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Energy-Smart Details

Slab on Grade Meets Partial Basement

A detail for ensuring thermal continuity when transitioning from a slab on grade to a partial-basement foundation

This slab-on-grade foundation was designed to receive a partial basement. The challenge was to ensure thermal continuity across all structural connections. The slab-on-grade portion of the house was a standard 4-in. concrete slab with a 32-in.-long by 18-in.-wide turn-down perimeter. The 10-in.-thick concrete perimeter foundation wall measured 7-ft. 10-in. high on a standard concrete footing.

The basement wall was the first portion of the foundation system to be poured. As seen in the detail drawing, the foundation wall stopped at roughly the bottom of the floor slab elevation. This allowed for the slab and the turn-down portion to be cast in a single pour and set atop the basement foundation wall. The edge form was then removed, and the rest of the assembly could follow.

Thermax-brand 2-in. foil-faced polyisocyanurate rigid insulation was installed to the inside of the foundation wall; Thermax is rated for exposed basement applications. It runs to the top of the concrete slab, which is insulated with 4 in. of Type IX EPS rigid insulation. Notice the EPS aligns with the interior face of the Thermax; this ensures thermal continuity to the inside of the foundation wall and slab.

A 2×4 16 in. o.c. wood-framed wall was built to the inside of the rigid-foam-insulated wall—framed flush to the face of the Thermax. A 2×8 16 in. o.c. wood-framed floor was installed atop the 2×4 wall. Note the flush condition between the 2×8 floor frame and the 4-in. EPS rigid insulation atop the slab. A 2×8 band joist on top of the wall’s double top plate gives a perimeter nailer for all the floor joists.

With the floor joists and EPS flush, it was easy to install two layers of 3/4-in. AdvanTech subfloor. With two layers, the 1-1/2 in. of subfloor above the EPS acts as a floating raft.…

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One Comment

  1. Expert Member
    Malcolm Taylor | | #1

    I'm curious what happens at the perimeter walls of the slab on grade portion. Presumably those walls are built directly on the slab? Did you consider moving the insulation under the slab and inside the concrete walls? To me that seems like it would simplify a lot of the detailing.

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