Air changes per hour: reading a blower door result

How does a blower door test work?

A frame with a fan and a flexible panel is fitted into an exterior doorway. All windows and exterior doors are closed, and flues and vents are prepared. The fan then draws air out of the house until the pressure difference between inside and outside reaches 50 pascals. A gauge records how much airflow the fan needs to hold that pressure, which equals the leakage through the shell.

The test takes under an hour. While the house is depressurized, the tester can walk through with a smoke pencil or thermal camera and find leaks you can feel as a draft at outlets, baseboards or the attic hatch.

What is ACH50?

ACH50 means the number of times the whole volume of house air would be replaced in an hour at 50 pascals of pressure difference. If a house of 500 m3 leaks 2,500 m3 of air per hour at that pressure, it has 5 ACH50. The pressure of 50 Pa is much higher than what wind creates normally, so the figure is a test condition for comparing houses, not the real hourly air exchange.

Some programmes report the leakage area, or airflow per area of enclosure, in addition to or instead of ACH50. Use the same metric when you compare two houses.

What counts as a good result?

Older houses are often many times leakier than new ones, and the span is wide. New code-built houses sit well below old stock, and high-performance standards ask for much less again. Targets differ by programme and code edition, so check the number that your programme or builder commits to, and ask for the written test result. Standards such as R-2000 and ENERGY STAR for new homes use testing as a verification step.

A lower number is better only if ventilation is planned. The final goal is controlled air supply, not a sealed box.

What the test cannot tell you

Leakage numbers do not show whether the insulation is complete, whether windows are efficient or whether the ventilation system works. They also do not say where moisture will go. A tight shell with a poor vapour and air barrier detail can still have problems, and a leaky one with great ventilation can still be comfortable in some weather. Use the number as one piece of evidence together with the energy rating and an inspection.

The result also shifts with the season and with how the house is prepared for the test, so repeat tests are best done under similar conditions.

Why airtight houses need ventilation

A leaky house gets fresh air through accidental gaps, but at an uncontrolled rate: too much on a windy cold day and too little in mild weather. Tightening the shell takes that random exchange away, so moisture, cooking smells and pollutants must leave through a designed path. A heat recovery ventilator does that while recovering most of the heat from the outgoing air.

Combustion appliances need a specific check. If a furnace, boiler, fireplace or water heater draws room air or vents with a natural draft, tightening the house can cause backdrafting. Have a qualified technician verify safe operation after air sealing.

When to book a test

For a new house, a test at the drywall stage, before the finish is on, lets the builder fix leaks while walls are open, followed by a final test. For an existing home, test before air-sealing work to find the targets and again afterward to confirm the result. This is the same before and after logic used in a home energy audit.

The idea comes up in district projects too: the 52 homes at Drake Landing were built to a standard that included airtightness testing, a main reason their small heat demand suited a low-temperature heating loop.

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