A blower door test explained in one line: a calibrated fan mounted in an exterior doorway pulls air out of the house until the indoor pressure drops by 50 pascals below outside, and a gauge measures how much air the fan has to move to hold that pressure. That reading is called CFM50, and it is the auditor's stand-in for "how much your house leaks." Divide CFM50 by the house's volume and the result, called ACH50, is the number that codes and rating programs actually compare, because it accounts for house size. Lower is tighter. The test itself only finds the total; a smoke pencil or thermal camera run during the same test finds where.
What a blower door test actually measures
The blower door does not measure insulation, and it does not measure comfort directly. It measures one thing: the total, combined air leakage of the building envelope, meaning every gap, crack, and unsealed penetration in the walls, floor, and ceiling added together, tested as a single number under a controlled pressure difference. The industry standard for how this is done, ASTM E779, defines fan pressurization testing so that one house's result can be compared to another's, tested on a different day, by a different technician, with different weather outside.
The fan itself sits in an adjustable frame that seals into the door opening, usually the front or back door, with a flexible panel closing off the rest of the frame around it. A digital manometer, a small pressure gauge with a hose running outside, reads the pressure difference the fan creates and the airflow needed to sustain it. Everything else in the house stays as it normally would: interior doors open, so the whole conditioned volume is being tested as one zone, while known intentional openings like a working exhaust fan duct may be temporarily sealed so the reading reflects the envelope and not the ductwork.
How the test works, and the safety step that comes before the number
Running a fan that depressurizes an entire house is not neutral to anything else that shares the same air. If your home has a gas water heater, a furnace, a boiler, or a fireplace that vents by natural draft rather than a sealed power vent, pulling the house pressure down can pull those vents backward. The Department of Energy calls this backdrafting, and the practical risk is combustion byproducts, including carbon monoxide, moving into the living space instead of up the flue. Carbon monoxide has no color and no smell, so a working CO alarm on every level is the minimum, not the safeguard, during any test that changes house pressure.
This is the reason a certified home energy auditor runs a combustion safety check as part of the visit, not as an afterthought: verifying that atmospherically vented appliances still draft correctly once the test is done, and knowing when to stop and call in an HVAC technician or licensed gas fitter if they do not. If you are hiring the test, ask whether combustion safety testing is included. If you are running a basic DIY depressurization check yourself with a window fan, per the Department of Energy's guidance on detecting air leaks (accessed 2026-09-18), turn off combustion appliances first and do not leave a fireplace or wood stove burning during the test.
Reading the number: CFM50 and ACH50
CFM50 is cubic feet of air per minute moving through the fan at the 50-pascal test pressure. On its own it tells you almost nothing, because a 4,500 cubic foot house and a 24,000 cubic foot house can post the same CFM50 and be nowhere near equally leaky. That is what ACH50 fixes: it is CFM50 multiplied by 60, then divided by the house's conditioned volume in cubic feet, giving the number of times the entire volume of air inside would theoretically be replaced by outside air in one hour, at that test pressure. As an illustration only, not a real result: a 1,000 CFM50 reading in a 15,000 cubic foot house works out to 4 ACH50, while the same 1,000 CFM50 in a 6,000 cubic foot house works out to 10 ACH50, a much leakier house for the identical fan reading.
Auditors also sometimes report CFM50 per square foot of envelope area (walls, ceiling, and floor combined) rather than by volume. Either normalization exists for the same reason: raw CFM50 cannot be compared between two different houses, only ACH50 or the per-area figure can.
What counts as tight, and why the code table isn't the whole answer
Multiple editions of the International Energy Conservation Code have set a maximum ACH50 for new construction, tightened over successive code cycles, with recent editions moving toward stricter limits than the code required a decade ago. ENERGY STAR's own certified new home requirements likewise set ACH50 thresholds that vary by climate zone. None of that is a number this article will assign to your house, because code adoption is a state and local decision: many jurisdictions amend the model code or adopt an older edition, and some have not adopted an energy code at all. Your local building department, not a blog post, is the authority on which number applies to your address and which permit, if any, your project needs.
What the report should give you regardless of which code applies is the comparison that matters for a homeowner deciding whether to seal: the technician's professional judgment of whether your ACH50 sits in a leaky, average, or tight range for a house your age and size, stated as a range, not a pass or fail grade borrowed from a new-construction code that was never written for an existing home.
The blower door test explained: what the number still won't tell you
This is the part most explainers skip. The number is real, but it has real limits.
It does not say where the leaks are. Two houses with an identical ACH50 can have completely different leak maps, one concentrated in the attic and rim joist, one spread through window and door weatherstripping. Finding location requires walking the house with a smoke pencil or infrared camera while the fan is still running, which is a separate step from getting the number.
It does not measure insulation. A house can test reasonably tight and still be under-insulated, or the reverse: heavily insulated with batts that were installed around, not against, every wire and pipe penetration, leaving the air leakage high. Airtightness and insulation are two different jobs done in a particular order, sealing first, because insulation installed over an unsealed gap does not stop the air moving through it.
It is sensitive to test conditions. Wind, indoor-outdoor temperature difference, and which doors and dampers were open or closed at the time all shift a reading by a meaningful margin if the test protocol wasn't followed carefully, which is part of why ASTM E779 specifies the setup rather than leaving it to preference.
DIY blower door kits versus a certified test
Consumer-grade blower door fans exist, and renting or borrowing one to run your own before-and-after check while you air seal is a legitimate way to see whether your work is having an effect. Where it stops being a substitute for a certified test: qualifying for a weatherization assistance program, a HERS rating, or documenting compliance with a building code all require a test performed and reported by a certified professional, typically a RESNET-certified rater or a BPI-certified building analyst, using calibrated equipment and the ASTM E779 protocol. A number from an uncalibrated consumer fan, read without correcting for wind and temperature, will not match, and shouldn't be presented as, a certified result. Use the DIY version to track your own before-and-after; use a certified test for anything that needs to be reported to someone else.
What to do with your results
A blower door result is a starting point, not a finished project. If the technician's leak-mapping pointed at the attic plane, the practical next step is usually sealing the top of the house before adding more insulation over the leaks, which is exactly the sequence covered in our guide to insulating a rim joist, one of the highest-leakage spots in most homes with an unfinished basement or crawl space. If the leaks concentrated around trim, baseboards, and small gaps rather than large bays, the fix is usually a sealant job matched to gap size, which is what we cover in matching caulk, foam, or backer rod to the size of the gap. A bonus room over a garage that never holds temperature often shows up on the test too, and that has its own cause, covered in knee wall insulation and why a bonus room never warms. If your crawl space is part of the tested volume, how it is currently insulated matters for the number, which our comparison of a vented versus an encapsulated crawl space walks through.
Whatever the report shows, the payoff for sealing and insulating together is documented, not guessed at: ENERGY STAR (accessed 2026-09-18) estimates that homeowners can save an average of 15 percent on heating and cooling costs, and about 11 percent on total energy costs, by air sealing the home and adding insulation in attics, floors over crawl spaces, and basements. That estimate is an average across many homes, not a promise for any one house, and your own result depends on your starting point, which is exactly what the blower door number establishes.
Frequently asked questions
What is a good blower door test score?
There is no single pass or fail number that applies to every house, because "good" depends on house age, size, and which code, if any, your jurisdiction has adopted. A certified auditor should compare your ACH50 to homes of similar age and size and explain the range, rather than hand you a bare number.
Is it safe to run a blower door test in a house with a gas furnace or water heater?
Yes, when a certified auditor runs a combustion safety check alongside it. Depressurizing the house can backdraft an atmospherically vented appliance, so the technician checks that vents draft correctly once the test ends, and working carbon monoxide alarms should already be installed regardless.
Can I run a blower door test myself?
A consumer-grade fan can give you a useful before-and-after comparison while you air seal. It won't match a calibrated, certified test, and it isn't accepted for a weatherization program, a HERS rating, or code documentation, all of which require a certified tester and the ASTM E779 protocol.
Does a tight blower door result mean my insulation is fine?
No. Airtightness and insulation are measured and installed separately. A house can test tight and still be under-insulated, or well insulated with air still leaking around every penetration in the batts.
How long does a blower door test take?
The test itself, once the fan is set up, typically takes a portion of a visit that also includes walking the house to find leak locations and, where relevant, a combustion safety check, rather than being a single quick reading and nothing else.
Sources
- U.S. Department of Energy, Energy Saver, guidance on detecting air leaks and DIY depressurization testing, accessed 2026-09-18: energy.gov/energysaver/detecting-air-leaks
- ENERGY STAR, "Why Seal and Insulate," savings estimate for air sealing and insulating a home, accessed 2026-09-18: energystar.gov/saveathome/seal_insulate/why-seal-and-insulate
- ASTM E779, Standard Test Method for Determining Air Leakage Rate by Fan Pressurization, referenced by name for the test protocol; standard text is not freely hosted, cited by title only.
Code adoption and enforcement vary by state and locality. Ask your local building department which energy code edition, if any, applies to your address.