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Plywood and Foam Assembly for Finished Basement Floor

Brian F | Posted in General Questions on

We are finishing a basement and trying to end up with a floor that is carpeted. I’m considering foam board + plywood/osb subfloor approach in Martin’s article: https://www.greenbuildingadvisor.com/article/installing-rigid-foam-concrete-slab

The floor level has a slope of about 1.5″ across the room but with some local hills and valleys of maybe 3/8″.

How flat does the floor need to be for the foam + plywood/osb approach if the goal for the finished surface is just carpet? Can 1″ thick of XPS tolerate hills of 3/8″ and perhaps some shallow valleys? It seems pretty stiff, but I’m not sure if it conforms to the floor a bit over time.

I’ve read several articles about the foam + plywood approach but I’ve not seen the flatness requirements of the floor dealt with in any of them.

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