Randy Williams of Willcon Inc. recently contacted me regarding a project he is wrapping up in northern Minnesota. I was intrigued because he has eliminated the concrete slab foundation altogether, which, given the material’s carbon load, is a design decision I fully support. There were additional aspects of his approach that were equally interesting, including his use of cutting-edge products in a residential building market generally unfamiliar with them; his success building directly off insulated concrete forms (ICFs); and his choice of a plenum truss for running duct work. To inform his decisions, Williams had several conversations with Steve Baczek and Jake Bruton, both of whom are experienced in the science of the “slabless slab.” Ultimately, in lieu of a slab-on-grade foundation, Williams used two layers of 2-in. Type IX EPS, a 6-mil. reinforced poly vapor retarder, and two layers of 3/4-in. AdvanTech subfloor.
In large part, Williams’s approach was a response to the clients’ desire for a heated floor. “I wanted a system that didn’t need heat in the floor,” he explains. “In our climate, the heat moves downward and warms the ground, not just the upper living spaces. The system I used creates a warm floor without having to incorporate a heat source.”
Though similar to Michael Maines’s frost-protected shallow foundation, Williams’s approach starts with excavating down 5 ft. Additionally, Maines puts sleepers on top of the EPS foam, and his flooring extends to the exterior walls. Here the floor system butts up to the double bottom plate of the wall framing. “My concern with Maines’s method was moisture,” says Williams. “We couldn’t build it fast enough to get it shelled in before the floor assembly would get wet. With two layers of AdvanTech, two layers of foam, and poly, we would have trapped in…
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