Am building an outdoors sauna on a limestone skid foundation.

This is the layup plan from the top down.

Ceramic Tile

Cement board

(Floor box construction)

Top layer of 3/4 Plywood

2 x 6 non treated floor joists

Insulation fiberglass with vapor barrier under 3/4 ply

Poly vapor barrier under the fiberglass

Bottom layer of 1/2 plywood seals box

(Floor box sits on)

Treated 4 x 4 skids sitting on

#57 limestone compressed 3 inches deep

Weed block

Soil

What do you think about the poly between the insulation and the bottom layer of ply nearest the limestone foundation?

Will it block ground moisture from getting into the fiberglass? Or will it hold moisture in the fiberglass?

Other option is to do away with the fiberglass and bottom layer of 1/2 inch ply , leave it open and use hard foam insulation for the floor. But I don't see any foam that thick. Usually it is 1 inch or so. (In the past when I tried hard foam on an RV I had problems with it falling down after being glued in place.)

Thanks for any help with this topic.