A Net-Zero Addition on Cape Cod

Contractor Mike Horgan installs a PVC vapor barrier onto the concrete wall footing for the new addition. To Horgan’s left we see the underside of the sill plate, with the flexible gasket already stapled onto the 2x6.

After cutting the plastic vapor barrier with a knife, Horgan slips the membrane over the foundation bolt.

Left, framing subcontractor Mike Hill unrolls the EPDM gasket, supplied by Conservation Technology. Right, Hill and Horgan apply gasketing to the underside of a sill plate.

Hill and Horgan set the sill plate into place onto the foundation sill.

Mike Hill holds the sill plate in position while carpenter Russ Laffin drills a pilot hole for a hand-driven split shank fastener.

Hill drives the split-shank fastener to hold the sill plate tight to the foundation and compress the air-sealing gasket. The wall assembly will later be bolted down, but Hill wants to make sure the sill is already tight to the concrete before he lays out the wall. Hill swears by the hand-driven split-shank fasteners, saying, “They’re great as long as you never want to pull them out. Once it’s in there, forget it.”

Russ Laffin holds a 2x6 in plumb position while Mike Hill drills a pilot hole for another split-shank fastener. This member is also gasketed to the existing concrete foundation using A BG65 flexible EPDM gasket supplied by Conservation Technologies. The EPDM gaskets offer several advantages: They conform readily to irregular surfaces, are flexible enough to install in sub-zero temperatures, and stay flexible indefinitely in service.

Hill drives a split-shank concrete anchor into the existing home’s concrete basement wall. With a correctly sized pilot hole, the split-shank fasteners are hard to drive into concrete, requiring a full swing with a framing hammer. Once driven, the fasteners are essentially impossible to remove, says Hill.

Hill and Laffin test-fit the bottom plate for the new wall.

After marking the position, Hill drills bolt holes for the walk-out basement wall’s bottom plate.

Hill begins to frame the new wall, starting with the window opening.

Hill sets a triple 2x10 header for the window opening in the new wall.

Hill completes the framing of the ground-level window’s rough opening.

Hill and Laffin stand the new wall.

The carpenters set temporary bracing before anchoring the new wall to its foundation sill.

Russ Laffin sets three-inch square foundation bolt washers in place on the new wall, as required by recent versions of the building code for walls in coastal locations that may be subject to high wind forces.

Laffin tightens down a foundation anchor bolt.

Hill and Laffin frame the first main floor system of the new addition. Here, they work on a header set at the level of the floor frame in the band joist area.

Hill installs an engineered lumber rim joist into the new floor system.

Plans for the new addition make the high-performance energy package clear: An extensive array of rooftop photovoltaic solar panels will cover the south-facing side of the existing house, as well as the new addition, powering air-source heat pumps as well as plug loads for the entire building. Given the owner’s plans to occupy only the new air-tight addition during the coldest part of the year, Horgan hopes that summertime production will balance out winter power consumption, and that the owner may avoid paying any power bill on a net annual basis.

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