Progress continues on the high-end custom Passive House under construction in Wayland, Massachusetts, by Auburndale Builders. In September, JLC visited the job again to catch up on the latest developments.

By mid-August, the Auburndale crew had completed rough framing and sheathing for the house's double stud wall frame. With plywood sheathing installed and taped (including over window and door openings), the team did a preliminary blower door test, but fell far short of the required Passive House airtightness spec (0.6 ACH-50). Conducted on a rainy day, the blower door test revealed that plywood, like OSB, is far from airtight, production manager Mike Dutra told JLC (see slideshow).

Insulated Roof Hip Strapping
A framer nails reinforcing steel straps to 2x4 furring at the hip, applied over six inches of Roxul rockwool insulation.
Ted Cushman/JLC A framer nails reinforcing steel straps to 2x4 furring at the hip, applied over six inches of Roxul rockwool insulation.

"When we had the house under positive pressure," said Dutra, "you could see the air bubbles coming through the plywood." When the team reversed the fan to apply negative pressure, Dutra said, "It was like it was raining inside" as the fan sucked rainwater through the plywood — proof, if any is needed, that wind-driven rain poses a moisture threat to wood framed walls.

For Passive House, of course, the air infiltration is also a significant energy concern. But Auburndale had already planned on covering the plywood skin with air-tight, waterproof, vapor-open Majcoat fabric membrane. Once the membrane was applied and taped, and after window openings were cut and the Unilux Passive House windows were installed, the house scored .28 ACH-50, easily tighter than the Passive House spec.

Next the crew moved on to installing exterior insulation: six inches of Roxul rock wool batts. They pinned the batts to the walls using the T3 Insulfast system from Ramset, which uses six-inch hollow tube fasteners pinned to the wall with spiral-shank hardened steel nails for a connection with close to zero thermal bridging.

Carpenter James Stewart applies Pittsburgh Corning PC 99 waterproofing sealant to joints in a Foamglas insulated brick shelf at the base of the Passive House wall. The horizontal base piece, which will support a brick masonry exterior cladding, is high load bearing "HLB" Foamglas.
Ted Cushman Carpenter James Stewart applies Pittsburgh Corning PC 99 waterproofing sealant to joints in a Foamglas insulated brick shelf at the base of the Passive House wall. The horizontal base piece, which will support a brick masonry exterior cladding, is high load bearing "HLB" Foamglas.

With the insulation applied, framers began to strap over the roof insulation in preparation for a roof deck. Ultimately, the house will get a slate roof. Meanwhile, Auburndale's crew worked on building an insulated brick shelf to support the building's brick veneer cladding.