A blower door test is how an energy auditor turns “this place feels drafty” into a number — and how they prove, after the work is done, that the air sealing actually worked.
A blower door is a calibrated fan mounted in an adjustable panel that temporarily replaces an exterior door. Once sealed into the door frame, the fan either pulls air out of the house (depressurizing it relative to outdoors) or pushes air in (pressurizing it). A digital manometer measures the resulting pressure difference between inside and outside, in Pascals, while also tracking the airflow the fan is moving. Together, those two readings let a technician calculate how much air is leaking through every unintentional gap in the building envelope — without ever having to find each gap individually first.
The test procedure itself is standardized by ASTM International, primarily ASTM E779, “Standard Test Method for Determining Air Leakage Rate by Fan Pressurization,” with ASTM E1827 covering single-zone testing of individual rooms. Building performance certification bodies — RESNET and the Building Performance Institute (BPI) — base their field procedures on these ASTM methods, which is why the process looks consistent no matter which certified technician performs it.
ASHRAE plays a related but distinct role. ASHRAE Standard 62.2, “Ventilation and Acceptable Indoor Air Quality in Residential Buildings,” doesn't define how the blower door test itself is run — but it uses the test's results. Once a home's air leakage rate is measured, ASHRAE 62.2's calculation determines how much of the home's fresh-air ventilation need is already being met by natural infiltration, and how much mechanical ventilation (an exhaust fan, an ERV/HRV) needs to make up the difference. A very tight, well-sealed and well-insulated mobile home can end up needing mechanical ventilation it didn't need before the retrofit — which is one reason a qualified energy auditor evaluates air sealing and ventilation together rather than air sealing in isolation.
Close all exterior windows and doors, open interior doors so the whole house acts as one zone, close fireplace and woodstove dampers, turn off the HVAC system, and flag any combustion appliances (gas water heater, propane furnace) for a safety check.
The blower door frame and adjustable panel are fitted into an exterior doorway, and the calibrated fan is mounted in the panel with its speed controller and pressure tubing connected.
A digital manometer is set up with one pressure tap referencing the inside of the home and one referencing outside air, so it can continuously read the pressure difference the fan creates.
The fan speed is ramped up until the manometer holds a steady 50 Pascal pressure difference — the industry-standard reference pressure, roughly equivalent to a 20 mph wind hitting every side of the house at once.
The fan's flow rate at that pressure — CFM50, or cubic feet per minute at 50 Pascals — is read directly or calculated from the fan's calibrated flow curve. This is the home's total measured air leakage rate.
With the house held under negative pressure, the technician walks the interior with a smoke pencil or infrared camera. Outside air rushing in through gaps becomes easy to feel, see, or image — around window and door flanges, electrical and plumbing penetrations, the attic hatch, and, in a mobile home, the marriage line and belly cavity access points.
CFM50 is converted to air changes per hour at 50 Pascals: ACH50 = (CFM50 × 60) ÷ conditioned home volume in cubic feet. This normalizes the result so homes of different sizes can be compared on the same scale.
The ACH50 result is checked against current energy code requirements, the home's pre-retrofit baseline (if this is a post-upgrade verification test), and the ventilation calculation in ASHRAE 62.2 to determine whether the home needs more air sealing, more mechanical ventilation, or both.
| ACH50 | Rough characterization |
|---|---|
| 15+ | Very leaky — common in older, un-retrofitted mobile homes with a damaged belly board or unsealed marriage line |
| 7–15 | Leaky to average for an older home; air sealing will typically deliver a noticeable comfort and cost improvement |
| 3–7 | Reasonably tight; approaching the range required by current energy codes for new construction |
| Under 3 | Very tight; mechanical ventilation is generally required to maintain indoor air quality |
These bands are general reference points, not a formal code table, and the appropriate target depends on the home's size, exposure, and existing ventilation. A technician typically also converts the 50-Pascal result into a natural infiltration estimate (natural ACH is roughly ACH50 divided by a shielding-and-climate factor, commonly in the 15–20 range for Maine's exposure and terrain) to estimate how the home actually behaves under everyday wind and stack-effect conditions, rather than under the artificial 50-Pascal test pressure.
Every cubic foot of heated indoor air that leaks out has to be replaced by a cubic foot of cold outdoor air leaking back in somewhere else — and that incoming air has to be heated from outdoor temperature back up to room temperature. Unlike R-value, which describes a slow, steady conductive loss, air leakage moves heat in bulk, and it also carries moisture with it. In a mobile home, high air leakage is frequently concentrated in a few specific, findable locations: the belly cavity where ductwork and plumbing penetrate the floor, the marriage line on a double-wide, and the perimeter where the home's floor structure meets the exterior walls.
That's why a pre-retrofit blower door test does double duty: it quantifies how much of the home's heat loss is coming from air leakage versus conduction, and it pinpoints exactly where crews should focus air sealing before insulation goes in. A second, post-retrofit test then verifies the work actually reduced the leakage rate — the same before-and-after comparison Efficiency Maine's registered vendors use to document a completed project.