I'm building bedrooms where a basement garage used to be. The floor of the former garage is a concrete slab that I have resurfaced. My plan is to install OSB subfloor and paint it for now, probably add carpet in the future. Two options for how to install the subfloor have been recommended to me:
Lay down 2x4 studs as "sleepers" at 16" centers, and place 1" thick styrofoam insulation on the concrete in the gaps between studs. Then add a plastic vapor barrier, then add 19/32 OSB subfloor on top. I happen to have lots of 2x4 (and 2x6) pieces left over from framing, and lots of this 1" insulation left over from my renovation, so this would be more economical.
Lay down whole sheets of 3/4" styrofoam insulation, with 1x3 strapping on top at 16" centers, then plastic, then OSB subfloor. I would have to buy all the materials. 3/4" styrofoam is an unusual thickness I might have to special order, but since I've already framed my doorways with the assumption that the subfloor would come up to 2 1/8", I can't use the 1" insulation. My sense is that having styrofoam between the concrete and the wood might be beneficial in terms of insulation, though, which would be the advantage of this approach.
For context, the house is located in eastern Maine and we have at least three months of cold, hard, winter. The question is: Is one of these options clearly better than the other? Criteria include: insulation value, cost, and comfort/performance as a bedroom floor.