As a HERS rater working in western New York, Matt Bowers is well acquainted with blower-door testing. Since he started working for Airtight Services (Marion, N.Y.) in 2009, Bowers has tested a lot of houses for air leakage—some of them for Energy Star verification and others just to meet the New York state building code.

House Airtightness Testing Using a Duct Blaster

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HERS rater Matt Bowers builds a custom frame and mounts his Duct Blaster fan in a window so he can use the device to test his whole house for airtightness.

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But Bowers is also a certified Passive House consultant, and when he started to design his own house, he made up his mind to shoot for Passive House certification—and the Passive House standard for airtightness is much more restrictive than the Energy Star standard or the building code. The Energy Star standard in effect in New York state requires a house to test at 3 air changes per hour at 50 pascals (3 ACH50); the New York building code, when Bowers started his house, allowed 7 ACH50, he said, although the state’s new code, which takes effect this summer, will match Energy Star’s 3 ACH50 limit. But the Passive House airtightness spec is 0.6 ACH50—five times tighter than the new code or Energy Star. So Bowers knew that building an airtight shell was going to be a critical issue for his Passive House project.

On the phone with JLC, Bowers described the air barrier and superinsulation strategy for his 2,800-square-foot house. “It’s a double-stud-wall assembly,” Bowers explained (for a more detailed look, see Bowers' blog posts, "Wall Assembly 1" and "Wall Assemby 2"). “The interior 2x4 stud wall is load-bearing, and that wall frame is sheathed with the Zip System. Then we have 8 inches of cellulose, and another 2x4 insulated wall over that. So the overall thickness of the insulated wall is 16 inches, and the air barrier in that double wall assembly is the Zip sheathing on the exterior side of the interior load-bearing wall.” The exterior 2x4 wall’s drainage plane is ProClima Solitex Mento membrane, which allows the dense-pack-cellulose insulation in the wall to dry readily to the exterior. Meanwhile, the airtight Zip sheathing is located well toward the conditioned interior of the house, largely eliminating any risk of vapor condensation on the Zip face in winter.

(Passive House design and construction methods emphasize not just air-tightness, by the way, but also the elimination of thermal bridging. Toward the end of the construction process, Bowers inspected his new home with a Flir infrared imaging device, viewing the interior walls with the scanner as outdoor temperatures soared during a New York heat wave. Below is the video of that inspection, demonstrating the thermal integrity of Bowers' chosen wall system under extreme ambient conditions.)

Following the advice of the Passive House community, Bowers decided to test his enclosure for airtightness at various stages of construction, starting when the interior load-bearing wall was complete, but before windows or doors were installed. “That way, if we got a large jump in blower-door numbers between one test and the next, we would know where to look,” he explained. If the house got leakier after he installed the windows, for example, he would know that the windows, the way they were installed, or both, were the source of the new leaks. And if the wall itself turned out to have leaks early on, Bowers would find out while the structure was still accessible for repair.

But for his first tests, with the door and window openings sheathed over and no doors or windows installed, Bowers had only one opening for placing his test fan: a basement window hole that was too small to hold a standard blower door. That’s when Bowers decided to use a Duct Blaster fan instead. “So for our first couple of tests, we taped the Duct Blaster to a sheet of rigid foam insulation over the basement window hole, and the foam board was foamed in place with the one-part fire-rated foam,” said Bowers.

As the job progressed, Bowers installed a window in the basement opening, along with the rest of the home’s doors and windows. But the operating doors in the main walls were too large for a standard Minneapolis Blower Door’s telescoping framework. So Bowers stuck with the Duct Blaster, crafting a custom testing assembly from a scrap piece of Zip sheathing and some other handy materials. He made his Duct Blaster frame to fit a window unit that he had installed in six places, so that he would have a choice of test fan locations.

On his first test, Bowers was hoping to score well below 0.6 ACH50. But his results far exceeded those expectations. “The first test came back at .15 ACH50,” he said, “and to be honest with you, I thought it might be a mistake. I was concerned that we had set the test up wrong, because I had never done a blower-door test with a Duct Blaster. So I talked to Energy Conservatory, which supplied the device, just to be sure that everything was set up right. And they verified that the setup was correct.”

That first successful test didn’t leave Bowers with a punch list—everything seemed to be fine. He ran another test after installing windows and doors, and again after installing air ducts and lines through the envelope for the Zehnder energy-recovery ventilator and a pair of Mitsubishi heat pumps. “And it was funny,” said Bowers, “but—everything I had read said, ‘Your tightest blower door is going to be the first one that you do, when there are no holes in the system.’ But not us—the more holes we made, the tighter it got.” Bowers gives credit to the project general contractor, builder Tad Garbacik: “I would not have been able to achieve these results without him.”

As the house nears completion, Bowers is continuing to test. In June, he ran another Duct Blaster test, as well as a standard blower-door test (using extra tape to fit the blower door into the building’s oversized door). At 50 pascals of pressure, airflows measured from 50 to 65 CFM—about 2 1⁄2 square inches of equivalent air leakage area, Bowers calculated. It’s hardly worth worrying about, but Bowers said there were a few things to keep an eye on: “We’ve got temporary holes for temporary power outside. We’ve got plumbing that hasn’t been finished yet. We’ve got windows that need to be opened and the frames need to be wiped clean because of construction dust.” Still, with results this good, Bowers is into the zone where the test apparatus itself may be responsible for most of the air leakage. “Making sure that the compression on the gasket was tight to the window, and everything was taped and sealed,” he said, “that’s where we actually spent most of our time.”