Blower door test

A tester sets up a blower door test to measure the air leakiness of a home. 

FAIRBANKS — The “Ask a Builder” series is dedicated to answering some of the many questions Fairbanks residents have about building, energy and the many other parts of home life

Q: You’ve probably heard us mention the “blower door test” when talking about energy efficiency or retrofitting. So what is it?

A: A blower door test measures how airtight a building is — or how much air leaks through openings in the walls, roof and floor. During Alaska’s long winters, warm air leaking out of small openings in a building’s envelope represents significant energy lost to the outdoors. The blower door test determines how much air is leaking and where it’s coming from, so you can take steps to make the building more airtight. On the other hand, if the blower door test shows the building is substantially airtight, you should make sure your ventilation system is providing enough fresh air to keep occupants healthy.

Building inspectors, energy raters and other professionals are trained in using a blower door apparatus. The test takes about an hour, including prep and breakdown, though it can take longer in larger or leakier houses.

First, the tester makes sure all exterior windows and doors are closed and turns off the combustion appliances, such as boilers, furnaces and wood stoves; ventilation systems; and exhaust fans, such as the bathroom fans or clothes dryers. These appliances all exhaust air intentionally and do not represent the air leakage being measured. Finally, it’s important to open interior doors so air can flow freely within the house.

Next, the tester will set up the blower door in an exterior door of the home. Like the name implies, the blower door consists of a large fan placed in the doorframe with nylon stretched over the remaining opening, making it airtight. The device uses a manometer that compares the air pressure outside, the air pressure inside and the fan speed to calculate the air leakage rate. The manometer can be combined with a computer running software that runs multiple instantaneous tests to ensure greater accuracy than a single manual test.

During testing, the fan blows air out of the house, making the interior a lower air pressure than the outdoors. Typically, the professional performing the test will report the air leakage rate of the building as the number of air changes per hour at a pressure of 50 Pascals (ACH50). For instance, if the blower door test shows an air leakage rate of 4 ACH50, that means all the indoor air would leave the building four times per hour to be replaced by outdoor air.

For reference, an air leakage rating of 4 ACH50 or less is required for a home to meet Alaska’s Building Energy Efficiency Standard. The state established this standard to promote energy efficient construction, and it is required by some financing institutions. In fact, the most airtight home in the world is in Alaska. Dr. Tom Marsik and Kristin Donaldson’s home in Dillingham measured at 0.05 ACH50 in 2013.

In addition to measuring the quantity of air, a blower door test also can identify where a building is leaking. In this case, the tester often walks around the interior of the home near walls, windows and doors with a smoke pencil. During a depressurization test, air leaking into the building will blow the smoke and allow the tester to identify spots that can be better sealed.

For instance, common air leakage spots are around windows and doors, plumbing and duct openings and electrical outlets. The insights gained from a blower door test can help you save money for winters to come.

Ask a Builder articles promote home awareness for the Cold Climate Housing Research Center (CCHRC). If you have a question, contact us at or 457-3454.