What is considered a good blower door score depends on climate, house age, and code, but is measured in CFM50 cubic feet per minute at 50 Pascals and ACH50 air changes per hour at 50 Pascals. For older existing homes built before 1980, a typical score is 2500 to 5000 CFM50 or 10 to 20 ACH50, which is very leaky. A moderate improvement target for weatherization is below 2500 CFM50 or below 8 ACH50. For newer homes built to recent energy codes, a good score is below 1500 CFM50 or 3 to 5 ACH50. Energy Star and modern codes require 3 ACH50 or less in many climate zones, and passive house standard requires 0.6 ACH50, which is extremely tight. Most building scientists consider 1500 to 2000 CFM50 or 3 to 5 ACH50 as good for a comfortable efficient home, tight enough to control drafts and save energy but not so tight that it requires extensive mechanical ventilation, though under 3 ACH50 you should add ERV or HRV for fresh air. Lower numbers mean tighter house, so 800 CFM50 is excellent tightness, while 4000 CFM50 is poor and indicates significant air sealing opportunities that would improve comfort and reduce bills.
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