Airtightness Explained
Airtightness is the foundation of every high-performance home. Here's what it means, how it's measured, and what you should be targeting.
What is airtightness?
Airtightness refers to how little uncontrolled air leaks through a building’s envelope — the walls, roof, floor and openings. A more airtight building has fewer gaps and cracks through which air can move uncontrolled.
Airtightness is different from breathability. A well-insulated, airtight home can still be healthy and well-ventilated — it just achieves ventilation through controlled means (like MVHR) rather than through uncontrolled leakage.
How is it measured?
Airtightness is measured using a blower door test. A calibrated fan is fitted into an external doorway and used to pressurise the building to a reference pressure of 50 Pascals. The airflow required to maintain this pressure is then converted into a standardised metric.
The most common metric is ACH50 — Air Changes per Hour at 50 Pascals. It expresses how many times the building’s air volume leaks out per hour when pressurised.
Typical airtightness benchmarks
Why does airtightness matter?
- Energy efficiency: Air leakage is one of the largest sources of heat loss in buildings. Reducing leakage directly reduces heating and cooling loads.
- Draught elimination: Draughts are caused by air moving through gaps. An airtight building is a draught-free building.
- Moisture control: Air carries moisture. Uncontrolled air movement through the building fabric can deposit moisture in walls and cause condensation and mould.
- Acoustic performance: Airtight construction also reduces sound transmission — particularly important for homes near roads or flight paths.
- MVHR effectiveness: MVHR only works as designed in an airtight building. In a leaky building, the MVHR system is fighting against uncontrolled air movement.
Achieving airtightness
Airtightness is achieved by making one continuous layer around the building envelope that is carefully sealed at every junction, penetration and connection. There are two common approaches — often used in combination depending on the construction type.
Airtight membrane (e.g. Intello Plus)
A dedicated airtight membrane — such as Proclima Intello Plus — is installed on the warm side of the insulation and lapped and taped at all edges and penetrations. This is the most reliable approach for timber-framed construction and is standard practice for Passive House builds.
Plasterboard as the airtight layer
In masonry or slab construction, the plasterboard itself can form the airtight layer — provided all gaps at perimeters, penetrations and junctions are carefully caulked or taped. This requires close attention to detail at every electrical, plumbing and structural penetration through the lining.
HiPer Haus can assist with airtightness strategy from early design stage, and carry out blower door testing to verify construction quality and identify any remaining leakage paths.
Verifying airtightness — the blower door test
Airtightness can’t be verified by eye. The only reliable way to know how airtight a building actually is — and where it’s leaking — is a blower door test. A calibrated fan depressurises the building to 50 Pascals and measures the airflow required to maintain that pressure, giving an objective ACH50 result. At the same time, a smoke pencil is used to locate the specific leakage paths.
Testing mid-construction (before plasterboard lining) is the most cost-effective approach — leaks can still be sealed before they’re buried in the structure. A final test at completion provides a documented result for Passive House certification, NCC compliance, or handover records.
Our blower door testing service →