How to Prepare a House for a Blower Door Test

Why this matters

A blower door test is a fan mounted in an exterior doorway that pulls the house to a fixed pressure below outdoors, usually 50 pascals, and measures how much air it takes to hold it there. That airflow, CFM50, is the number a code inspector, an energy program or a builder's contract is waiting on. The rater runs the fan, but the house is set up by whoever was there last, and on new construction and remodels that is usually a trade. A house left half-prepared either fails for reasons that have nothing to do with the building, or produces a number nobody can compare to the next test, and the return visit lands on someone's schedule.

This card is the setup side only. What the number means belongs to Blower Door Numbers Explained for Other Trades; finding the leaks once the fan is running belongs to How to Find Air Leaks While the House Is Depressurized.

The one principle behind every step

Prep has one goal: the test measures the building's air barrier as built, and nothing else. Every step below either removes a temporary opening that is not part of the house (an open window, a dry floor drain), keeps something dangerous from happening while the house is pulled negative (a burner firing, ash in a firebox), or keeps the permanent openings in the state the test standard expects so the result is repeatable.

That is why the rule cuts both ways. You close what a person would normally close. You do not tape, plug or stuff anything that is part of the finished house, because that hides a leak the house will have every day after the rater leaves. A taped-over bath fan housing passes the test and leaks for the next thirty years, and if anyone notices the tape, the result is void and the visit is wasted.

Steps

1. Confirm with the rater which standard and which test. Raters work to a written method, most often ASTM E779 or RESNET/ICC 380, and the method decides some details, such as how a ventilation intake or a dedicated combustion air opening is handled. Ask by phone the day before: depressurization or pressurization, single point or multipoint, rough-in or final. Skipping this is how two trades prepare the same house two different ways for a rough test and a final test, which produces two numbers that cannot be compared.

2. Close every window and exterior door, and latch them. A window closed but not latched can sit a sixteenth of an inch open on its sash, and at 50 pascals that gap whistles. Include the garage door to the house, the basement bulkhead, attic hatches and pull-down stairs that lead to unconditioned space. If one will not close (a broken sash lock, a door with no slab yet), tell the rater on arrival rather than taping it; they decide whether it can be temporarily sealed and noted, or whether the test waits.

3. Open every interior door, closets included. The test treats the conditioned space as one volume. A closed bedroom door turns that room into a separate zone, its exterior leaks get measured through a door crack, and the number comes out low. Prop doors that swing shut.

4. Turn off or set to pilot every combustion appliance. Furnace, boiler, water heater, gas fireplace, gas dryer, unvented heater. Set the water heater to pilot or vacation, turn the thermostat off, and switch off the furnace at its service switch. At 50 pascals of depressurization a natural-draft vent will run backward, pulling flue gas and possibly flame out of the burner area into the house. This is not a step that can be judged "probably fine." If an appliance cannot be turned off because nobody can find the control, the rater does not depressurize until it is.

5. Put out and cool every solid-fuel fire, then deal with the ash. No test runs with a fire, embers or a warm pellet stove. Depressurization pulls air down the chimney and into the room, so loose ash in an open firebox blows across the floor. Remove the ash to a lidded metal can outside, or cover it with damp newspaper, and only after confirming it is cold to the hand held just above it.

6. Close fireplace and wood stove dampers, and stove doors. A damper is a normal operating control the occupant closes, so it is closed for the test. A fireplace with no damper or a damper that will not seat is a real leak; leave it and note it.

7. Turn off every fan that moves air across the boundary. Bath fans, range hood, clothes dryer, whole-house fan, a heat or energy recovery ventilator, the air handler. A running exhaust fan fights the blower door and muddies the reading. Shut the dryer off at the machine, not just stopped mid-cycle.

8. Leave intentional openings as you found them. Bath fan grilles, the range hood, the dryer vent, a ventilation intake and supply registers stay as built. The test standard says how each is treated, and some raters set a few of them a particular way. Your job is not to pre-tape them. This is the step trades get wrong most often, usually with good intentions.

9. Fill floor drains, tub and shower traps, and any dry P-trap. On new construction a trap that has never held water is an open pipe to the sewer, and at 50 pascals the rater will hear it. Pour in a quart or two. This is the one plugging step that is legitimate, because a trap with water in it is the house as it will be used.

10. Clear the doorway the rater will use and the path to the attic hatch. They need a door with a frame they can seal the fan panel into, and about six feet of clear floor inside it. They will also go up to look at the attic during the test.

11. Tell the occupants, the crew and the neighbors who share a wall. Nobody opens an exterior door for the length of the test. Pets that can push a door, a tenant coming home, a crew member running in for a tool, each opening resets the reading. In an attached unit, a neighbor's running dryer or kitchen fan can show up in the number.

12. After the test, put the house back, and check each restore. Agree beforehand who does this, the rater or the site lead. The fan comes out of the doorway and the house is back at neutral before any burner is relit. Return each appliance to the setting written on the sheet, then confirm it actually came back: the water heater pilot or burner lights, the furnace fires on a call for heat, and any natural-draft appliance draws at its draft hood once it has run a minute (relighting a pilot or a gas valve is a gas-qualified person's job). Turn fans and the HRV back on, and close any doors you propped. What breaks if skipped: a water heater left on vacation, or a pilot that did not relight, is a no-hot-water call the next morning, and an appliance nobody watched fire is one nobody confirmed is venting.

The sheet you hand the site lead

The checklist travels with the job, filled in by whoever preps the house, and goes to the rater on arrival.

BLOWER DOOR PREP - JOB 4471            TEST DATE: TUE AM
Standard per rater: RESNET/ICC 380, final, depress.

ENVELOPE
[x] All windows closed AND latched (14)
[x] Exterior doors latched; garage-to-house shut
[x] Attic hatch seated, weatherstrip intact
[ ] Basement bulkhead - latch broken, NOTE TO RATER
INTERIOR
[x] All interior and closet doors open, propped
COMBUSTION AND FIRE
[x] Furnace off at service switch, tstat OFF
[x] Water heater on pilot/vacation
[x] Gas fireplace off; wood stove cold, ash removed
[x] Fireplace damper closed
FANS
[x] Bath fans (3), hood, dryer, HRV, AHU all off
INTENTIONAL OPENINGS - LEFT AS BUILT, NOT TAPED
[x] Fan grilles, hood, dryer hood, supplies, intake
TRAPS
[x] 2 floor drains + 1 unused shower filled
PEOPLE
[x] Owner told; dog crated; crew told no door use
Prepped by: J. Ortiz   Open items: bulkhead latch

The open item is the point of the sheet. A rater who arrives knowing the bulkhead latch is broken can decide in a minute; one who discovers it mid-test has a blown reading and a decision to make with the fan roaring.

What one open trade penetration does to the result

Most failed tests are not lost on big holes. They are lost on a small number of openings a trade made and did not close. Here is how that plays out with real arithmetic.

The house: 2,000 square feet of conditioned floor with 8 foot ceilings, so 2,000 times 8 is 16,000 cubic feet of volume. The local code, as adopted (the IECC tested-leakage provision, R402.4.1.2 in the 2021 edition), sets 3 air changes per hour at 50 pascals (ACH50) for this house. Air changes are CFM50 times 60 minutes, divided by volume, so the allowed CFM50 is 3 times 16,000 divided by 60, which is 800 CFM50.

The test reads 840 CFM50. Check it the other way: 840 times 60 is 50,400, divided by 16,000 is 3.15 ACH50. It fails by 40 CFM50, 0.15 ACH50.

The rater walks the attic and finds the plumber's 3 inch vent stack running through a 4-1/2 inch hole in the top plate, unsealed. The open ring is the hole's area minus the pipe's: pi times 2.25 squared is about 15.9 square inches, pi times 1.75 squared (3-1/2 inch outside diameter) is about 9.6, so roughly 6.3 square inches is open to the attic.

How much does that leak at 50 pascals? A rough sharp-edged orifice estimate, using a flow coefficient of 0.61 and air at about 1.2 kilograms per cubic meter, comes to about 7.6 cfm per square inch at 50 pascals. That is an approximation for a clean hole; one stuffed with insulation or with ragged edges leaks less. On that estimate, 6.3 square inches is about 48 CFM50.

Seal it and the house would read about 840 minus 48, which is 792 CFM50, or 792 times 60 divided by 16,000, about 2.97 ACH50, a pass by a thin margin. One unsealed stack was more than the whole 40 CFM50 miss. The re-test is a second trip for the rater, a second fee the builder did not plan on, and a schedule slip for every trade behind it. How to Close a Penetration You Made Through the Air Barrier covers the seal itself.

What changes the answer. On a 4,000 square foot house with the same 3 ACH50 target, the allowance doubles to 1,600 CFM50 and the same stack is a smaller share of it, but a trade rarely leaves just one: four or five unsealed chases scale with the house. In a jurisdiction whose code sets a looser limit, or on a remodel with no tested-leakage requirement, the number is informational and the stack still leaks every day.

Where prep goes wrong in the field

Taping intentional openings to help. A well-meaning trade tapes the bath fans and the range hood the afternoon before. The rater finds it, removes it, and the test runs late, or does not find it and the house passes on a number it will never perform to.

Leaving the dryer mid-cycle. A dryer is about 150 to 200 cfm of exhaust in many homes, a share of a small house's whole CFM50. If someone starts it during the test, the reading jumps.

Vermiculite in the attic. Vermiculite loose-fill, the granular gray-brown or gold insulation, may contain asbestos. Depressurizing a house can pull its dust down through ceiling gaps into the living space. Do not disturb it, and tell the rater when you book the test. Raters commonly test such a house by pressurizing it instead, or not at all, until the insulation is sampled.

A test scheduled before the trades are done. A rough-in test with one trade still to cut its holes measures a house that is about to change. Schedule the final test after the last trade that penetrates the air barrier, including low-voltage and the satellite installer.

How to confirm the house is ready

Walk it once before the rater arrives, with the sheet in hand. Every line is checked or carries a written open item. Every exterior window is latched, by hand. Every burner is off by its own switch, not just the thermostat, and the water heater dial reads pilot or vacation. The firebox is cold and empty. Nothing that belongs to the finished house has tape on it. If any of that is not true, the sheet says so in the open-items line, or the visit is rescheduled before the rater drives out.

References

• ASTM E779, Standard Test Method for Determining Air Leakage Rate by Fan Pressurization, and RESNET/ICC 380, the setup methods raters commonly follow
• IECC R402.4.1.2 (2021 edition numbering; check the current edition), tested air leakage, as adopted by your jurisdiction (the ACH50 limit varies by edition and climate zone)
• See related: Blower Door Numbers Explained for Other Trades
• See related: How to Find Air Leaks While the House Is Depressurized; How to Close a Penetration You Made Through the Air Barrier
• See related: What a Pascal Is and Why House Pressures Are Tiny