Shed to Tiny: insulating over subfloor – GreenBuildingAdvisor

The shed has some spray foam insulation underneath, but I don’t know the R value. Can’t really see much of the spray foam, so not sure what kind of condition it is in. I’d like to add a layer of rigid foam insulation over the existing subfloor. I’m thinking to use 2x3s (wide side down) 24in on center to create a support frame for the new floor. So the layers will consist of original subfloor (with the spray foam insulation), then the 2×3 grid with rigid foam, and finally birth plywood flooring. The shed will be on blocks to get it off the ground. Should I install a vapor barrier at some point? The location is Northern Maryland.