What is a blower door test?

A calibrated fan is sealed into an external doorway and used to lower the air pressure inside the house. How hard the fan has to work to hold that pressure is a direct measure of how much the house leaks. It takes an hour and a half to two hours and can be done in any season.

What does the blower door number actually mean?

On its own, not much. It means something next to the size of the house, and next to the same house after work has been done. That is why the useful version of this test is run twice — before and after. A single reading with nothing to compare it to is a fact without a use.

Will it tell me where my drafts are?

Not by itself, and that is its honest limit. It measures how much air is moving in total. Finding where takes walking the house while the fan runs, with infrared alongside, which is when leaks you could never feel on a still day become obvious at the back of your hand.

Can a blower door test be done in summer?

Yes. The measurement works in any weather and at any time of year. Infrared imaging alongside it works best when there is a real temperature difference between inside and out, so a cold morning gives a clearer picture, but the test itself does not care.

Do I need a re-test after the work?

If you want to know whether the money did anything, yes. The before reading and the after reading are what make either of them meaningful. If a quote includes a test but no re-test, it is worth asking why.

What does a blower door test cost?

Standalone, it is quoted in this market at $100 to $350 as of October 2026, depending on the size of the house and how many openings have to be sealed and accounted for. As part of a full energy assessment it is included rather than priced separately.

A Blower Door Number Only Means Something Twice

What the machine is

A blower door is a fan in an adjustable frame that seals into an external doorway. It pulls air out of the house until the inside sits at a set pressure below the outside, then measures how hard it has to work to hold it there.

A leaky house takes a lot of fan. A tight one takes very little. That is the whole idea, and it is the only method that turns "it feels drafty in here" into something two people cannot argue about.

The test takes an hour and a half to two hours. It works on any house and in any season.

The number by itself is nearly useless

This is the part the industry is quiet about, so here it is plainly: a single blower door reading, handed to you on a sheet, is a fact with nothing to compare it to.

It means something in two situations and no others:

Next to the size of the house. The raw airflow figure has to be normalized — against floor area, or volume, or envelope surface — before "4,200" tells you whether that is bad for a 1,100 square foot bungalow or good for a 3,400 square foot colonial.
Next to the same house after work. This is the one that actually matters.

So the useful version of this test is run twice: once before anybody touches anything, once after the air sealing is done. The second number is what tells you the money did something. Without it you are back to a feeling, which is where you started, except now you have paid for a feeling.

If you are having air sealing done and the quote includes a test but not a re-test, ask why. It is a reasonable question and the answer should not be complicated.

How much, not where

The honest limit of a blower door is that it measures total leakage. It will tell you your house loses a given volume of air per minute. It will not tell you that most of it is going out through the attic hatch and around the chimney chase.

Finding where takes two things done while the fan is still running:

Walking the house under pressure. With the fan pulling, leaks you could never feel on a still day become obvious at the back of your hand. This is the single most useful twenty minutes of the whole visit.
Infrared alongside it. The camera shows cold tracking in through the building rather than air moving past your hand, so it catches missing and slumped insulation that is not a leak at all.

Infrared works best when there is a decent temperature difference between inside and out, so a cold morning gives a clearer picture. The blower door measurement itself does not care what the weather is doing.

Why the furnace gets tested too

Any serious air sealing changes how much air is available to appliances that burn gas. A furnace or a water heater with a natural draft relies on air it can pull from the room to carry exhaust up the flue. Tighten the house far enough and that supply is no longer guaranteed.

That is why combustion safety testing — draft, spillage, carbon monoxide — belongs with this work rather than being an extra somebody offers. You do not seal a house blind. Where the testing finds a fault, the repair is a licensed trade and goes to a licensed heating contractor; the testing itself is part of doing the job properly.

What it is not

Not a mold test. Different measurement, different trade.
Not a radon test. Radon is measured over days with its own equipment.
Not an air quality test. It measures air movement, not what is in the air.
Not a verdict on your windows. It measures the whole envelope at once and does not attribute leakage to a component.

Where this stands today

This is a new service with no completed jobs behind it yet, and that is said here rather than left for you to discover. BPI Building Analyst certification is in progress and is not held — it completes in November 2026. Where a rebate program requires a tested result from a certified Building Analyst, that certification is the qualifying step; the energy rebates page states where each program stands.

Four things before a test

Do not weather strip anything the week before. The point is to measure the house as it actually is. Tidying up the obvious leaks first gives you a flattering number and a worse plan.
Decide whether you want a before-and-after. If sealing work is planned, a test with no re-test is a number with nothing to compare it to.
Have twelve months of energy bills to hand.
Expect the house to feel cold while the fan runs. It is pulling outside air in through every leak in the building at once. That is not a problem with the test; that is the test.

Book it

Book a free quote · call 517-505-7135 · or read more about a blower door test.
The full survey it sits inside: home energy assessment. What the result usually points at: air sealing, then attic insulation. Why that order: your house is a chimney.
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