What is a blower door test?

A calibrated fan is sealed into an external doorway and used to hold the inside of the house at a set pressure below the outside. How hard the fan has to work to maintain that is a direct measure of how much the house leaks. It takes an hour and a half to two hours and works in any season.

What is a good blower door result?

It depends on the size of the house, which is why the raw airflow is usually converted into air changes per hour — the volume of the house, how many times over, every hour. A well-sealed modern build sits in the low single digits; an older house can be several times that. What matters more is the difference between a reading before work and the same reading after.

Will a blower door test find my drafts?

Not by itself, and that is the honest limit of it. It measures how much air moves in total, not where it moves. Finding where takes walking the house while the fan runs, usually with infrared, which is when leaks you could never feel become obvious.

Do I need the test done twice?

If any air sealing is going to follow, yes — and that is an argument against our own margin. A single reading with nothing to compare it against cannot tell you whether the work changed anything. Feeling warmer afterward is not evidence; the second test is.

Can it be done in summer?

Yes. The measurement works in any weather. Infrared imaging alongside it gives a clearer picture when there is a decent temperature difference between inside and out, so a cold morning shows more, but the number itself does not care.

A Blower Door Number Means Nothing on Its Own

Q: Should anything be checked after air sealing?

Yes. Tightening a house reduces the leakage that used to supply replacement air to gas appliances, so a water heater or furnace that drafted adequately before can struggle afterward. Combustion safety testing belongs alongside air sealing, not instead of it.

The number people get sold

A blower door test produces a figure. A fan is sealed into a doorway, the house is brought to a set pressure below the outside air, and the equipment reports how hard the fan had to work to hold it there. Leaky house, lots of fan. Tight house, very little.

What usually gets handed over is that figure, on its own, with a sentence saying whether it is good or bad. And on its own it is very close to useless — not wrong, just not usable. Here is why, and what makes it worth paying for.

A number with no second number

Suppose your house tests at some value. Is that good?

You cannot answer that without knowing how big the house is, because a large house legitimately leaks more air than a small one at the same level of tightness. That is why the raw airflow usually gets converted into air changes per hour — the whole volume of the house, how many times over, every hour, while the fan holds that pressure. A well-sealed modern build sits in the low single digits. An older house with original windows and an unsealed attic hatch can be several times that.

So the figure already needs one piece of context before it means anything. But that is the small problem.

The real problem: nothing to compare it against

The question nobody can answer from a single reading is the one you actually care about: did the work I paid for change anything?

A test before the air sealing and the same test afterward answers that precisely. The house either moved or it did not, and the difference is a measurement rather than a conversation about whether it feels warmer. Feeling warmer is not evidence — October is warmer than January, a new furnace changes how a house feels, and everybody who has just spent money is inclined to notice an improvement.

This is an argument against our own margin and we make it anyway: a single test with no re-test is a number with nothing to compare it to. If any sealing work is going to follow, book both or do not bother with the first.

It tells you how much. It does not tell you where.

This is the honest limit of the equipment and it is the thing most worth understanding before you buy one.

The fan measures total leakage across the entire building. It has no idea whether that air is coming through the attic hatch, around the rim joist, under the back door or through sixteen recessed lights. Anyone handing you a number and calling it a diagnosis is selling you half the job.

Finding where is a different activity that happens while the fan runs: walking the house under pressure, where leaks you could never feel on a still day become obvious against the back of your hand, and infrared imaging alongside it, which shows cold air tracking into the building in places nobody would have thought to check. The fan creates the conditions. The walk-through and the camera do the finding.

What it is not

It is not a mold test, a radon test or an air quality test. It does not inspect your furnace. It does not tell you how much insulation you have. It measures air movement, and that is all it measures — which is plenty, as long as nobody implies otherwise.

One thing that must follow the work, not the test

Tightening a house reduces the accidental leakage that used to supply replacement air to gas appliances. A water heater or furnace that drafted adequately before the work can struggle afterward, with nothing about the appliance having changed.

That is not a reason to skip air sealing. It is the reason combustion safety testing belongs alongside it, which is why rebate programs require it and why we wrote separately about water heaters that only backdraft when the dryer is on and furnaces that pass a service and fail a safety test.

Before a test, do nothing

Genuinely. The commonest mistake is a burst of weather stripping the week before, which measures a house you do not live in. The point is to record the building as it actually is, so the plan afterward is built on the truth.

Two things that do help: have the last twelve months of energy bills to hand, and decide in advance whether you want the before-and-after. Also expect the house to feel cold while the fan runs — it is pulling outside air through every leak at once, which is the entire idea.

Where this stands today

Our BPI Building Analyst certification is in progress through the Consumers Energy program at Jackson College and completes in November 2026. It is not held yet and is not claimed as held anywhere on this site.

When it is: blower door testing across Greater Lansing, on its own or as part of a full home energy assessment — and with the re-test, because that is the half that proves anything.