A few years ago, I had the opportunity to interview Gary Nelson, one of the founders of The Energy Conservatory (TEC) and Minneapolis Blower Door. During that conversation, he explained how his original hope for the blower door was that every contractor would own one and that testing both new and existing homes would become common practice. That was 40 years ago.

We aren’t there yet, but there are more contractors purchasing the tool because there are several good reasons to own a blower door. There are many online videos and webinars showing how a blower door is set up and used. Here, I will discuss how to perform basic diagnostics on a home using a blower door. I will include methods for finding air leaks, some talk on zonal pressure testing, and tasks a manometer can perform—all topics I think a contractor who owns (or wants to own) a blower door should know about.

Invest in the test

There are two major manufacturers of blower doors in North America: TEC’s Minneapolis Blower Door, and Retrotec. A third, Infiltec, also manufactures blower door equipment, but I have yet to see any of their products in the field. Both major manufacturers have on-demand online classes; for anyone who either owns a blower door already or is considering buying one, I recommend taking a class on how to correctly operate it. (Go to: Minneapolis Blower Door training and Retrotec training.)

There are also agencies that teach blower door testing alongside building science principles. For example, I am co-teaching the Building Analyst Technician (BA-T) training at Building Science Institute (BSI). It is a three-day course that covers a range of building science topics and includes lab instruction on blower door testing a home.

Locating air leaks

One of the advantages of using a blower door to test the airtightness of a building is the equipment can also find leaks. Although sometimes it’s easier said than done. One of the best observations I’ve heard about building tightness testing and finding air leaks was from Dr. Joe Lstiburek. He was talking about complex three-dimensional air pathways. The takeaway was: think of an air leak like a water leak, which can manifest some distance from the actual leak.

The same can happen with an air leak. The leak might start at the exterior—say, through an electrical box—and travel inside a wall cavity, then move into a floor joist between two conditioned floors, and enter the living space through a ceiling light fixture. An air leak like this can be difficult to identify. On the other hand, air leaks at rim joists or from unconditioned vented attics can be more readily identified because the hole is a more direct pathway to the outside.

Note that timing is important. I prefer running a blower door and looking for air pathways when there is a temperature difference between inside and outside near 10°F.

Using thermal imaging

Thermal imaging cameras have become more affordable in recent years; $500–$600 buys an-easy-to-use, quality camera. It will reveal the telltale signs of an air leak, which appear as purple rays—because of hot or cold air blowing across room surfaces.

Using smoke to see the movement of air is another method for detecting leaks. Either positive or negative pressures inside a home can direct air currents away from or toward the leak. A small amount of smoke is often all that is needed. (TEC and Retrotec both supply tools that produce smoke.)

Visual cues can be a tool for finding air leaks, too. In basements and crawlspaces, moving spiderwebs during testing suggest an air leak in the area. Drapery can also flutter; I’ve even seen doors slam shut—also indications of air leaks.

Another method is to seal off areas within the house while the blower door is running. The equipment may register a reduction of airflow—in cubic feet per minute (CFM)—moving through the fan, suggesting there is a leak on the other side of the space that was closed off from the blower door testing area.

For example, closing the door between a conditioned basement and the rest of the house can reduce airflow, which will register on the manometer. The door probably isn’t the only pathway air takes to move between the basement and the rest of the house, so the CFM reading isn’t representing the actual air leakage, but you can get a sense of how much air is coming from the space.

Closing a door most of the way is another method. I put my face between the mostly closed door and door frame to feel the air moving between the spaces. When I show homeowners this trick, they are amazed at the amount of wind blowing on them.

The more experience you have in searching for air leaks, the easier they become to find. Just remember every house is unique. In some it is easy to find the majority of leaks; others have a thousand little holes—those homes are the frustrating ones to diagnose.

Zonal pressure diagnostics (ZPD)

ZPD testing can be helpful, but also misleading. When I teach this concept to students, we measure pressure differences with the blower door running between the conditioned space and an interstitial space, such as an unconditioned vented attic, attached garage, or vented crawlspace.

For example, let’s say the blower door is operating to negatively pressurize a home to -50 Pascals air changes per hour (ACH50). Using a manometer, we measure the pressure difference in the unconditioned vented attic. A reading of 50 ACH50 would suggest the ceiling separating the living spaces from the attic is a perfect air barrier. A reading of 25 ACH50 reading would mean the air leaks between the ceiling and the living space are equal to the air leaks between the attic roof and outside.

A reading of 25 ACH50 means nothing about airflow. It just means that leakage between the conditioned house and the attic is equal to the leakage between the attic and outdoors. There could be the equivalent of a 2-sq.-in. hole in the floor/ceiling and an equal size hole between the attic and outdoors.

A reading of 0 ACH50 would mean there is no air barrier between the house and the attic. In this case, the roof is the air barrier. Again, the reading has nothing to do with leakage rate; it indicates only that there is a relationship between hole sizes.

Lastly, the “add-a-hole” method estimates the amount of air leakage between conditioned and unconditioned spaces. This method is outside the scope of this article, but here is a video showing how it’s done: Zone Pressure Diagnostics.

Additional diagnostics

In addition to the zonal pressure diagnostics just described, there is another ZPD method used with forced-air heating and/or cooling systems. When this type of HVAC system is operating, a room without a good return-air pathway can become pressurized if the door is closed. This forces air into the building cavities. Conversely, if a room has more return air than supply air, then air gets pulled from outdoors into the wall assembly and into the room.

A manometer can be used to test if a room is being pressurized or depressurized when the force-air system is running.

To perform this test, connect a hose to the input (+) side of one of the manometer channels (manometers used for blower door testing have two channels). The (–) side is the channel that will be used for reference—in this case, the main body of the house in relation to the room being pressurized or depressurized.

As you can see in the lead photo, the room tested at 3.5 Pascals. Ideally, this number stays under 3. To remedy this situation, undercut the door or add a jump duct/transfer grill. This will create a pathway for air to move toward the return-air side of the system.

There’s also something called Combustion Appliance Zone (CAZ) testing, which is something Weatherization Assistant Programs (WAP) and RESNET’s HERS rating program use. The test simulates worst-case depressurization conditions in spaces with combustion appliances.

The building is put under depressurization with all exterior windows and doors closed, and with all exhaust fans—including the clothes dryer—operating at the highest exhaust rate. Additional testing conditions include closing interior doors that do not contribute to an improvement of the depressurization, and keeping open all doors to rooms with exhaust fans. (This test requires a manometer and the ability to reference the outside in relation to the CAZ.)

When those conditions are set, all atmospherically vented appliances are turned on to determine if there is any backdrafting. Atmospherically vented water heaters that are orphaned—meaning their venting system has become isolated or is no longer properly connected to the rest of the home’s ventilation system—can backdraft with as little as 2 Pascals of negative pressure in the CAZ.

Some WAPs require daily test-in and test-out procedures when work is being done on a house. In my opinion, this type of safety testing should be conducted by all crews performing renovation work on existing homes.

Final thought

Owning your own blower door equipment provides opportunities to do more than test airtightness. Of course, not every contractor has the time to learn every use for a blower door and its related equipment, but if you learn about the basic tests, you may have an edge over your competition.

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Randy Williams is a builder and energy auditor in Grand Rapids, Minn. Photos by the author.