How to interpret blower door test results focuses on understanding CFM50, ACH50, and what they mean for efficiency and comfort. The primary result is CFM50, cubic feet per minute at 50 Pascals, which is airflow needed to maintain 50 Pascal pressure, lower means tighter house. For example 1500 CFM50 is tighter than 3500 CFM50. To compare different sized houses, interpret ACH50, air changes per hour at 50 Pascals, calculated as CFM50 times 60 divided by building volume, so a 2000 square foot house with 16000 cubic feet at 2000 CFM50 equals 7.5 ACH50. Code requires 3 ACH50 or less in many climate zones for new homes, so 7.5 is leaky. Natural infiltration ACH natural is approximately ACH50 divided by 20, so 7.5 ACH50 equals about 0.375 natural air changes per hour under normal conditions. Also look at CFM50 per square foot of floor area or envelope area for comparison. Beyond numbers, interpret location of leaks identified with infrared and smoke during test, common leaks at attic hatches, recessed lights, plumbing penetrations, and rim joists. A result below 3 ACH50 means you need mechanical ventilation like ERV for fresh air per ASHRAE 62.2.
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