Airtightness describes how well a building's envelope resists the uncontrolled passage of air. It is not — and this is the single most common confusion — the same thing as ventilation. Airtightness is about eliminating accidental, uncontrolled leakage through cracks, gaps and penetrations. Ventilation is about providing deliberate, controlled fresh air. The Passive House maxim captures the relationship perfectly: build tight, ventilate right.
Every building leaks to some degree. The question is how much, where, and whether that leakage is controlled. Uncontrolled leakage is wasteful (you heat air and immediately lose it), unpredictable (driven by wind and temperature, so it varies hour to hour) and risky (it can carry moisture into the structure). A well-designed building replaces accidental leakage with a tight envelope plus a designed ventilation system.
How airtightness is measured — the blower door
We measure airtightness with a blower door: a calibrated, variable-speed fan sealed into an external doorway using an adjustable panel. The fan pressurises or depressurises the building to a reference pressure relative to outside, and the airflow needed to hold that pressure tells us precisely how leaky the envelope is. The tighter the building, the less airflow is required to reach the reference pressure.
The two metrics: ACH₅₀ and air permeability (q50)
Airtightness results are expressed in one of two ways, and confusing them leads to wildly wrong comparisons. Both are measured at 50 Pa, but they are normalised by different things:
| Metric | Symbol | Normalised by | Units |
|---|---|---|---|
| Air Change Rate | ACH₅₀ (n50) | Internal volume | air changes per hour @ 50 Pa |
| Air Permeability | q50 (qE50) | Envelope (surface) area | m³/(h·m²) @ 50 Pa |
- ACH₅₀ (also written n50) asks: how many times per hour does the entire internal air volume leak out at 50 Pa? It is the metric Passive House uses (≤ 0.6 ACH₅₀).
- Air permeability (q50) asks: how much air leaks per square metre of envelope at 50 Pa? It is the metric UK Building Regulations (Part L) uses, with a measured limit historically around 8 m³/(h·m²).
Why it matters — four reasons
1. Heat loss
Ventilation and infiltration heat loss can account for a large share of a building's total heat loss — in a leaky, well-insulated house it can rival the losses through the walls. Every cubic metre of warm air that escapes is replaced by cold outdoor air that must be reheated. As you improve insulation, air leakage becomes a proportionally bigger slice of what's left, which is why airtightness and insulation must improve together.
2. Comfort and draughts
Cold air leaking in at low level (around skirtings, floor-wall junctions and service penetrations) and warm air escaping at high level (loft hatches, ceiling penetrations) sets up a 'stack effect' draught you feel as cold feet and chilly rooms — even when the thermostat says the air is warm. Eliminating leakage removes that discomfort.
3. Moisture and condensation risk
This is the under-appreciated one. Warm, moist indoor air leaking out through the structure (exfiltration) cools as it travels and can deposit its moisture as condensation inside walls and roofs — interstitial condensation you cannot see until decay or mould appears. Air-transported moisture moves far more water than vapour diffusion does, which is why an airtight, well-ventilated envelope is also a moisture-safe one.
4. Indoor air quality and ventilation efficiency
A leaky building cannot ventilate efficiently. Mechanical ventilation with heat recovery (MVHR) only delivers its rated efficiency if the air it controls actually passes through the heat exchanger; in a leaky house much of the air bypasses the system, so you pay for the fans but lose the heat — and the recovery, filtration and humidity control all suffer.
Build tight, ventilate right
The phrase is not a slogan — it is the whole design logic. Make the envelope airtight to eliminate uncontrolled, wasteful, risky leakage; then design a ventilation system to deliver controlled, filtered, heat-recovered fresh air. The two are a matched pair. Doing one without the other causes problems: airtight without ventilation traps moisture and pollutants; ventilation without airtightness wastes energy and underperforms. We cover that relationship in depth in the dedicated 'airtightness vs ventilation' article in this guide.