What a Blower Door Test Actually Tells You About Your Home

When I set up a blower door in the front door of a house for the first time, homeowners usually react one of two ways. Some are curious — they want to know exactly what the machine does and what the numbers mean. Others are skeptical — they've been told their house is "pretty tight" and they don't see what a fan in the door frame could possibly tell them that they don't already know.

By the time I walk them through the results — and, more importantly, by the time I show them where the air is actually coming from while the fan runs — the skeptics are usually the more engaged ones. There is something viscerally convincing about holding your hand over an electrical outlet on an exterior wall and feeling outdoor air pour through it at pressure.

Here is what a blower door test is, what the numbers mean, and how to use that information.

How the Test Works

A blower door consists of a calibrated fan, a frame that mounts in an exterior door, and a pressure gauge called a manometer. The fan depressurizes the house to a standard reference pressure — typically 50 Pascals — and measures the airflow required to maintain it. That airflow measurement is the CFM50 number: cubic feet per minute at 50 Pascals of pressure difference.

The 50 Pascal reference pressure is about equivalent to a 20 mph wind blowing uniformly on all sides of the house at once. It's not a condition you'd actually experience, but it's a reproducible standard that makes results comparable between houses and over time.

From the CFM50 number, the test can calculate ACH50 — air changes per hour at 50 Pascals — which normalizes for house volume and is more useful for comparing differently-sized homes. Divide CFM50 by house volume in cubic feet, multiply by 60, and you get ACH50.

What the Numbers Actually Mean

Here is a practical reference frame based on my field experience and industry benchmarks:

Most older homes I test — built before 1980, never air sealed — come in between 8 and 15 ACH50. This is not unusual. It's simply what happens when a house is built without attention to the air barrier and sits for decades without anyone addressing the envelope systematically.

What the Test Tells You — and What It Doesn't

The blower door test gives you a single number that represents total building air leakage. It tells you how leaky your house is. It does not, by itself, tell you where the leaks are.

That is where the diagnostic process becomes more interesting. While the fan is running and the house is under negative pressure, a good auditor uses several techniques to find leaks:

• Hand feel. Running your hand around window frames, door frames, electrical outlets, and attic hatches while the house is depressurized makes air movement detectable that you'd never feel under normal conditions. This is more reliable than most people expect.
• Smoke pencils. A thin stream of smoke deflects visibly when it hits moving air. Useful for pinpointing leaks around penetrations and framing intersections.
• Infrared thermography. A thermal imaging camera, used while the house is under pressure and there's a temperature difference between inside and outside, shows cold air infiltration as blue patches in the image. This is the most powerful diagnostic tool available for locating distributed leakage, but it requires the right conditions to be useful.
• Zone pressure diagnostics. By partially isolating areas of the house — closing doors, sealing attic hatches — and measuring how the pressure responds, an auditor can attribute leakage to specific zones. This is how I determine whether a given house's leakage is primarily in the attic, the basement, or the walls.

How to Use the Results

If you're getting a blower door test as part of a home energy audit, the most useful output is not the single ACH50 number — it's the zone breakdown and the prioritized list of locations to seal. A good auditor will walk you through exactly where the air is coming from and rank the opportunities by ease of access and estimated impact.

If you're getting estimates for air sealing work, the blower door result gives you a baseline. A contractor who performs air sealing without a pre-work blower door test has no way to measure what they actually accomplished. Insist on a pre- and post-test if you're paying for significant air sealing work. The post-test is your receipt.

From a practical standpoint, homes that start above 10 ACH50 typically have enough distributed leakage that you can achieve meaningful reductions — often 30 to 40 percent — through a targeted attic and basement air sealing project without touching the walls at all. Walls in wood-framed construction are difficult and expensive to air seal from the interior, but attic and basement work is accessible and high-impact.

One Number That Changes How You Think About Your Home

The most valuable thing about the blower door test isn't any single result — it's what it does to your mental model of your house. Most homeowners think of air leakage as something that happens at the windows and doors: obvious gaps that you can feel on a windy day. After a blower door test, that model changes. The attic, the basement, and the utility penetrations typically account for far more leakage than all the windows and doors combined.

That shift in understanding is what leads to better spending decisions. It's why I consider the blower door test to be one of the most valuable tools in residential energy work — not because the number itself is magic, but because it makes the invisible visible and gives homeowners something concrete to act on.