Unfortunately if it’s not constructed properly during construction it’s almost impossible to correct later.

The Devil's in the Detail

While there are a variety of ways to handle this, in twenty years of insulating slab floors I have found the "bevel-cut" detail (Figure #5 below) to be the best compromise of achievability and performance. The key to achievability is not interrupting the subcontractors from their normal practices.

The stem wall is built with at least the last course being 6” block. This allows the rigid foam with a bevel cut top to be hidden below the wood framed or SIPS wall above. I prefer Dow Blue foam for its higher density and durability during construction. The only challenge is coordinating the installation of the foam between trades.

This is certainly one of those details that is often over looked and no one can see the difference when the floor is poured. But please don’t brag about your energy efficient walls when your house is built on an uninsulated slab floor.