Airtightness is the degree to which a building envelope resists uncontrolled air flow. It is a measurable property: the less air passes through the joints at a given pressure difference, the more airtight the building.
Where airtight construction is about workmanship, airtightness is about the number that comes out of it.
How it is measured and expressed
Measurement is by blower door test (fan pressurisation to BS EN ISO 9972): a fan in a doorway puts the building under positive and negative pressure.
Different countries use different indicators:
| Indicator | Definition | Used in |
|---|---|---|
| Air permeability, q50 | Air flow per m² of envelope area at 50 Pa, in m³/(h·m²) | United Kingdom |
| n50 | Air changes of the internal volume per hour at 50 Pa, in 1/h | Germany, Passivhaus |
| qv;10 | Air flow per m² of floor area at 10 Pa, in dm³/(s·m²) | Netherlands |
Indicative benchmarks: the Passivhaus standard requires n50 of no more than 0.6 air changes per hour. UK Building Regulations set a maximum air permeability for new dwellings, with typical designed values well below that limit; the exact figure depends on the current edition of the approved documents in your jurisdiction, so check the version in force.
Because reference area and test pressure differ, these figures cannot be converted into one another directly.
Why testing pays
- The report does not just give a number; it shows where the leaks are. Under negative pressure they can be pinpointed with a smoke pen or thermal imaging
- Test during the shell stage, while the airtight layer is still reachable. A test at handover is a check, not a repair opportunity
- In a retrofit, a test shows whether the money is better spent on sealing or on more insulation
> Note: airtight is not the same as vapour-tight. A construction can be airtight and vapour-open at the same time — with many timber frame details that is exactly the intention.
Related terms
- Airtight construction
- Blower door test
- Vapour control layer
- Airtight tape
- Condensation
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