What it is called, by market
One concept, four markets. Where a name means something else locally, the card says so rather than leaving you to find out on site.
United Kingdom
Same word, different thingair permeability
also airtightness, q50, air leakage, air pressure test, m³/h·m² @50Pa
AIR PERMEABILITY divides leakage by the ENVELOPE AREA. It is a different quantity from the air change rate used in North America, not the same quantity in other units.
Australia
Close, not identicalair permeability
also air tightness, ACH50, blower door test, air leakage rate
Both conventions appear, with air changes per hour common in voluntary programmes and envelope-area figures in commercial work — so the unit must be stated rather than assumed.
United Statesyours
Same word, different thingair leakage
also ACH50, air changes per hour, blower door, CFM50, envelope tightness
ACH50 divides leakage by the internal VOLUME. CFM50 is the raw flow before any division. Neither converts to a British air permeability figure without the building's own area and volume.
Canada
Same word, different thingair leakage
also ACH50, NLA, normalized leakage area, blower door
NORMALIZED LEAKAGE AREA expresses the leak as an equivalent HOLE rather than a flow, which is a third convention again and the most intuitive of the three to explain.
The governing standard, by market
United Kingdom
ATTMA TSL1 / Approved Document L
Measuring air permeability of building envelopes in dwellings, and the maximum permitted figure and testing requirement.
United States
ASTM E779 / IECC R402.4
Determining air leakage rate by fan pressurization, and the prescriptive air leakage limit expressed in air changes per hour at 50 pascals.
Calculators for this
Each works in either measurement system, and the terminology on the page follows whichever market you have selected.
Frequently asked questions
- Can an ACH50 figure be converted to air permeability?
- Only for a specific building, and only if you know both its envelope area and its internal volume — which means the conversion is a property of the building, not of the units. Air permeability divides the leakage flow by the area of the envelope through which it leaks; ACH50 divides the same flow by the volume the building encloses. The ratio between the two is therefore volume over area, which is a length: a tall, compact block has a large volume per unit of envelope, a single-storey sprawl has a small one. The practical consequence is that the same physical envelope quality produces a BETTER-looking ACH50 in a compact building and a worse one in a spread-out building, while air permeability is unaffected by shape. So a rule of thumb quoted for converting one to the other is a rule of thumb about typical house shapes, and it fails exactly where the shape is unusual.
- Why is the test done at 50 pascals when no building ever sees that?
- Because it is the smallest pressure at which the answer is repeatable. At the pressure differences a building actually experiences — a few pascals from wind and the stack effect — the flow through the leaks is tiny, and the measurement is swamped by the weather changing while you take it. Raising the pressure to 50 pascals makes the leakage flow large compared with the noise, so two testers on two days get the same answer. Fifty pascals is roughly what a stiff breeze would exert, so it is high enough to be measurable and low enough not to damage anything or to force leaks open that are normally shut. The number is a test condition, not a prediction: translating it into the leakage a building actually experiences needs a further step, and the common divide-by-twenty rule is a rough correlation from field data rather than physics.
- Does a tighter building need more ventilation?
- It needs ventilation that is DESIGNED rather than accidental, which is not quite the same claim. A leaky building is ventilated by its defects — at a rate set by the weather, mostly through the parts of the envelope nobody chose, and at its highest on the coldest, windiest days when it costs the most. Sealing it does not remove the need for fresh air; it removes the uncontrolled supply, and the fresh air then has to come from openings and fans that were specified. This is why airtightness targets appear in codes alongside ventilation requirements rather than instead of them, and why a retrofit that seals an old house without adding controlled ventilation reliably produces condensation and mould within a year or two. The phrase that captures it in the trade is *build tight, ventilate right* — and the second half is a requirement, not advice.