Launch Slideshow

Insulating the Foundation & Setting Custom Trusses

Insulating the Foundation & Setting Custom Trusses

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    Chris Corson applies 6-inch polystyrene foam insulation to the basement wall after forming, pouring, and waterproofing the basement walls.

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    Sub-slab insulating foam and a poly vapor barrier in place before the slab is poured.

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    His crew is calling it the "Corson truss" — a hybrid member built up out of ripped 2x stock and short pieces of wood I-joist, used to fatten walls for super-insulation. Here, carpenter Luke Small stands on a truss web to test the system's strength. It's plenty rugged, says Corson — but on balance, he's not sure it's practical for the entire wall.

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    A crane flies a set of roof trusses into place for the Surry Passive House. Achieving the necessary R-values will require packing the cathedral truss voids full of cellulose insulation.

Last year, Coastal Connection followed the story of an affordable Passive House under construction in the Maine hamlet of Knox, near the coastal town of Belfast. It was builder Chris Corson's first Passive House, and he set a tough goal: bring in the completed home for under $135/sqft, and exceed the stringent Passive House airtightness and energy consumption criteria. In the end he succeeded at both, and, with the inclusion of a seven-panel photovoltaic array, got the house down to zero net energy as well (on paper only). JLC covered the project in a two-part article: see "An Affordable Passive House Part I" (May 2012) and "An Affordable Passive House Part II" (June 2012).

This year, Corson is working on another Passive House — this time, with a more ample budget, and a more complex design. Located in Surry, Maine, on the rocky shore of the Union River Bay, the cape-style house offers a few more construction challenges than the simple, boxy Knox design. And with the Knox house under his belt, followed by another custom home and a whole-house retrofit, Corson is applying the lessons learned to improve building assemblies and cut costs.

The Surry house's foundation is a full basement with a bulkhead entry, rather than the simple raft slab of last year's house. So instead of using the thick EPS foam as an insulated form system, Corson had to form and insulate in separate steps. It's a full basement, but that comes at a price: says Corson, "Including the concrete and the sub-slab and perimeter insulation, the Knox house has an $8,500 foundation. This is a $20,000 basement. But for the difference, the Surry homeowners are getting 1200 square feet of clear-span useable space."

Even though the basement floor is below the frost line, Corson insulated under the slab with the same 8-inch polystyrene insulation. Seams between blocks of insulation were sealed with injectable foam sealant, and then taped with 3M tape. Then, a poly vapor barrier was applied over the insulation — again, taped at all joints and seams.

Corson's frame wall system has undergone a few small improvements. Window bucks have been downsized from 1.25 LSL stock to 1.125-inch rim-board material, to cut costs and reduce thermal bridging. And where the I-joist outer wall framing passes by the windows, Corson no longer uses full-length I-joists — instead, he combines short sections of I-joist with ripped 2x stock to create a member similar to Larsen trusses. This eliminates a hard-to-insulate narrow gap that occurred next to the windows in the previous arrangement.

For a project last summer, Corson used the combination I-joist and sawn-lumber combination for all the exterior walls of a house, alternating with full-height I-joists, to save material and reduce thermal bridging (see photo below). But he says he's on the fence about that idea. "It's less material cost, but the labor of making up the pieces adds cost," he says. "And I don't think it's justified by the energy savings." And, he adds, blowing the wall cavities with dense-pack cellulose was difficult when he had created four-foot-wide cavities instead of two-foot spaces. So for the Surry house, he says, it's back to full I-joists — except at the window locations.

The complex roof on the Surry house added some challenges to the job. Instead of a simple flat attic piled high with cellulose insulation, Corson had to devise a more intricate truss assembly (below). But he was still able to achieve R-80 to R-123 insulation values throughout — enough to satisfy the Passive House design criteria.

Next spring, Corson will present back-to-back morning and afternoon sessions at the JLC Live conference at the Rhode Island Convention Center in Providence, R.I. (March 20-23). The morning session will cover on-site construction details and methods from four Passive House jobs, including the affordable house in Knox, two higher-budget custom houses (including the Surry project), and a retrofit project. The afternoon session will cover technical issues, explaining how the computerized Passive House Performance Package (PHPP) was used to design and detail the four projects and to verify compliance with the protocol.