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Amherst DER - First Blower Door Test

Updated: Jun 18, 2019

With the shell nearly complete, we staged our first test of our robust air barrier

temporary air barrier in place, when needed

We'd held off for weeks after most of the windows were in. We just needed a few more things done before we'd be confident that the test would mean something.

A blower door test is a pressure test on a building's enclosure. It's a common method for testing airtightness. I think of it as a way to test quality, and craft. The equipment is simple: a large-capacity fan (blower) and a collapsible frame with a canvas cover (door). With the building closed up we use the fan to depressurize the building - to blow all the air out of it.

When the building has been de-pressurized to -50 Pascals (roughly the effect of a 30mph wind whipping across all six sides), we read the pressure gauge and air flow calculator. The higher the reading, the more air is moving through the building - the leakage. The more leakage, the more heat loss, the more chance for moisture in the walls and roof, the greater the drafts, the higher the energy bills.

Nobody wants a high leakage number.

And on this day in May, we were testing the Amherst DER for the first time.

We knew it was going to need further work - the skylights weren't in, the basement sill hadn't been sealed yet, and the big bathroom window upstairs was just covered with a piece of plastic. It was a make-shift air barrier set up, and we knew it. But sheetrock was coming soon, and we didn't want to miss the chance to test the walls before they got all covered up. Once they were insulated and covered, we'd lose our best chance to find the cracks in the wall.

With all the installed windows closed and our temporary air barrier parts in place, we turned on the fan and depressurized the building. The fan's number showed the result of the crew's diligence: 1600cfm50 (cubic feet a minute, at 50 Pascals).

For a building like this, we intend to get it down to about 500cfm50, so we had a ways to go.

But our chief goal today wasn't the end number - it was to try to identify any big single leaks, or any installation failures that we could fix and prevent as we went forward.

So we stopped the fan and flipped it around. Now we'd pressurize the building. And fill it with fog. That way, if there were any areas that leaked more than others, we'd find them by following the trails of fog.

Holden Builders watching for motion

We started inside, watching the rooms full of fog to spot motion (leaks!) in the places we thought most likely. And then moved outside to search for wisps of fog along the outside of the building. [Yes, I called the fire department, in case well-intentioned neighbors thought the building was on fire. No fire trucks were dispatched in the filming of this blower door test.] We found a spot under an eave ("Oh, yeah, that connects to the ceiling of the hall, where we only had that board over a gap there - we'll go seal that next," the insights come tumbling out), and the mechanical penetrations at the basement sill, but those are easy to catch, and no surprise.

We'll come back once the basement's done and the last window and skylights are installed, and test again. By then, the fog will likely be unneccesary - the big holes will be sealed and only the tiny cracks will remain. And if that number's good, we'll feel confident that -with good insulation and a high efficiency ventilation system - these owners will enjoy a quiet, comfortable, draft-free, low-maintenance home for a long time to come.