Definition
Air Changes Per Hour (ACH)
A measurement of how many times the entire volume of air in a building is replaced in one hour, indicating how leaky the structure is.
The Full Picture
Air Changes Per Hour (ACH) quantifies building air leakage. ACH50 specifically refers to the number of air changes per hour when the building is depressurized to 50 Pascals (during a blower door test). It's calculated by dividing the blower door CFM50 reading by the building volume, then multiplying by 60. A lower ACH50 means a tighter building. Current energy codes typically require ACH50 between 3.0 and 5.0 depending on climate zone. Older homes commonly test at 8-15 ACH50, meaning the entire volume of air leaks out and is replaced 8-15 times per hour under test conditions.
Why It Matters
Why field professionals need to document this
ACH50 is the universal language of building envelope performance. It lets energy auditors compare buildings objectively, set improvement targets, and measure the impact of air sealing work. For homeowners, it translates 'your house is drafty' into a specific number with a clear target. Utility rebate programs often require achieving a specific ACH50 threshold to qualify for incentives.
In a Report
How this shows up in findings
Here's how an air changes per hour (ach) finding looks in a professional field report generated by ReportWalk:
8.2 ACH50 — significantly above 5.0 target, air sealing is highest priority
Reduced from 8.2 to 4.0 ACH50 after attic and rim joist air sealing — 51% improvement
3.1 ACH50 — meets 2021 IECC requirement, no additional air sealing needed
Relevant For
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