Question on workshop floor insulation
I am planning on building a 20×30’ woodworking workshop with “pretty good” energy performance. The building will have a slab-on-grade foundation because of ledge very near the surface. Because I don’t want to stand on a concrete floor for extended periods of time, I’m planning on using PT 2×4 sleepers over the concrete (with the 3 ½” dimension vertical) and a plywood floor and possibly a wood floor over the plywood in the future (funds permitting!)
I’m considering the usual 2” of foam under the slab, but am aware of the difficulties of insulating the edges especially since I would prefer not to have exterior foam around the perimeter of the concrete. Foam at 2” thick is about $1.00/SF for R-10. As an alternative, I’m also considering using Roxul 3 ½” ComfortBats (mineral wool) between the sleepers at a cost of $0.65/SF and an R-value of 15 (i.e. better R-value and lower cost – notwithstanding some thermal bridging which will degrade the R-value somewhat.)
Will the Roxul insulation system work (especially if I include a vapor barrier above the insulation and below the plywood), or will condensation on the concrete or some other issue be a problem? I’m guessing that the concrete directly over the ledge will moderately cold, but I’m not sure how cold (Climate Zone 6, Maine coast).
Note: the shop will be relatively dry (at least compared to a house) and will be heated 24/7 with a mini-split/grid-tied solar PV system. Walls will likely be around R-26 and the ceiling will be at least R-40. I’d go to higher R-values with these, but I don’t think the higher values are cost-effective for a workshop setting.
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