A blower door test measures how leaky your house is. The auditor fits a panel and a calibrated fan into an outside doorway, closes the windows and outside doors, and pulls air out until the house sits 50 pascals below the pressure outside. Air comes back in through every gap, and the gauge measures how much.
During the test
- Inside doors stay open and fireplace dampers are closed.
- The furnace, water heater and gas fireplace are set so they can't fire, then put back afterward.
- With the fan running, an infrared camera shows where the air comes in.
- It can take an hour or more.
Get the house ready
- Put out any wood fire, coals included, and clean out the ashes.
- Clear a way to closets, the attic and any crawl space.
- Tell the auditor about vermiculite attic insulation. Xcel's Squad Plus skips the blower door test in homes with vermiculite or a dirty wood-burning fireplace.
Reading the number
CFM50 is the air the fan moves, in cubic feet per minute, to hold that 50-pascal difference. Higher means leakier. ACH50 is CFM50 × 60 divided by the volume of the house, so you can compare houses of different sizes.
Ask for both in the report. Air sealing is judged against the same number: Xcel's air sealing rebate needs a 20% drop in CFM50.
New homes in Denver
Denver wants a blower door test on new homes, run by a third-party tester, before the certificate of occupancy. Under the 2025 Denver Energy Code, additions don't need one, though the building official can still ask for a third-party inspection. A HERS rater can run it.
Common questions
How much does a blower door test cost in Denver?
Xcel's Squad Plus visit includes one and is $100 for a limited time, normally $150. Independent auditors set their own price, and Xcel pays back 60% of a blower door audit, up to $160.