Airtightness describes how well a building's envelope resists the uncontrolled passage of air. It is not the same as ventilation — a distinction that causes endless confusion. Airtightness is about stopping accidental, uncontrolled leakage; ventilation is about providing deliberate, controlled fresh air. The Passive House maxim captures it: 'build tight, ventilate right'.
We measure airtightness with a blower door — a calibrated fan sealed into an external doorway that pressurises or depressurises the building to a reference pressure of 50 pascals. The fan flow needed to hold that pressure tells us exactly how leaky the envelope is, expressed as air changes per hour (ACH₅₀) or as air permeability (m³/h·m² at 50 Pa). The tighter the building, the less airflow is required.
Why airtightness matters
Air leakage is invisible but expensive, and its consequences go well beyond energy. In a well-insulated building, uncontrolled leakage can rival the heat lost through the walls — and as you improve insulation, leakage becomes a proportionally bigger slice of what remains. But the under-appreciated role of airtightness is moisture safety: warm, moist indoor air leaking out through the structure carries far more water into walls and roofs than vapour diffusion ever does, risking hidden interstitial condensation, rot and mould.
| Building | Air permeability (q50) | Comment |
|---|---|---|
| Old, un-refurbished housing | 10–15+ | Very leaky; draughty, hard to heat |
| Typical UK new build (Part L) | ~5 | Meets regs; still relatively leaky |
| Good airtight new build | 1–3 | Deliberate air-barrier strategy |
| EnerPHit retrofit (≈ 1.0 ACH₅₀) | ~1–1.5 | Passive House retrofit standard |
| Passive House (≤ 0.6 ACH₅₀) | below ~1 territory | Order of magnitude tighter than regs |
What this guide covers
Work through the cluster below for the full picture — what airtightness is and how it's measured, how the blower door test works and how to read its results, the UK regulations and Passive House targets, where buildings actually leak, how we locate those leaks with smoke and thermal imaging, how to design a continuous air barrier, and why airtightness is ultimately a moisture-safety measure as much as an energy one.