Blower doors are variable-speed fans that are installed into a tightly fitting shroud in an exterior door opening. With the rest of the building closed up tight and interior doors open, the fan can be used to depressurize (or pressurize) the building and evaluate its airtightness.
Building scientists did not fully understand until the late 1970s that 25%–40% of the total heat loss or gain in a conditioned building can be attributed to air leakage. Not coincidentally, that is the same time the blower-door test was invented, independently, by two groups of researchers—the “Princeton house doctors” in New Jersey and a team led by Harold Orr in Saskatchewan.
You can actually use any powerful fan to create pressure that exaggerates and helps to identify air leaks. But without the precision of an accurately calibrated variable-speed fan and
manometer (pressure gauge) in a blower-door test kit, the air leakage can’t be quantified. And when pressure differences are measured at various fan speeds, the resulting curve of results can tell a bit about the nature and size of the air leaks in a building.
A single blower-door kit works for most homes and small commercial buildings; for very leaky smaller buildings and larger commercial buildings, blower doors may need to be “stacked”—meaning multiple fans are used in the same or multiple doorways—to sufficiently depressurize the building for a meaningful result.
To compare blower-door results, a standard pressure difference of 50 pascals is used. A
pascal is a unit of pressure equal to one newton per square meter; it’s roughly the amount of pressure exerted when you pronounce the letter “p.” A 20 mph wind exerts about 50 pascals.
The final result of a blower-door test is commonly expressed as either air changes per hour (the number of times in an hour that indoor air is replaced with fresh air) at 50 pascals (ACH50) or cubic feet per minute at 50 pascals (cfm50). The advantage of ACH50 is that the number is independent of the volume or the size of the building. A “tight” new building is considered anything at or below 2.5 ACH50; getting existing homes lower than 4 ACH is often very challenging. For comparison, the Canadian R2000 program requirement for new homes is less than 1.5 ACH50; the Passive House standard is 0.6 ACH50.
Measuring airtightness in buildings is not just about energy savings. As air leaks into and out of buildings, any drop in temperature across the building assembly can cause moisture in the air to condense inside structural walls. Uncontrolled air leakage patterns can also contribute to poor indoor air quality: combustion exhaust may fail to vent properly, or outdoor air may be pulled into interior spaces through moldy basements or other unintended locations.