Details for a Complicated Airtight Floor System

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Where the parallelogram-shaped floor passes over a basement wall and onto piers, Thompson Johnson Woodworks crew provides for a continuous air barrier across the transition.

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This fall, JLC has been following the construction crew's progress on a custom house on Peaks Island, in the Portland, Maine, harbor. Last month, carpenter Mark Pollard and the crew from Thompson Johnson Woodworks formed and poured a complicated foundation: half insulated forms and concrete basement, half piers. Adding to the challenge were two big wrinkles: The house is supposed to be air-tight and net-zero energy; and the building's basic footprint is a parallelogram, not a rectangle.

This week, we take a look at the floor framing. As they framed the wood I-joist floors, Pollard and his crew had to attach some I-joists at a steep angle to the perimeter, to match the angled wall of the parallelogram-shaped foundation. The other puzzle was to detail the transition from the basement half of the foundation to the pier-supported half of the floor system, in a way that would let the crew maintain near-perfect airtightness at the joint between the two portions of the floor.

Pollard described the under-floor transition detail in an email: "For the floor system air control layer over the piers, I used Henry Blueskin VP100. I cut 18-inch pieces from the 48-inch roll, and removed 12 inches of the backing to expose the adhesive. I sprayed Blueskin Prep on the exposed portion of the mud sill, and some on the OSB rim board, and then stuck the VP100 to the foundation waterproofing, 3M tape, mudsill, and about 2 inches up the OSB rim-board. The remaining 6 inches of VP100 will be stapled to the underside of the first I-joist, the backing removed, then the membrane that will air seal the whole floor system will be adhered to that 6-inch portion. The insulators will cut through the membrane to dense pack the joist bays, and we will tape those holes." It's a hard description to follow, for sure. For a better understanding, take a look at the slideshow.