Cabin Floor assembly

This one's more of a builder forum exchange than a policy or program story, but it's worth a quick note if you're building or renovating a cabin, shed, or similar structure with a floor over open joists.

A contractor building an off-grid cabin in Maine asked whether skipping insulation between the floor joists and instead sandwiching rigid foam (XPS) between two layers of plywood, on top of the joists, would hold up over time. The idea is similar to a structural insulated panel (SIP) laid flat: half-inch plywood on the joists, then 1.5 to 2 inches of foam board, then three-quarter-inch plywood as the finished floor. His worries were whether the foam would compress and make the floor bouncy, and whether the foam poses a fire risk near a wood stove.

An experienced builder replied that he's used this approach successfully on two backyard structures, with no sagging or softness issues once the foam extends under the exterior walls and a wood perimeter takes the structural load. He noted the same method is used for foam-insulated floor slabs without problems, though the floor can feel slightly soft right where screws compress the foam at the edges. He also suggested sealing the foam's edges and taping the plywood seams underneath to keep insects out.

If you're weighing a similar floor build over an unheated crawlspace or on piers, this kind of foam-sandwich approach is a known, workable option, though the fire-safety question near a wood stove wasn't addressed in the discussion.