Blower door test
A blower door test measures building air leakage by using a calibrated fan, temporary door panel, and pressure gauge to pressurize or depressurize a building. The result helps builders, auditors, and homeowners understand airtightness, find leaks, verify code targets, and plan air-sealing work.
What a blower door test is
A blower door test is a controlled way to measure how much air leaks through a building enclosure. A technician mounts a calibrated fan in an exterior doorway, seals the remaining doorway with a temporary panel, and uses a pressure gauge to measure the pressure difference between indoors and outdoors. The fan moves air until the building reaches a target pressure, commonly 50 pascals, and the airflow needed to hold that pressure becomes the leakage measurement.
How the test works
The test can depressurize the building by blowing indoor air outward, or pressurize it by blowing air inward. Depressurization often makes leaks easier to find because outdoor air is pulled through cracks and gaps. Before testing, technicians may close windows, open interior doors, set combustion appliances safely, prepare dampers, and document which intentional openings are left as normal operating conditions or temporarily sealed according to the test protocol.
CFM50 and ACH50
CFM50 means cubic feet per minute of airflow at a 50-pascal pressure difference. It is an airflow number, so larger buildings often have larger CFM50 values even if they are relatively tight. ACH50 converts that leakage to air changes per hour at 50 pascals by using the building volume. ACH50 makes buildings easier to compare, but it can still be misleading when buildings have very different sizes, shapes, or enclosure areas.
Finding leaks
A blower door is also a diagnostic tool. While the fan is running, technicians can use smoke pencils, theatrical fog, infrared cameras, pressure pans, anemometers, or simple hand checks to locate leakage paths. Common sites include attic hatches, rim joists, electrical penetrations, plumbing chases, recessed lights, window rough openings, duct chases, fireplaces, sill plates, and connections between garages and living spaces.
Code and performance targets
Many energy codes and green-building programs use blower-door results to verify airtightness. The exact target depends on jurisdiction, building type, climate zone, program, and test standard. A passive-house project may aim for a much tighter result than a code-minimum home. The number should be read in context: it shows leakage under test pressure, not natural air exchange under every weather condition.
Why timing matters
Testing during construction can catch leaks while they are still accessible. A rough-in or pre-drywall test can reveal missing air-barrier connections, unsealed penetrations, and framing bypasses before finishes hide them. A final test verifies the completed building, but repairs may be harder. Large projects may use zone-by-zone testing or guarded tests to isolate specific parts of the enclosure.
Safety and preparation
Blower door testing changes building pressure, so technicians need to consider combustion appliances, fireplaces, attached garages, radon systems, exhaust fans, ventilation equipment, and fragile interior conditions. In existing homes, a test may be paired with combustion-safety checks and ventilation assessment. Very leaky, dusty, smoky, or mold-damaged spaces may need extra care before depressurization.
Limits of the test
A blower door test measures enclosure leakage, not insulation quality, indoor air quality, duct leakage by itself, or whether ventilation is adequate. It does not automatically tell which leak should be fixed first unless diagnostics are performed. Weather, baseline pressure, setup choices, building volume estimates, and test method can affect results, so reports should document assumptions and conditions.
Why it matters
Blower door testing matters because airtightness is hard to judge by sight. A building can look finished while hidden bypasses waste energy, move moisture, create drafts, and undermine comfort. Testing gives builders and owners a number, but more importantly it turns invisible air paths into problems that can be found, prioritized, and fixed.