A blower door test is a standardised method for measuring how airtight a building is. A calibrated fan is temporarily sealed into an exterior doorway and used to create a pressure difference between the inside and outside of the building. The airflow required to maintain that pressure difference reveals how much uncontrolled air leakage exists in the building envelope.
How it works
The test equipment consists of three components: a variable-speed fan, an adjustable panel that fits into a doorway frame, and a pressure gauge (manometer) connected to a data-logging device. During the test, all intentional ventilation openings — such as trickle vents and extractor fans — are sealed. The fan then pressurises or depressurises the building to a reference pressure, typically 50 Pascals above or below ambient.
The manometer measures the pressure difference while the fan records the volume of air it moves. The result is expressed as air changes per hour at 50 Pa (n50) or as air permeability in cubic metres per hour per square metre of envelope area (q50). A lower number indicates a tighter building.
Application
Blower door tests are required by building regulations in many countries as part of energy performance compliance. Standards such as Passive House demand extremely low leakage rates — typically below 0.6 air changes per hour at 50 Pa. Standard new-build requirements in Europe range from 3 to 6 n50 depending on the country and building type.
The test is valuable not only for compliance but also for diagnostics. While the building is pressurised, technicians can use smoke pencils, thermal imaging cameras, or their hands to locate specific leakage points around windows, doors, service penetrations, junctions between walls and floors, and around electrical outlets. Identifying and sealing these leaks improves comfort, reduces energy consumption, and prevents moisture-related damage within the building envelope.
Related terms
- Air barrier
- Thermal bridge
- Passive House (Passivhaus)
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