Glossary · Testing & air leaks
ACH50
Also called: ACH@50Pa · ACH50 rating · air changes per hour at 50 Pa · air changes per hour at 50 pascals · air changes per hour · n50
An airtightness score: how many times an hour the house's whole volume of air leaks out while a test fan pulls on it; lower means tighter.
What it means
ACH50 takes the leakage measured in a blower door test (CFM50) and divides it by the size of the house, so big and small homes can be compared fairly. It's calculated as CFM50 × 60 ÷ house volume in cubic feet. An ACH50 of 3 means the fan pulled out three houses' worth of air per hour at test pressure; on a normal day the house leaks far less.
Why it matters in an audit
It is the main airtightness score used by energy codes and programs, and comparing before-and-after ACH50 shows whether air sealing worked.
Typical numbers
Reference values with their sources. Your energy advisor’s report gives the figure for your own house.
| What | Value | Source |
|---|---|---|
| NRCan R-2000 Standard (2012 edition) | 1.5 ACH50 or less | natural-resources.canada.ca ↗ |
| Passive House (Passive House Institute) | 0.6 ACH50 or less, checked with an on-site pressure test | passivehouse.com ↗ |
In Canada
NRCan's R-2000 Standard uses the same air-changes-at-50-Pa measure, so Canadian and US ACH50 numbers can be compared directly.