©2013 Green Building Advisor. From The Taunton Press, Inc., publisher of Fine Homebuilding Magazine.
Leaky homes are hard to heat and hard to cool. The only way to know whether your home is leaky or tight is to measure its air leakage rate with a blower door. A blower door is a tool that depressurizes a house; this depressurizationSituation that occurs within a house when the indoor air pressure is lower than that outdoors. Exhaust fans, including bath and kitchen fans, or a clothes dryer can cause depressurization, and it may in turn cause back drafting as well as increased levels of radon within the home. exaggerates the home’s air leaks, making the leaks easier to measure and locate.
An energy-efficient house must be as airtight as possible. Many older U.S. homes are so leaky that a third to a half of the home’s heat loss comes from air leaks.
There is no such thing as a house that is too tight. However, it’s also true that there is no such a thing as an airtight house. Every house leaks, and that’s why we perform blower-door tests — to measure a building’s leakage rate.
Blower-door testing is useful for both new construction and existing homes. By testing a new home, a builder:
If you’re building a new home, the best time to conduct a blower-door test is after the home is insulated but before the drywall is hung. If the test reveals major problems, the leaks will be easier to fix at that point than later on.
Duct Leakage Testing 
There are at least two reasons to conduct a blower-door test on an existing house: to determine how leaky it is, and to help locate and fix the leaks.
When a blower door is used to help an air-sealing contractor locate and fix leaks in an existing house, the procedure is called “blower-door-directed air sealing.” (To see a GBA video of blower-door-directed air sealing, click here. )
During the 1960s, energy experts didn’t realize the extent to which air leakage contributed to residential heat loss. During the early 1970s, however, a few researchers in Sweden, Saskatchewan, and New Jersey began studying air leakage in homes. In spite of these efforts, most early airtightness researchers still didn’t understand how air was leaking out of most existing homes.
The Eureka moment came in 1977. A Princeton University researcher named Gautam Dutt was frustrated because he couldn’t account for all of the heat escaping from a group of townhouses he was studying in Twin Rivers, New Jersey. According to a July 22, 1979 New York Times article, “Of 30 or so houses he [Dutt] checked, all were losing three to seven times as much heat to the outside as the models predicted.” After Dutt spent hours investigating the homes’ nooks and crannies, he eventually pulled back some attic insulation and discovered a huge air leak through an unsealed utility chase. Dutt is now credited as the discoverer of the “thermal bypass.”
The blower door was originally a research tool. It was simultaneously and independently invented in the early 1970s by two groups of North American researchers — the so-called “Princeton House Doctors” (David Harje, Ken Gadsby, Frank Sinden, and Dutt) in New Jersey and a group in Saskatchewan that included Harold Orr. The first commercially available unit, the Gadsco blower door, hit the market in 1980.
In 1981, Harry Sherman and his son Max — Max is now a senior researcher at the Lawrence Berkeley National Laboratory — started selling blower doors under the Harmax brand. A year later, Gary Nelson, the founder of The Energy Conservatory, started selling the Minneapolis blower door. Of these three pioneer companies, only The Energy Conservatory is still in business.
A blower door kit includes several components:
Three U.S. manufacturers (listed at the end of the article) sell residential blower-door kits for prices ranging from $2,500 to $3,200.
Before a blower-door test can begin, the following preparation is necessary:
In most cases, the following openings are not sealed:
Ventilation system intake or exhaust vents (and passive air inlets) are usually (but not always) sealed, depending on the aims of the blower-door test. If the test is being performed to comply with section N1102.4.2 of the 2009 International Residential Code, the section requires that "Exterior openings for continuous ventilation systems and heat recovery ventilators shall be closed and sealed." Moreover, in many cases a builder will seal passive air intake vents during a blower-door test to determine the theoretical leakiness of the building’s envelope without any passive inlets.
According to an anonymous document, “Blower Door Basics,”  posted online, “An old energy-auditor trick is to leave your truck keys on the water heater so you remember to turn the water heater and furnace back on” when the test is completed.
Once the house has been prepped, the blower-door technician starts up the fan slowly to depressurize the house. Before cranking the fan all the way up, it’s a good idea to walk through the house to make sure that nothing unexpected is happening — for example, to be sure that fireplace ashes aren’t being pulled across the living room hearth.
The fan speed is then turned up until the pressure difference between the indoors and the outdoors reaches 50 Pascals. At that point the technician reads and records the fan’s airflow as indicated on the airflow manometer. (Airflow is measured in cubic feet per minute, or cfm).
The pressure difference at which blower-door tests are conventionally performed — 50 Pascals — is arbitrary but useful. By establishing 50 Pascals as a standard pressure difference, a wide variety of houses can be usefully compared. Leaky houses require a high airflow to maintain this 50-Pascal pressure difference, whereas tight houses require a low airflow, so the airflow of the fan (in cfm) during 50-Pascal depressurization provides a number that correlates directly with a home’s leakiness.
There are two main ways that blower-door tests are reported: airflow at a pressure difference of 50 Pascals (cfm50) or air changes per hour at a pressure difference of 50 Pascals (ach50).
The first number — cfm50 — can be read directly off the airflow manometer at the time of the test.
The second number — ach50 — can only be calculated once the building’s volume has been determined. To calculate ach50, multiply cfm50 by 60 minutes per hour and divide the product by the building volume, including the basement, measured in cubic feet.
Some blower-door technicians estimate a home’s “natural infiltration” or “natural air change rate” (ACHnat). This number shouldn’t be taken too seriously, since it is only an estimate. Natural infiltration rates (and rules of thumb for calculating ACHnat) vary by climate. In Minnesota, ACHnat approximately equals ach50 divided by 17, while in Florida, ACHnat approximately equals ach50 divided by 30. According to Gary Nelson, the president of The Energy Conservatory in Minneapolis, “ACHnat is probably only accurate plus or minus a factor of two.”
Here are some comparison points to help interpret an ach50 reading:
David Keefe, the manager of training services for Vermont Energy Investment Corporation, recently wrote an article  on blower-door testing. “Houses with less than 5 or 6 ach50 are considered tight, and those over 20 are quite leaky, though these numbers can be misleading without considering other variables such as climate, house size, and old versus new construction,” Keefe wrote. “Tight houses tend to measure less than 1,200 cfm50, and moderately leaky homes measure between 1,500 and 2,500 cfm50. Homes that measure over 3,000 cfm50 are considered leaky.”
According to The Homeowner’s Guide to Renewable Energy by Dan Chiras, “A really good measurement is around 500 to 1,500 cfm50. The older houses we work on typically fall in the 6,500 to 8,500 cfm50 range.”
Any competent energy auditEnergy audit that also includes inspections and tests to assess moisture flow, combustion safety, thermal comfort, indoor air quality, and durability. of an existing home must include a blower-door test. Once you know your air leakage rate, you can formulate a plan for improving your home’s performance.
The leakier a home, the more economic sense it makes to hire an air-sealing contractor. “Homes with more than 6,000 cfm50 may merit days of labor and hundreds of dollars of materials,” write energy experts John Krigger and Chris Dorsi in their book, Residential Energy. “Homes with 1,500 cfm50 are difficult to improve.”
If your house is leaky enough to justify air-sealing work, you’ll need a blower door to efficiently locate and fix the leaks. Blower-door-directed air sealing is done while the house is depressurized to about 30 Pascals.
Once the blower-door has been set up, it usually makes sense to leave the fan running for several hours. By walking from room to room, many leaks can be found by feeling around with your bare hands. Subtler leaks can often be found using a smoke pencil or smoke bottle. In cold weather, an infrared camera can also be used to find air leaks.
The most important areas to seal air leaks are down low — in the home’s basement or crawl space — and up high — at the attic floor. Because of the stack effectAlso referred to as the chimney effect, this is one of three primary forces that drives air leakage in buildings. When warm air is in a column (such as a building), its buoyancy pulls colder air in low in buildings as the buoyant air exerts pressure to escape out the top. The pressure of stack effect is proportional to the height of the column of air and the temperature difference between the air in the column and ambient air. Stack effect is much stronger in cold climates during the heating season than in hot climates during the cooling season., leaks in these areas matter much more than leaks in the middle of the house, where there isn’t as much of a difference in air pressure between the indoors and outdoors.
Many homeowners assume that gaps around windows and doors are responsible for most of a home’s air leaks. In fact, air leaks in the following areas are usually much more significant:
With the blower door running, air-sealing work begins, using a variety of materials, including spray foam, caulk, and rigid foam board. Workers first attack the largest and most obvious leaks. As they proceed, they periodically check the blower-door fan’s air flow to determine whether the air-sealing work is effective.
After air-sealing work in an existing house is complete, it’s vitally important to conduct a combustion safety test. This usually involves a worst-case depressurization test: all of the home’s exhaust fans, including the clothes dryer, are turned on at once, and every combustion appliance is checked to be sure there is no spillage of flue gas into the home.
Sealed-combustion appliances are immune to spillage and therefore preferred for tight homes.
Air-sealing contractors need to have a good understanding of “house as a system” principles to be sure that their work doesn’t cause or exacerbate indoor humidity problems, radonColorless, odorless, short-lived radioactive gas that can seep into homes and result in lung cancer risk. Radon and its decay products emit cancer-causing alpha, beta, and gamma particles. exposure, or a variety of other potentially hazardous conditions.
Many air-sealing contractors aim to lower the air-leakage rate in an existing home to somewhere in the range of 1,000 to 2,000 cfm50. If air-sealing work continues until the house is tightened below 1,000 cfm50, it’s advisable to install a whole-house mechanical ventilation system.
So why would anyone want to first tighten a house and then turn around and ventilate it with a fan? For several reasons:
For those who can’t afford to buy a blower door, there are other ways to locate air leaks. In his excellent book, Insulate and Weatherize, energy consultant Bruce Harley advises, “You can make your own blower door if you can obtain a powerful fan (a regular box fan won’t work). You won’t be able to measure the air leakage, but you can use it to feel the air leaks.”
Another method requires a theatrical fog machine. For more information on this technique, see “Pinpointing Leaks With a Fog Machine.” 
To see a GBA video of blower-door-directed air sealing, click here. 
Last week's blog: “HRV or ERV?”