ACH50 means air changes per hour at 50 pascals: the number of times the entire interior air volume of a building would be exchanged in one hour under a controlled 50 Pa pressure difference created by a blower door. It is usually the first number builders and homeowners see on an airtightness test report, and it is also one of the most frequently misread. It is often mistaken for a ventilation rate or for the building’s real-world leakage, when it is neither. Understanding what the number does and does not describe makes it far more useful.
Breaking Down the Name
Each part of the abbreviation describes one element of the measurement.
Air Changes
One air change is a quantity of air equal to the building’s entire conditioned interior volume. If a home holds 500 cubic metres of air, one air change is 500 cubic metres of air moving through the envelope.
Per Hour
The rate is normalized to one hour. This puts every building on the same time basis, so the result for a small townhouse and a large detached home can be expressed in the same units.
At 50 Pascals
The measurement is taken while the blower door holds a pressure difference of 50 Pa between inside and outside. The “50” refers to this test pressure, not to a target or a score.
How ACH50 Is Calculated
During a blower door test, a calibrated fan mounted in an exterior doorway moves air into or out of the building until the pressure difference reaches 50 Pa. The airflow through the fan at that pressure equals the air leaking through the envelope. ACH50 converts that airflow into air changes using the building’s volume.
- Imperial formula: ACH50 = (CFM50 × 60) ÷ conditioned volume in cubic feet
- Metric formula: ACH50 = airflow at 50 Pa in m³/h ÷ conditioned volume in m³
The imperial version multiplies by 60 because CFM50 is measured in cubic feet per minute and ACH50 is an hourly rate.
Illustrative example (metric):
- Conditioned floor area: 200 m²
- Average ceiling height: 2.5 m
- Conditioned volume: 200 × 2.5 = 500 m³
- Measured airflow at 50 Pa: 1,000 m³/h
- ACH50 = 1,000 ÷ 500 = 2.0
These figures are chosen for clarity, not as a typical result. The calculation also shows why volume matters so much: the same airflow divided by a larger or smaller volume produces a different ACH50. How conditioned volume is measured is defined by the test standard used, which in Canada is typically CGSB-149.10 and in the US is typically ASTM E779.
Why 50 Pascals?
A 50 Pa pressure difference is strong enough to overwhelm the small, constantly changing pressures created by wind and by the stack effect, where warm air rises and pushes out through the upper parts of a building. Testing at this pressure keeps the result stable and repeatable regardless of the weather on test day.
For a sense of scale, 50 Pa is roughly comparable to a moderate wind pressing on every side of the building at the same time. Because it is a widely shared reference point, results taken at 50 Pa can be compared across buildings, programs, and building codes.
What ACH50 Does Not Tell You
“It is how leaky the house is every day.” It is not. Under ordinary conditions, pressures across the envelope are far lower than 50 Pa, so natural air leakage is much smaller than the test figure suggests. A long-standing rule of thumb divides ACH50 by about 20 to estimate natural infiltration, but this is only an approximation. Actual infiltration varies with climate, wind exposure, building height, and where the leaks are located.
“It is a ventilation rate.” It is not. In HVAC design, air changes per hour describe intentional fresh air supplied by a ventilation system. ACH50 describes unintentional leakage through the envelope under test pressure. A tight building with a low ACH50 still needs mechanical ventilation, usually a heat recovery ventilator, to supply fresh air.
“It shows where the leaks are.” It does not. ACH50 is a single whole-building number. Locating individual leaks requires diagnostic tools such as smoke pencils or infrared thermal imaging, ideally during a mid-construction blower door test, while the air barrier is still exposed and repairs are inexpensive.
One further clarification: ACH50 has no connection to the AH50 and CH50 medical laboratory tests, which share similar abbreviations but measure immune system activity.
Reading the Number
A lower ACH50 indicates a tighter building. In broad terms, results fall into four bands:
| Range | What it typically indicates |
|---|---|
| High | Older or unrenovated buildings with no continuous air barrier |
| Moderate | Newer construction built to a basic code level |
| Low | Carefully detailed new construction with a continuous air barrier |
| Very low | Passive House level, where 0.6 ACH50 is the certification limit |
Whether a result is “good” depends on the target that applies to the project. A number that comfortably passes a code minimum may fall well short of the requirements for Passive House design. Each result should be judged against the code step or standard the building is designed to meet, not against an abstract ideal.
ACH50 vs. NLR50 vs. NLA10
ACH50 has a known weakness. Because it divides leakage by interior volume, a large home can score better than a small one even when both have the same amount of leakage per square metre of envelope. Small buildings have more envelope surface relative to their volume, so they are penalized by the calculation. This affects compact homes, laneway homes, and individual units in townhouses and multi-family buildings.
To address this, BC recognizes alternative airtightness metrics that are normalized by envelope surface area rather than volume:
| Metric | Normalized by | Best suited to |
|---|---|---|
| ACH50 | Interior volume | Detached homes of typical size |
| NLR50 (normalized leakage rate) | Envelope surface area | Small homes and attached units |
| NLA10 (normalized leakage area) | Envelope surface area, expressed as an equivalent leakage area | Comparisons across different building shapes |
For many smaller buildings, choosing a surface-area metric gives a fairer picture of how well the envelope was actually built.
ACH50 Under the BC Energy Step Code
Airtightness under the BC Energy Step Code is verified on site with a blower door test, and the results feed into the energy model used to show compliance. Targets become stricter at higher steps and depend on which airtightness metric is used. For an overview of what the current provincial minimum requires at each permit stage, see the guide to BC Step Code 3 requirements.
The most practical way to protect the final number is to test before it is locked in. A mid-construction test, run before drywall covers the air barrier, shows whether the building is on track and where the leaks are while they are still easy to fix.
Conclusion
ACH50 is a standardized stress-test number. It makes buildings comparable with one another, but it is not a measure of everyday leakage, a ventilation rate, or a map of where air escapes. Monolith Housing Solutions performs blower door testing on homes and multi-unit buildings across the Lower Mainland and helps builders interpret ACH50 and alternative metrics against their project’s target. Testing is led by founder Shahin Arvandi, a Certified Air Tester and CHBA Net Zero Energy Advisor.
Frequently Asked Questions
Definition
- What does ACH50 stand for? ACH50 stands for air changes per hour at 50 pascals, the rate at which a building’s full interior air volume would be replaced while a blower door holds it at a 50 Pa pressure difference.
- What does 3 ACH50 mean? It means that, under the 50 Pa test pressure, air leaking through the envelope would replace the building’s entire interior air volume three times in one hour.
- Is ACH50 the same as air changes per hour? No. Air changes per hour without a pressure qualifier usually refers to ventilation or natural infiltration under normal conditions. ACH50 is measured only under the 50 Pa test pressure.
Calculation and Testing
- How do you calculate ACH50 from CFM50? Multiply CFM50 by 60 to convert it to an hourly rate, then divide by the building’s conditioned volume in cubic feet.
- Why are blower door tests done at 50 pascals? 50 Pa is high enough to overwhelm the effects of wind and temperature differences during the test, which makes results stable, repeatable, and comparable across buildings.
- Does house size affect the ACH50 result? Yes. Because ACH50 divides leakage by interior volume, larger homes tend to score better than smaller homes with the same quality of air sealing.
Interpretation
- What is a good ACH50 for a new home? It depends on the target that applies to the project. The relevant benchmark is the code step or performance standard the home is designed to meet.
- What ACH50 is required for Passive House? Passive House certification requires a maximum of 0.6 ACH50.
- How do you convert ACH50 to natural air changes? A common rule of thumb divides ACH50 by about 20, but this is only an estimate. Actual natural infiltration depends on climate, exposure, building height, and leak locations.
Alternatives and BC Context
- What is the difference between ACH50 and NLR50? ACH50 normalizes leakage by interior volume. NLR50 normalizes it by envelope surface area, which gives a fairer comparison for small and attached buildings.
- Why does BC allow airtightness metrics other than ACH50? Volume-based metrics penalize small buildings. Surface-area metrics measure the quality of the envelope more evenly across building sizes and shapes.
- What ACH50 does the BC Energy Step Code require? Requirements vary by step and by the metric chosen. The current values are set out in the BC Building Code and should be confirmed with the project’s energy advisor.
