GBA,

I am building to zone 6 standards, living at zone 5/6 boundary…

I am building a 20′ X 8′ X 4″ concrete detached slab, under a 9′ tall deck and plan to enclose to create a storage space/shed under the deck.

I plan to use rebar and wire screen reinforcements in 4k psi concrete. There will also be (6) precast piers set 1′ inset in the corners, buried 48″ below grade, so the top 4″ of the piers will be even with the thickness of the slab and the top surface of the slab. I plan to install expansion foam wrap around the piers to isolate the piers from the slab.

Should I add 1″, 2″ or no ESP foam board under the slab? Shed will not be insulated.