Blower Door Numbers Explained for Other Trades
Why this matters
On new construction and on a growing share of retrofit jobs, somebody runs a blower door, writes down a number, and the number decides whether the house passes. Electricians, plumbers, HVAC installers, low-voltage crews and insulators all make holes in the envelope, and when the number comes back high, the builder starts asking which trade made them. A sub who can read the result sheet knows whether the problem is real, whether the number was computed on the right volume, and whether their own work is a plausible cause. A sub who cannot is left defending their work against a figure they do not understand.
What the test actually does
A blower door is a calibrated fan mounted in an exterior doorway in an airtight frame. It blows air out of the house (or in, for a pressurization test) until the inside sits 50 pascals below outdoors, then measures how much air it is moving to hold that difference. With the house sealed except for its leaks, every CFM the fan pulls out must come back in through the cracks and holes, so the fan's flow at 50 pascals is a direct measure of how leaky the shell is.
Fifty pascals is far above anything a house sees naturally. It is roughly the stagnation pressure of a 20 mph wind against a flat wall, while stack and wind effects on an ordinary day are a few pascals. The test pressure is set that high on purpose, so the fan's push swamps the weather and the reading repeats from day to day. The scale of a pascal itself belongs to What a Pascal Is and Why House Pressures Are Tiny.
The three numbers on the sheet
CFM50 is the airflow the fan moved at 50 pascals, in cubic feet per minute. It is the raw measurement and it does not depend on anyone's idea of how big the house is. If you only keep one number, keep this one.
House volume is the conditioned volume inside the pressure boundary, in cubic feet. It is not measured by the fan. Somebody computes it from plans or a tape, and the decisions in that computation (is the basement in, are vaulted ceilings counted at full height, is the attached garage out) move the next number by a lot.
ACH50 is air changes per hour at 50 pascals: CFM50 times 60 minutes, divided by the volume. It normalizes leakage to house size so a large house and a small one can be held to one standard. Because it divides by a computed volume, it is only as good as that computation.
A result sheet, read line by line
BLOWER DOOR TEST - single point, depressurized
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Test pressure -50 Pa (house WRT out)
Fan flow at test press. 1,600 CFM50
Conditioned floor area 2,000 sq ft (2 story)
Avg ceiling height 8 ft
Volume used 16,000 cu ft
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ACH50 = CFM50 x 60 / volume
= 1,600 x 60 / 16,000
= 96,000 / 16,000
= 6.0 ACH50
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Basement (1,000 sq ft, 8 ft) not in volume
Basement door OPEN during test
Check the arithmetic both ways: 6.0 air changes of a 16,000 cubic foot house is 96,000 cubic feet per hour, and 96,000 divided by 60 minutes is 1,600 CFM, which matches the fan. The calculation closes. The last two lines on the sheet are where the trouble is.
Worked example: one test, two verdicts
The basement door was open during the test, so the fan was pulling air through the basement's leaks too: the rim joist, the sill plate, the foundation penetrations. That basement leakage is inside the 1,600 CFM50. But the volume line left the basement out. The leakage and the volume describe two different houses.
If the basement is inside the pressure boundary, which it is when it is conditioned or simply open to the house with no insulated, air-sealed floor between them, then the volume should include it. 1,000 square feet at 8 feet adds 8,000 cubic feet, for 24,000 in total. Same fan reading:
- ACH50 = 1,600 times 60 divided by 24,000, which is 96,000 divided by 24,000, or 4.0.
Same house, same fan flow, and the reported tightness moves from 6.0 to 4.0 on one line of the sheet.
Against the energy code. The IECC's mandatory air leakage test, as adopted by your jurisdiction, caps new homes at 5 ACH50 in climate zones 1 and 2 and 3 ACH50 in zones 3 through 8, in the 2012 through 2021 editions (R402.4.1.2 in those editions; the current edition renumbers the section and adds alternative compliance limits, so check it). Some states adopted the code with a different number or an earlier edition, so read the one your jurisdiction adopted.
- In zone 2, the cap is 5. At 6.0 the house fails; at 4.0 it passes. The volume decision is the whole verdict.
- In zone 5, the cap is 3. At 6.0 or at 4.0 the house fails. The volume decision changes how badly, and the shell still needs work.
What a trade does with that. Before anyone starts hunting for leaks, the first question on a failed test is whether the volume matches the space the test actually depressurized. If the basement was open and counted, the volume is right. If it was open and not counted, the result overstates the leakage. If it was shut off behind a closed door and not counted, the test measured a smaller house and both numbers are consistent, though then the basement's own leakage is untested. None of this lets a trade off the hook for real holes, but it keeps a builder from blaming the plumbing for a spreadsheet error.
Why ACH50 is not how much air the house actually changes
The most common misreading by other trades is treating ACH50 as the house's real ventilation rate. It is not. It is leakage under an artificial 50 pascal push. Under ordinary weather the same holes see a few pascals, and the air that actually moves through them is far smaller.
A common rough conversion divides ACH50 by a factor often taken between about 10 and 30, depending on climate, wind exposure and the number of stories, with about 20 a frequently used middle value for a typical one or two story house. On that basis the 4.0 ACH50 house above leaks roughly 4.0 divided by 20, about 0.2 natural air changes per hour, averaged over a year. That is an estimate with a wide band, not a measurement. It is good enough to make the point that a house can be "leaky" by code and still under-ventilated for its occupants, which is why tight houses get mechanical ventilation. See related: Why Air Sealing Changes What a House Needs for Ventilation.
Notice also that the conversion works the same on CFM50: 1,600 divided by 20 is about 80 CFM of average natural leakage, whatever volume anyone picks. The cubic feet per minute leaking through the shell are a property of the holes. The air changes per hour depend on the volume you divide by.
What changes the number between tests
Test setup. Interior doors open, exterior doors and windows shut, fireplace dampers closed, exhaust fans off, combustion appliances turned off or to pilot so the fan does not pull flue gas down a vent, and intentional openings such as a ventilation intake handled the way the governing test standard (ASTM E779, ASTM E1827 or RESNET/ICC 380) says. The setup checklist belongs to How to Prepare a House for a Blower Door Test. A different setup between a rough test and a final test produces two numbers that cannot be compared.
Stage of construction. A test at pre-drywall finds the air barrier before it is buried, and the leaks are easy to fix. A final test includes everything every trade did after that. A drop between them is normal. A rise means something was opened after the rough test: a recessed can, a bath fan, a dryer vent, a new plumbing chase.
Depressurization versus pressurization. Some leaks behave like flaps and pass more in one direction than the other. A test run the other way may read differently, which is one reason the standard procedures specify the method.
What a trade's work does to CFM50
Every trade-made hole that crosses the air barrier adds to CFM50, and the biggest contributors are usually not the obvious ones. A single unsealed bath fan housing, a recessed can that is not airtight-rated, a plumbing stack chase open to the attic, an electrical home run through a top plate drilled oversize, a dryer vent sleeve, a ducted range hood with no damper, and an unsealed attic hatch each add a measurable share. On a house that missed by a small margin, one trade's untreated chase can be the whole miss. The trade that cut through the barrier is the trade best placed to seal it, and the sealing method belongs to How to Close a Penetration You Made Through the Air Barrier.
The useful habit is to ask, for every hole you make through a top plate, a ceiling or an exterior wall, which side of the air barrier you just connected to which. See related: Where the Air Barrier Actually Is in a House.
Reading the sheet with confidence
You have read a blower door result right if you can say where CFM50 came from and whether it changed between tests, what volume was used and whether it matches the space the fan actually depressurized, which code number applies in that climate zone under the edition adopted where the house stands, and what the result does not tell you, namely the house's actual ventilation rate. If the volume line and the test setup disagree, raise it in writing before anyone starts a leak hunt.
References
- International Energy Conservation Code, Section R402.4.1.2 in the 2012 through 2021 editions, as adopted by your jurisdiction
- ASTM E779 and ASTM E1827, and RESNET/ICC 380, the test methods the code points to in recent editions
- See related: What a Pascal Is and Why House Pressures Are Tiny; How to Prepare a House for a Blower Door Test
- See related: How to Find Air Leaks While the House Is Depressurized; Where the Air Barrier Actually Is in a House