Extruded foam over slab
I have an existing pole barn that I’m converting to a workshop. Since this building will be heated I would like to insulate the existing slab which is pitched to middle so that it could be washed down back in the day. I will be putting down a layer of plastic with treated sleepers over it ripped from 1 1/2” at the end to 2 1/2” in the middle and then covered with plywood . If I use 1 1/2” XPS will the 1” air gap in the middle be inefficient? Am I better off having someone spray open cell spray foam that fills the void completely? Thank you.
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Consider leveling the floor with more concrete, omitting the sleepers, using epoxy as a vapor barrier and using EPS (possibly of higher than usual density). See below (figure 7) regarding epoxy vs plastic:
What he said- the sleepers aren't necessary. Also consider using EPS rather than XPS, since the performance of XPS will eventually decline to that of EPS of similar density as it loses it's high global warming potential HFC blowing agents.
Re Dana's comment. You might be able to source reclaimed EPS at quarter to a third the cost of new product. Check Craig's List and/or do a general browser search.