In New Hampshire, Chris Roche is pulling together plans for a new energy-efficient home. The slab-on-grade design calls for double-stud walls insulated with dense-packed cellulose, what Roche believes is the most economical and practical approach for getting the walls to R-40, or beyond.
Roche has paused at an important detail: the layer of rigid foam insulation that separates the concrete foundation wall on the outside from the slab on the inside.
Roche writes in a recent Q&A post that just about every foundation design for a slab that he’s seen either puts all of the rigid insulation on the outside of the foundation, or splits it half-and-half between inside and outside. In either case, Roche sees a problem.
The aim is to insulate the foundation wall with a total of 4 inches of expanded polystyrene (EPS) insulation. If the foam goes on the outside of the wall, it creates a difficult flashing detail, plus he’ll have to protect the outer surface of the foam from damage. If he puts 2 inches of foam on the outside and 2 inches on the inside, the concrete may stay to cold, and transmit that to the interior slab.
Roche saw a solution to this dilemma in a recent GBA article on double-stud walls by Dan Kolbert in which the entire perimeter of the slab was insulated with EPS to the interior.
“This would effectively create a cold foundation wall but warm interior slab,” Roche says. A potential weakness with this approach is that the interior stud wall would be partially resting on foam, so Kolbert had suggested placing plywood under the bottom plates to provide some additional support.
Roche has followed that up with a design detail of his own, shown in the illustration at the top of this article. A video…
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