Blower Door Basics

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 depressurization 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.

Who needs a blower door?
Blower-door testing is useful for both new construction and existing homes. By testing a new home, a builder:

• Can determine whether a certain airtightness target — for example, the Passivhaus airtightness standard — has been met;
• Can document airtightness levels needed to qualify for certain home labeling programs, including Energy Star;
• Can do a better job calculating heat loss and heat gain the next time he or she builds a similar house;
• Can brag about the home’s airtightness to prospective homebuyers or drinking buddies.

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.

Testing existing homes
There are at least two reasons to conduct a blower-door test on an existing house: to determine how leaky it is, and the 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.)

A short history of the blower door
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.

What’s a blower door look like?
A blower door kit includes several components:

• A frame and a flexible panel designed to temporarily fill a doorway;
• A powerful variable-speed fan that is attached to the blower-door frame; and
• At least two pressure gauges (manometers): one to measure the pressure difference between the home’s interior and the outdoors, and another (the airflow manometer) that deduces the fan’s airflow.

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 you turn on the fan, go through the checklist
Before a blower-door test can begin, the following preparation is necessary:

• Close all windows and storm windows.
• Close all exterior doors (except, of course, the door with the fan).
• Open all interior doors. In most tests, any doors between the basement and the first floor are left open.
• Disable heating equipment and non-electric water heaters by turning down their thermostats (and in some cases by shutting off electrical power to the equipment).
• If there are any wood stoves, verify that all fires are out. Cover the ashes in the wood stoves and fireplaces with damp newspaper.
• Shut all wood-stove dampers and fireplace dampers. Close glass fireplace doors.
• Turn off the clothes dryer and all bathroom and kitchen exhaust fans.
• If there is any reason to believe that plumbing traps are dry, fill all the plumbing traps with water.

In most cases, the following openings are not sealed:

• Combustion flues;
• Dryer vents;
• Ventilation system intake or exhaust vents

There are exceptions to these rules, however. For example, in some 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.

Crank up the fan
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.

Interpreting your results
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.”

Is my house tight?
Here are some comparison points to help interpret an ach50 reading:

• A 2002 study of 24 new Wisconsin homes showed a median air leakage of 3.9 ach50.
• New home builders in Minnesota routinely achieve 2.5 ach50.
• The Canadian R-2000 program has an airtightness standard of 1.5 ach50.
• The Passivhaus airtightness standard — a tough standard to achieve — is 0.6 ach50.

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.”

Blower-door-directed air sealing
Any competent energy audit 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 effect, 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:

• Basement rim joist areas;
• Holes cut for plumbing traps under tubs and showers;
• Cracks between finish flooring and baseboards;
• Utility chases;
• Plumbing vent pipe penetrations;
• Kitchen soffits;
• Fireplace surrounds;
• Recessed can lights; and
• Cracks between partition top plates and drywall.

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.

Don’t forget a combustion safety check
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, radon exposure, or a variety of other potentially hazardous conditions.

Do you need mechanical ventilation?
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:

• Leaky homes don’t provide dependable volumes of fresh air. In a leaky house, air infiltration rates are very high in some conditions (when it’s cold outdoors and when it’s windy) and very low in other conditions (when outdoor temperatures are mild and there is little wind.)
• Leaky houses tend to be overventilated in zones that are leaky and underventilated in zones that are relatively tight.
• Air leakage through wall and ceiling assemblies can lead to condensation, mold, and rot.
• Leaky homes are uncomfortable.
• Tight homes use less energy than leaky homes — even taking into account the electrical energy used for ventilation.

Alternatives to blower-door testing
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.”

Manufacturers of blower doors
There are only three U.S. manufacturers of blower doors:
Infiltec of Waynesboro, Virginia.
Retrotec of Bellingham, Washington.
The Energy Conservatory of Minneapolis, Minnesota.

To see a GBA video of blower-door-directed air sealing, click here.

Last week's blog: “HRV or ERV?”