Mind the Gap: Take Three

July 9, 2020: The Day of Reckoning had arrived. When I first set out to build an energy-efficient house, I knew that I had to solve three key problems:

  1. How much insulation to use in the slab, walls, and ceiling
  2. How to seal the inevitable gaps between different building materials and where mechanicals penetrated the building shell
  3. With such an air-tight envelope, how to get adequate fresh air inside

While still in the planning phase, I contracted with Jim Kjorlie of Kjorlie Design Services to test my progress at the completion of three milestones. His first visit was after we were “dried in” (roof on and windows in) and I had meticulously taped, caulked, or foamed gaps in the exterior shell. I reported his test results and what we found out in my blog post “Mind the Gap: Take One”.

Jim’s core business is testing Focus on Energy (Energy Star) homes for large builders—the kind who put up look-alike homes across cornfields on the outskirts of Madison. To be certified, they need to test out at 4.0 air changes per hour (literally the amount of times the volume of air in the home is replaced through leaks in the building shell). To their tribute, these builders are averaging 2.0-2.5 air changes per hour. Code won’t red-tag you unless you test leakier than 7.0 air changes per hour.

The lowest number Jim has seen was 1.2 air changes per hour. I told him at the outset that my goal (for the final blower door test) was 1.0. Here’s how I did:

Blower Door #1=590 cfm or 2.20 ach@50Pa or 0.13 cfm50/sf shell

Several months later, Jim came back when the ceiling was insulated and drywalled and the mechanicals were in and sealed to the exterior shell—but before the walls were insulated and drywalled. It was my last chance to find any holes before they were forever covered up. We did find holes. See “Mind the Gap: Take Two”.

Blower Door #2=230 cfm or 0.86 ach@50Pa or 0.05 cfm50/sf shell

Today’s test was not so much about finding holes as it was to see how well the fresh air systems were working and to award me a final score for all my efforts. The walls were insulated and drywalled. Cabinets and other finishing touches were going in.

Jim tests with a contraption called a “blower-door”. A fan and related gauges simulate a 20 mile per hour wind bearing down on all sides of the house—and while a good proxy for our Wisconsin winters—it’s the industry standard for measuring air-tightness. The moment of truth was at hand.

Blower Door #3=101 cfm or 0.38 ach@50Pa or 0.02 cfm50/sf shell

Reader, this is a VERY GOOD number. My house will save energy because it will lose so little, but the more important point is that the wall and ceiling cavities will stay dry. Any hole, however big or little, lets in moisture-laden air. Warm moist air from inside the house in the winter makes its way through and condenses on the cold surface of the exterior shell. In the summer (when air-conditioning) it’s the reverse: warm moist air migrates inward toward the cool surface of the interior drywall and condenses. The walls and other structural cavities of an airtight home stay dry and don’t degrade; they are free of mold, mildew, dust, allergens, and pests.

Why is this obvious lesson in physics still a new concept in residential building? Take a look at this recap of What We Knew back in 1979.

An airtight home can of course trap indoor pollutants. The pandemic has raised awareness about the quality of our indoor air. We now know to worry about viral transmission as well as the more familiar but still vague dangers of mold, CO2 (from breathing), formaldehyde, VOC’s (volatile organic compounds) and other additives embedded in building materials and everyday household products that can off-gas, causing both short-term breathing problems and long-term health concerns.

Codes and the residential building industry are way behind on implementing best practices now recommended by building scientists. While improvements in building materials and demand for energy-efficiency have made homes more air-tight, eliminating harmful products is rarely discussed, and properly sized mechanical ventilation for good health is hit-or-miss.

See my blog posts for detailed information about the non-toxic products I chose at each step of the way. The EPA has some good information about IAQ (indoor air quality). Also check out Healthy Building Network.

The standard market-rate home relies on exhaust-only bathroom fans—the kind that exhaust stale air but pull makeup air through the cracks and crevices of the building envelope. This accelerates moisture dump in wall and ceiling cavities, and delivers “dirty” air to the inside.

Three fan locations circled in red: west bedroom, great room, east bedroom
Three Lunos fans circled here in red: west bedroom, great room, east bedroom

A better choice—and the one I used—is what’s called a “balanced heat recovery ventilator” or HRV. These fan systems exhaust and draw in the same volume of air. This balanced air flow neither pressurizes or depressurizes the building shell—a phenomenon that helps stop unwanted air infiltration through holes. These types of systems also recover the heat energy of exhausted air for further comfort and energy savings. See my blog post “Electrical & Mechanical” for a detailed look at the Lunos HRV’s we installed.

Two Lunos fans circled here in red: loft and great room

There’s nothing more satisfying than learning something new, seeing how it can apply to the situation at hand, defying all nay-sayers and bean-counters, executing said new thing to the best of one’s ability, and having a hard number that proves success!

2 Comments on “Mind the Gap: Take Three

  1. Amber,
    That 0.38 “air changes per hour” number is jaw dropping! Had Jim the blower door guy ever encountered a house as airtight as yours in his years of testing buildings for air infiltration? I’ll bet not. More builders should go on your house tour so they can see what’s actually possible. Prospective new home owners, too. Congratulations on your success and fine example!


    • Yeah, it’s a really really good number. Everything I did I copped from other builders who post on the forums I follow or who published similar projects in magazines like Fine Homebuilding. Jim once tested a house at 1.2—the lowest he’s seen. The air-sealing didn’t take tools, advanced skills, or hard work. It just took time and patience, which I had. Thank you Eric—your finish carpentry, design expertise, and furniture pieces made this house look and feel solid, secure, strikingly handsome, and loved! Amber


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