Building Science
Air Changes per Hour at 50 Pascals
ACH50
A building's measured air leakage, expressed as the number of times its interior volume would be exchanged per hour while held at 50 pascals of pressure difference. A test condition, not a ventilation rate.
In more detail
Fifty pascals is a convention chosen for repeatability. It is far above the pressures a house experiences from wind and stack effect on an ordinary day — roughly the pressure of a 20 mph wind on all sides at once — which is precisely the point: at that pressure the weather no longer influences the reading, so two tests of the same house agree and two different houses can be compared.
The raw measurement is CFM50, the airflow needed to hold the pressure. ACH50 divides that by the house volume, which makes it a normalized figure but also builds in a bias: dividing by volume flatters large houses, because volume grows faster than enclosure area. A big house and a small one at the same ACH50 do not have equally well-built enclosures. Leakage per square foot of enclosure area is the fairer comparison and the one building scientists tend to prefer, though ACH50 is what codes and programs mostly cite.
Benchmarks give the number meaning. Many current U.S. energy codes require roughly 3 to 5 ACH50 depending on climate zone and edition, older housing commonly tests in the double digits, and the Passive House airtightness criterion is 0.6 ACH50. Below roughly 3, mechanical ventilation stops being advisable and becomes structural.
Why it matters
It is the only enclosure property that gets measured rather than assumed. R-values are specified, assemblies are drawn, and both are believed; airtightness is tested, and the test regularly disagrees with the intent. That makes ACH50 the single most useful number for telling whether a house was built the way it was designed — and if it is measured mid-construction rather than at completion, the disagreement can still be fixed.
The common misunderstanding
That it describes how much the house breathes in service. It does not. ACH50 is measured at an artificial pressure roughly ten to twenty times what the house sees naturally, and the natural air change rate is a small fraction of it — the old rule of thumb divided by somewhere around 20, varying with climate and exposure. A house at 3 ACH50 is not getting three air changes an hour; it is getting a fraction of one, which is why it still needs a ventilation system.
Related terms
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Sources
Last reviewed July 2026 by Compound Editorial. Definitions are educational and not project-specific engineering or medical advice.