What ACH50 Measures
ACH50 stands for air changes per hour at 50 pascals. During a blower door test, a calibrated fan depressurizes or pressurizes the building, and the equipment measures the airflow needed to maintain the test pressure.
A lower ACH50 generally means a tighter enclosure, but it does not automatically mean better indoor air quality. Tight homes need planned ventilation, combustion safety checks where relevant, and moisture control. Air sealing and ventilation should be designed together.
- Energy raters use ACH50 for code and program documentation.
- Builders use it to verify air-sealing work.
- Homeowners use it to understand drafts and energy loss.
How to Calculate ACH50
The formula is: ACH50 = CFM50 x 60 / building volume. CFM50 is the blower door airflow in cubic feet per minute at 50 pascals. Building volume is the conditioned volume in cubic feet.
For example, if the blower door reads 1,200 CFM50 and the conditioned volume is 18,000 cubic feet, ACH50 = 1,200 x 60 / 18,000 = 4.0. That means the test airflow equals four building air changes per hour at the test pressure.
ACH50, Natural Air Change, and Code
ACH50 is a test condition, not the natural leakage rate on a normal day. Natural air change depends on wind, stack effect, weather, shielding, building height, and leakage distribution. Rules of thumb that convert ACH50 to natural ACH are only approximations.
Energy codes and green building programs may set maximum ACH50 values, but the required limit depends on the adopted code edition, climate zone, building type, and local amendments. Always check the rule that applies to the project, not a generic internet number.
- Use conditioned volume, not total lot size or floor area.
- Confirm whether attached garages or unconditioned spaces are excluded.
- Record test direction, pressure, and any guarded or multifamily setup.
Common Blower Door Mistakes
A common error is using the wrong volume. Small volume mistakes can move the ACH50 result enough to affect pass-fail status. Another is testing before the enclosure is ready, with missing weatherstripping, open dampers, or incomplete penetrations.
The result should lead to diagnostics, not just a score. Smoke, infrared imaging, zone pressure testing, and room-by-room inspection can locate the leakage paths that matter most for comfort, durability, and energy use.
- Do not forget intentional openings required by the test protocol.
- Do not compare CFM50 between homes without considering volume.
- Do not tighten a building without confirming ventilation strategy.
Frequently asked questions
Is lower ACH50 always better?
Lower leakage is usually better for energy and comfort, but very tight buildings need appropriate mechanical ventilation and moisture management.
What is the difference between CFM50 and ACH50?
CFM50 is the measured fan airflow at 50 pascals. ACH50 converts that airflow into air changes per hour using the building volume.
Can ACH50 be converted to natural ACH?
Only approximately. Natural leakage depends on weather, building height, exposure, and leakage locations, so a single conversion factor is not precise.
Why do code limits differ?
Air leakage limits are set by adopted energy codes and local amendments. Jurisdictions may enforce different editions or program requirements.