What the test is
A blower-door test measures how much air leaks through the shell of a house. A tester fits a frame with a fabric panel into an exterior doorway and mounts a calibrated fan in it. Every window and outside door is closed. The fan pulls air out of the house until the pressure inside is a set amount lower than the pressure outside. Air then pushes in through every gap it can find: around window frames, where the wall meets the floor, through the holes drilled for wires and pipes, at the attic hatch, behind the tub. A gauge records how hard the fan has to work to hold that pressure. The harder it works, the leakier the house.
The test pressure is the same everywhere, so one house can be compared with another. It feels roughly like a steady wind of about twenty miles an hour pushing on every side of the house at once. Nobody lives in those conditions. The point is to make small leaks big enough to measure.
The result is usually written as air changes per hour at the test pressure: how many times the full volume of air inside the house would be swapped for outside air in one hour under that push. A lower number is a tighter house. Building codes set a ceiling. High-performance programs set a much lower one.
What the number tells you
Air leaks are not just drafts. They touch nearly everything you will notice about living in the house.
- Comfort. Cold floors in January and rooms that will not hold a temperature are often leaks, not the heating system.
- Moisture. In winter, warm indoor air carries water vapor. When it leaks out through a wall and meets a cold surface inside it, the vapor condenses. Over years, that is how framing stays damp and mold gets its start. A tight shell keeps that air out of the wall.
- Energy. Heat you paid for leaves through the gaps, and in a cold climate like ours, air leakage is one of the biggest heat losses in an ordinary house. Tightness is one of the least expensive energy upgrades there is, because it is mostly care and tape, done once, before the walls are closed.
- Air quality. In a leaky house, the air you breathe arrives through the garage wall, the crawlspace and the attic, unfiltered. In a tight house it arrives through a ventilation system with a filter, on purpose.
- Quiet. The gaps that leak air leak sound too.
What it does not tell you
The number is a grade, not a diagnosis. Three things to know:
- It does not say where the leaks are. For that, the fan runs while someone walks the house with a smoke pencil or an infrared camera and finds each one. That is why it is worth asking for a test once the air barrier is finished and before anything covers it, when every leak can still be fixed. The final test, after the house is done, confirms the result.
- It does not measure insulation. A house can be tight and thinly insulated, or thickly insulated and leaky. They are separate things, and both matter.
- It does not make a house stuffy. A tight house needs mechanical ventilation that brings in fresh, filtered air and sends stale air out. A tight house without ventilation is a mistake. So is a leaky house with it, because you are paying to condition air that leaves anyway.
Who runs the test matters
A builder who owns a blower door and tests their own work is doing something right. But the number you keep should come from someone who does not work for the builder: a third-party rater who sets up the fan, runs the test to the standard procedure and signs the report. Ask for it in writing, with the date, the tester's name and the house volume the number is based on.