I’m currently in the process of putting up a small cabin (~14×16) in southern michigan. It’s about 150 ft up hill from an epa flood zone, which does get standing water when it rains heavily.

It’s a stick building – floor is 2x12s set on 4×4 sleepers and 10″ piers, so it’s set quite a bit above ground. 

I’ve almost finish building the floor system, but I’m starting to second guess my choices. In order to reduce issues relating to moisture retention (specifically relating to fiberglass), I decided to go with 2″ XPS double stacked in between the floor joists, so basically 4″ of XPS. These are held up by small blocks on the inside of the joists so they will be held flush against the subfloor at the top of the joists. There is no air gap. I am sealing the corners and edges of the XPS with spray foam to close gaps.

Do I (or rather should I) throw a poly sheet on top of the foam board before I nail down the subfloor?

This is really meant more as a 3 season cabin, in so far as that usage will be year around, but probably less so in the summer. The current thought process meaning there (probably) will be more time spent heating than cooling the building.