How airtight is the house?

Our car thermometer dropped to -28 degrees Fahrenheit as we drove through the Root River valley on our way to the house Tuesday morning. Steam rose from areas of icy water flowing over river rapids, somehow underscoring the irony that we were about to test just how air-tight the construction is that will insulate us from the exact cold we were feeling in years ahead. And it proved  to be a big day for everyone involved!  

After months of careful construction and attention to sealing every seam and penetration in the house building envelope (floor, ceiling, walls, windows and doors), building system technology instructor and Certified Passive House Consultant Josh VandeBerg and five of his Wisconsin Technical College students came from La Crosse to do a blower door test on the house—an increasingly popular approach to identifying air leaks that result in unintended winter heat loss.

I couldn’t imagine exactly what the day would be like, but knew it couldn’t hurt to clear up the clutter. As I worked I looked at all the ways the builders have made the building envelope airtight.

They caulked nail holes in the interior sheathing…


…foam insulated, caulked and taped every window installation…


…applied sealant at, and taped, every seam — everywhere…


…caulked and taped around every plumbing vent and PVC conduit penetrating ceilings, floors and walls…

tape plumbing

…taped the sub-floor moisture barrier to the wall to form a continuous air barrier…


…adjusted doors and windows to fit properly…


…and temporarily sealed (red tape in photo) all open plumbing vents. (Well, almost all. More on that later!)

plumbing vents

Josh and students Genevieve Mortenson, Bob Carlson, John Kroll, Ben Heyer and Josh Martell arrived loaded with equipment, ready to perform the blower door test and detect air leaks (which could then be addressed by the builders). Joe Deden (Director, Eagle Bluff Environmental Learning Center), and Brad Pecinovsky (Director of Member Services, Tri-County Electric) joined us to observe the testing.

Our house is heated by the sun with back-up heating (or cooling) only for the most extreme days; the tightly sealed and insulated building envelope is key to maintaining these temperatures. The blower door test uses air pressure to measure the airtightness of the building envelope. Before the house is finished we’ll have two tests: one now, when leaks can be easily fixed in the exterior structure, and a second test once the additional mechanical penetrations, insulation, sheetrock, and siding are complete. Essentially what we’ll know is: How airtight is the building?

In the days leading up to Tuesday, Jeff, JR and Troy double-checked to see that everything was sealed. On Tuesday, before the test took place, the testers also did a visual inspection of the interior and exterior of the house.

The blower door itself is a piece of tightly woven nylon cloth held firmly in the door frame with a collapsible frame. Here, we’re looking down on the blower door frame as it’s affixed to the cloth.


After the door is installed, a fan is placed in a hole in the lower half of the door. In this photo it’s the black circle near the floor. Collars of different sizes were slipped onto the fan during the test to adjust the size of the opening for pressurizing the house.


The door is firm, but here Bob and John hold the door in place as pressure is increased, to be sure it doesn’t pop out and hit the house door. It happened to them once, so they were being cautious!


House interior volume, exterior surface, and square footage of interior livable space were entered into a computer program connected to the door, fan and monitor. A click on the computer starts the fan, runs the test, and registers measurements.

Ten measurements are taken for each test. Two tests are completed: 1) depressurized (sucking air out of the house); and 2) pressurized (pushing air into the house), then scores are averaged. The final score is measured in air changes per hour (ACH). Our goal is 0.4 ACH which would land us well within the Passive House standard of 0.6 ACH. (By comparison, conventional construction results in 2-4 ACH, and some existing houses can have 5-10 air changes per hour. That’s a lot of costly heated or cooled air escaping every hour of every day!).

Pressure can be measured using the monitor alone (in this photo), but our testers interconnected their monitor with a computer and used a program called Tectite to automate the tests.


We performed the two tests. The averaged first round was .55 air changes per hour (ACH)—a good number, but surprisingly high for a building this tight. So we all grabbed hand-held infrared cameras and scanned the walls, joints, and windows looking for leaks. In the next photo temperature variations show as different colors on the small screen. Heat differences are natural, but moving purple forms on the screen indicate cold drafts. In this photo a leak has been found; it looks like a tornado dropping from the center purple blob.


When we found a leak, someone taped it up. A retest showed a lower score, but still not what we expected. THEN, someone noticed a 1.5″ plumbing vent that had been overlooked and not sealed, leaving it essentially wide open to the outdoors! That prompted us to take an even closer look and a few minutes later a second open plumbing vent was found hidden in the floor joists. After those were taped, we retested.

Needless to say, we were somewhat anxious at that point, having fixed two truly gaping holes in the envelope and anticipating a much more promising test. The minutes ticked by as the testing commenced; Jeff and Troy, especially, were glued to the screen. Along with JR, they’d invested a lot of energy, focus and careful craftsmanship in building this house well and it was an important moment.


The score finally popped up; .32 ACH—lower than the goal by good measure! All were thrilled. But beyond achieving the goal, on a practical front, this holds great promise for us living comfortably in all seasons thanks to good design, careful work—and the sun.




    1. Hi, Loni…thanks for your comment and question! Indoor air quality is regulated by an air exchanger — pulling air from the bathrooms and kitchen, and refreshing the house through bedrooms and project rooms. Specifically, we’re installing an Energy Recovery Ventilator (ERV), so not only does this exchange the air, but it transfers 90% of the heat in outgoing air to the incoming air, essentially ‘preheating’ it so we’re not losing the heat. Similarly, it transfers a fair amount of moisture so that we’re not constantly drying the place out. But it’s a controlled system so we can adjust it up or down if it gets too humid or too stale or too hot, etc. [Watch for an upcoming post on the installation of our Zehnder ComfoAir 350.]

  1. Tom Bassett-Dilley

    Great explanation of air sealing–thanks for the photos and description! Would love to have you and/or Christi come talk to the Chicago PH group if possible.


Lanesboro, Minnesota
Climate Zone 6 (cold/moist)
Latitude: 43° 44' 18'' N
Longitude: 91° 54' 48'' W

House Size

Net Treated Floor Area: 1,514 SF
Gross Square Footage (House only): 2,210 SF

Building Envelope

Roof: R-99
Wall: R-61
Ground: R-53

Windows & Doors

Glazing: U-0.10 BTU / hour / sq. ft.
Solar Heat Gain Coefficient (SHGC): 0.48”
Frame: U-0.19 BTU / hour / sq. ft.

Modeled Performance

Specific Primary Energy Demand (Source Energy Demand): 12.1 kBTU / sq. ft. / year

Specific Space Heat Demand: 7.0 kBTU/sq. ft. / year

Peak Heating Load: 7,047 BTU / hour

Space Cooling Demand: 0.44 kBTU / sq. ft. / year

Peak Cooling Load: 3,625 BTU / hour

Pressure Test Goal: Whole House Air Changes Per Hour (ACH) = 0.4 ACH 50


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