An airtightness result such as n₅₀ = 1.5 h⁻¹ tells you nothing about any single product you specified. It is a measurement of the completed building as a whole, taken by pressurising and depressurising the structure with a fan (blower door) at ±50 Pa, following BS EN ISO 9972 (which superseded EN 13829). The test quantifies how much air escapes through every joint, seal and penetration in the envelope once it is fully assembled — not how good any individual component is on its own.
What do n₅₀ and q₅₀ actually measure? Two figures come out of a blower-door test, and they measure different things: n₅₀ — the air change rate per hour at 50 Pa, expressed as h⁻¹. q₅₀ — air permeability per unit area, expressed as m³/(h·m²). Neither number is fixed by the materials chosen at design stage. They are the product of how the whole envelope — walls, roof, openings, service penetrations — was detailed, built and sealed on site.
How is airtightness estimated before the building is finished? You cannot test what hasn't been built yet, so at the design stage airtightness can only be estimated. Estimates draw on three sources: Results from component-level laboratory tests, experience from comparable completed projects, and reasonable safety factors to cover site variability. This is why the final blower-door result can diverge from design assumptions.
What does good airtightness deliver on a completed building? A low, well-controlled n₅₀ or q₅₀ result is associated with several measurable benefits for the finished structure: lower uncontrolled energy losses through the envelope, reduced risk of condensation and associated corrosion, improved internal comfort and acoustic performance, and better long-term durability of the building envelope.
Practical takeaway for site teams: Treat n₅₀ as a verdict on execution, not a product spec. Before the blower-door test takes place, review every junction, service penetration and seal against the design intent — the better each connection is understood and detailed at the design stage, the higher the likelihood the final test will confirm what was intended rather than expose what went wrong on site.
