Building Science
Blower-Door Test
Also known as fan pressurization test
A diagnostic that measures how leaky a building is by sealing a calibrated fan into an exterior doorway, holding the house at a set pressure difference, and recording the airflow required to maintain it.
In more detail
The fan mounts in a fabric panel in an open doorway and depressurizes the house, typically to 50 pascals below outdoors. The airflow needed to sustain that difference is the leakage: a house that needs 2,000 cfm has twice the effective leakage area of one that needs 1,000. Interior doors are opened so the whole conditioned volume is connected, combustion appliances are shut down, and fireplaces are dealt with before the fan starts.
The number is the smaller half of the value. With the house depressurized, every leak becomes a detectable inbound draft, so an infrared camera, a smoke pencil, or a bare hand will locate them. That turns an abstract figure into a punch list — this top plate, that chase, those four can lights.
Timing determines whether the test is useful or merely documentary. Run at the end for code compliance, it produces a number and a certificate. Run once after the air barrier is installed and before insulation and drywall, it produces a list of leaks that can still be reached and sealed, then a second test to confirm the sealing worked. Same equipment, entirely different value.
Test procedures are standardized — ASTM E779 and ISO 9972 are the usual references — which is what makes results comparable between operators and over time.
Why it matters
It is the mechanism that converts an intended enclosure into a verified one, and it is cheap relative to what it catches. It also has a safety function that gets overlooked: the same equipment, run with exhaust appliances operating, reveals whether the house can be depressurized enough to backdraft a water heater or furnace. That is a life-safety finding, not an energy finding, and nothing else in a residential handover produces it.
The common misunderstanding
That it is a final exam. Treated that way it produces a number nobody can act on, because by the time the test happens every leak it found is behind finished surfaces. The test earns its fee in the middle of construction. A single test at completion tells you what you built; a test before insulation lets you change it.
Related terms
On Compound
Where this term does real work — and where the decisions get made.
Sources
Last reviewed July 2026 by Compound Editorial. Definitions are educational and not project-specific engineering or medical advice.