, fewer drafts, better comfort, less dust and pollen infiltration, reduced moisture issues, and lower heating and cooling bills. However, very tight homes below 3 ACH50 need mechanical ventilation like HRV or ERV to ensure fresh air, so good score also means tight but with proper ventilation strategy.