0.6 ACH means that under a 50 Pascal pressure difference, the air leaking through the building fabric amounts to no more than 0.6 times the building’s internal volume every hour. It is the Passivhaus airtightness limit, verified with a blower-door test to EN ISO 9972, and it is achieved by design and sequencing, not by any single product.
What does n50 actually measure?
n50 is the air leakage rate of a building held at a 50 Pa pressure difference, expressed as air changes per hour of the internal volume. At 0.6 ACH, the volume of air leaking through the fabric in one hour equals 0.6 of the building’s air volume, with the building deliberately pressurised well beyond normal conditions.
That last point matters. 50 Pa is an artificial test pressure. Nobody’s house experiences it on an ordinary day. The pressure is applied because it swamps wind and stack effects. That makes the measurement repeatable from one test to the next and from one building to another. The n50 result is a property of the fabric, not a measure of how much air moves through the building in normal service.
Why does Passivhaus use air changes instead of permeability?
Because the standard cares about the heating energy carried away by leaking air, and that scales with the volume of air in the building. Hence n50, with internal volume as the denominator.
Many regulatory regimes instead use air permeability, usually written q50: leakage per square metre of envelope area at the same 50 Pa. Same fan, same test, different denominator. The two are not interchangeable, because the ratio of a building’s volume to its envelope area changes with size and shape. A large, simple building and a small, articulated one can post the same q50 and very different n50 figures. Converting between them is a geometry calculation, not a rule of thumb, which is exactly what our airtightness converter does.
n50 vs q50 at a glance
| Metric | Denominator | What it measures | Where it is used |
|---|---|---|---|
| n50 | Internal air volume | Air changes per hour at 50 Pa | Passivhaus: 0.6 ACH Classic, 1.0 ACH EnerPHit |
| q50 | Envelope area | Leakage per m² of fabric at 50 Pa | Common regulatory compliance metric |
How does the blower-door test work?
A calibrated fan is sealed into an external doorway, the deliberate ventilation openings are closed off, and the fan drives the building to a 50 Pa difference, both pressurised and depressurised. The airflow the fan must move to hold that pressure equals the leakage through the fabric. EN ISO 9972 sets out the procedure, and for Passivhaus certification the test result is part of the verified evidence, not a self-declaration. Failing it at completion is one of the most painful outcomes in construction, because by then the leaks are behind the finishes.
How do you actually build to 0.6 ACH?
Airtightness is a continuity-of-layer problem. The question to ask of any design is simple: can you trace one unbroken airtight line around the entire heated volume on every drawing, without lifting the pen? If the line breaks at a junction, that junction leaks.
- Designate the airtight layer and name it. Decide which material does the job in each element: the membrane, the plaster, the board. Ambiguity here becomes leakage later.
- Mark it on every drawing. Every section, every detail, the same coloured line. If a drawing cannot show where the layer runs, the site cannot build it.
- Detail every junction and penetration. Window-to-wall, wall-to-roof, soil pipes, flues, cables. The field of a wall is rarely the problem. The edges are.
- Brief the trades. Most airtightness damage is done by people who were never told the layer existed. Every electrician and plumber who penetrates it needs to know what it is and how to seal around their work. This is a core module of our Certified Passivhaus Tradesperson course.
- Test before finishes close in. Run a first blower-door test as soon as the airtight layer is complete but still exposed. Leaks found now are an hour with tape. The same leaks found after plastering are demolition.
- Retest at completion. The final test to EN ISO 9972 produces the certified figure.
Is 0.6 ACH hard to achieve?
It is far tighter than typical regulatory backstops, and on a leaky-by-default site culture it can look intimidating. In practice it is routinely achieved, including at volume on large housing schemes, by teams that treat the airtight line as a design element rather than a finishing trade. The difference between a building that passes and one that fails is almost never the tape. It is whether the continuity question was answered on paper before anyone opened a toolbox.
And the number is not arbitrary. At 0.6 ACH, uncontrolled leakage is small enough that the ventilation system controls the air quality and recovers the heat, which is what lets an MVHR unit with at least 75% heat-recovery efficiency do its job. Tightness without designed ventilation is a mistake; the two are specified together. A tight envelope also protects the fabric itself, because moist indoor air forced through random gaps in the construction is a moisture risk no membrane manufacturer will warranty against.
Where Mosart fits
Most of what we contribute on airtightness happens at the drawing stage, long before anyone seals a fan into a doorway. If you are converting between n50 and q50 for a live project, use the airtightness converter; if you are taking a building to the certified standard, start with our Passivhaus certification service.