Blower Door Testing for Contractors, Should You Own One?

This post first appeared on the Green Building Advisor Website.

I had the opportunity to interview Gary Nelson, one of the founders of The Energy Conservatory and Minneapolis Blower Door a few years ago.  An Interview with Gary Nelson-NorthernBuilt.  During the interview, he made the comment that his hope for the blower door when it first started appearing in the market was that every contractor would own one and that testing both existing and new homes would become a common practice.  That was 40 years ago.

We haven’t got there yet, but there are more and more contractors purchasing the tool and there are many online videos and webinars showing how a blower door is set up and used.  In this blog, I’ll discuss how to perform basic diagnostics on a home using a blower door.  Methods used to find air leaks, a short discussion on zonal pressure testing, and different tasks that the manometer can perform, all topics I think a contractor that owns (or wants to own) a blower door should be familiar with.

If you currently own a blower door or are looking to purchase one in the near future, my advice is to take a class on how to correctly operate the equipment.  There are two major manufacturers of blower doors in North America, Minneapolis Blower Door from The Energy Conservatory (TEC) 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 online classes that can be taken at your own pace.

TEC BUILDING AIR TIGHTNESS TRAINING (energyconservatory.com)

Online Courses – Training (retrotec.com)

There are also organizations that teach blower door testing along with building science principles and other safety testing.  One class that I recently began co-teaching is Building Science Institute’s (BSI) Building Analyst Technician (BA-T).  A good three-day class that covers a range of building science topics and testing, the class includes learning blower door operation by actually testing a home.

Locating Air Leaks

One of the advantages of using a blower door to test the air tightness of a structure is the equipment is also very well suited to find the air leaks.  Sometimes easier said than done.  One of the best observations I’ve heard about building tightness testing was from Dr. Joe Lstiburek and his comment about complex three-dimensional air pathways.  An analogy for the concept would be a water leak in a roof.  The roof might be leaking in one spot but presents some distance from the actual leak.

The same can happen with an air leak.  The leak might start at an outside receptacle, travel inside an exterior wall cavity, move into a floor joist area between two conditioned floors, and enter the building at a ceiling light fixture.  An air leak like that can be very difficult track.  On the other hand, air leaks at rim joists or from unconditioned and vented attic spaces can be easier to identify.  The hole has a more direct pathway to the outside.

Using thermal imaging (when you have a temperature difference between inside and out of around 10°F) is my first choice to find air pathways during blower door testing.  Thermal imaging cameras have become very affordable in recent years, $500 gets an easy to use, quality camera.

Another method is using smoke to “see” the movement of air.   Using either positive or negative pressures inside a home can direct air currents either way from, or into the leak.  A small amount of smoke is often all that is needed.  Equipment that produces smoke is available from both TEC and Retrotec.

Looking for air movement can also be a tool.  In basements and crawlspaces, I often see spiderwebs that are moving during blower door testing suggesting an air leak is in the area.  Drapery can also flutter during testing.  I’ve even seen doors slam shut, all indications of air leaks.

Another method I use is to seal off areas within the home while the blower door is running.  You may register a reduction of air flow (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.  An example would be to close the door between the basement (if the basement’s space can be counted as part of the conditioned volume of the home) and the rest of the home.  Often, you’ll see a reduction of air flow on the manometer.  The door probably isn’t the only pathway for air to move between the basement and the rest of the home, so the CFM reduction shown on the manometer typically isn’t the actual air leakage in the basement space, but you can get a sense of how much air is coming from the space.

Closing a door most of the way between two spaces inside the home with the blower door running is another method.  I like to put my face between the mostly closed door and door frame to “feel” the air moving between the spaces.  I’ve shown homeowners this trick, they are often 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.  Some are easy to find the majority of the leaks, others have leaks by a thousand little holes, those homes are often the frustrating ones to diagnose.

Zonal pressure Diagnostics (ZPD)

ZPD testing can be helpful, but also misleading if not well understood.  I teach this concept to students taking the Building Performance Institute Building Analyst-Technician certification.  We measure pressure differences with the blower door running between the conditioned space of the home and an interstitial space, such as an unconditioned and vented attic, attached garage, or vented crawlspace.

Let’s say we have the blower door operating to negatively pressurize the home to -50 Pascals.  If we use a manometer to measure the pressure difference in an unconditioned and vented attic space, (by extending a hose from the manometer into the attic space), a reading of 50 pascals on the manometer would suggest the ceiling separating the home from the attic is a perfect air barrier.

A 25 Pascal reading would mean the air leaks between the ceiling and living space equal the air leaks between the attic roof and outside.  There could be an equivalent of a 2 square inch hole in the ceiling separating the home from the attic, and an equal size, 2 square inch hole between the attic and the outside.  Or the size of the holes could be 20 square inches in both.  The reading of 25 pascals means nothing about air flow, only that leakage between the home and attic is equal to the leakage between the attic and outside.

A reading of 0 Pascals would mean that there is no air barrier between the home and attic and the roof, not the ceiling separating the home from the attic is acting as the primary air barrier.

This concept can be difficult to get your head wrapped around.  Again, the reading means nothing about the leakage rate, only that there is a relationship in hole sizes.  There is a method to estimate the amount of air leakage between the conditioned space of the home and a space like an attic called the add-a-hole method.  This is outside the scope of this article, here is a good video showing how this type of testing can be done: TEC – Zone Pressure Diagnostics (youtube.com).

Other uses for a blower door manometer

The manometer used for blower door testing is the heart of the system, but the tool can be used for many other purposes.  We just touched on zonal pressure diagnostics while blower door testing.  There is another type of zonal pressure diagnostics that involves a forced air heating and/or cooling system.

When a force air system is in operation, a room without a good return air pathway can become pressurized if the door is closed.  This can result in air being forced into building cavities.  The same is true if a room has more return than supply, air can be drawn from outside, into wall assemblies and into the room.  A manometer can be used to test if a room is either being pressurized or depressurized while the force air system is in operation.

The test is simple, connect a hose to the input or plus side of one of the manometer channels (remember, manometers used for blower door testing will have two channels).  The minus side is the channel will be used as the reference, in this case, we are referencing the main body of the home with relation to the room we are checking for pressurization or depressurization.  As you can see in the photo, the test shows a pressurization of 3.5 Pascals.  Typically, we like to see this number limited to under 3 Pascals.  How is the conditioned fixed?  Undercut the door or add a jump duct/transfer grill to allow a pathway for air to move toward the return air side of the forced air system.

A test commonly used by weatherization assistant programs (WAP) and RESNET’s HERS rating is called Combustion Appliance Zone or CAZ testing.  Part of the test creates a worst-case depressurization of the area where combustion appliances are located in the home.  The test requires the home be placed under its worst natural depressurization state, all exterior windows and doors are closed.  All exhaust fans, including the clothes dryer are in operation at their highest exhaust rate.  Interior doors that do not contribute to an improvement of the depressurization of the home be closed.  All door to rooms that have exhaust fans must remain open.  The door to the CAZ zone may be open or closed and any forced air furnace may or may not be in operation depending on what makes the depressurization of the CAZ most negative.  We then start any atmospherically vented appliances to determine if the appliance “spills” or backdrafts after a given amount of time.  This test requires a manometer and the ability to reference the outside with relation to the CAZ.  To give an example of what can happen, orphaned atmospherically vented water heaters can backdraft with as little as 2 Pascals of negative pressure in the CAZ zone.  Some WAP programs will require test-in and test-out procedures to occur every day when work is being done on a home.  In my opinion, this type of safety testing should be conducted by all crews performing renovation work on existing homes.

We can also use the manometer for other purposes.  Both TEC and Retrotec have additional tools that require a manometer.  Duct tightness testing uses a manometer, so does exhaust fan testing. How to Test a Bath Fan-NorthernBuilt.  Both TEC’s Exhaust Fan Flow Meter tool or Retrotec’s FlowBox are handy tools to have.  A pressure pan is another diagnostic tool that requires a manometer.

Owning your own blower door equipment gives you a lot of opportunity to do more than simply testing the tightness of a new home.  Not every contractor has the time to learn every use for a blower door and its related equipment, but if you can figure out a few of the basic tests, you may find yourself with an edge over your competition.