How to read a blower door test involves understanding key metrics CFM50, ACH50, and CFM per square foot. The main number is CFM50, cubic feet per minute of air flow required to maintain 50 Pascals pressure difference between inside and outside, lower means tighter. For example 2000 CFM50 means 2000 cubic feet per minute leaks at test pressure. To compare houses of different sizes, read ACH50, air changes per hour at 50 Pascals, calculated as CFM50 times 60 divided by house volume, so a 2000 square foot house with 16000 cubic feet volume at 2000 CFM50 equals 7.5 ACH50, meaning house air would be replaced 7.5 times per hour at 50 Pascals, which simulates about 20 mph wind. Natural air leakage ACH natural is estimated as ACH50 divided by 20, so 7.5 ACH50 equals about 0.375 natural air changes. Building codes use ACH50, with 3 to 5 ACH50 required for new homes. Also read CFM50 per square foot of envelope or per 100 square feet of floor area to compare. During test, auditor also notes baseline pressure and cannot reach 50 Pa if house is very leaky. Reading report also includes locations of leaks found with infrared and estimated savings from air sealing.
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