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Question on workshop floor insulation

user-3311655 | Posted in Energy Efficiency and Durability on

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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  1. GBA Editor
    Martin Holladay | | #1

    I wouldn't pour a slab in Maine without continuous horizontal subslab insulation (at least 2 inches) and vertical insulation at the perimeter. Just do it; you won't have a chance to change it later. The perimeter insulation is the most important, not the least important.

    That's what I did on my garage, even though I don't really plan to heat it -- just because. I protected the rigid foam on the exterior with PT plywood, and my 2x6 wall studs overhang the slab by 2 inches to cover the perimeter foam.

    Insulating between your 2x4 floor joists is much less effective than insulating under (and around) your slab. But you can do both.

    By the way, the polyethylene belongs under the slab, not under the plywood subfloor.

  2. user-4524083 | | #2

    Stuart - Martin is right. Also,you preferably don't want to put the concrete right on the ledge. One way around avoiding the perimeter vertical insulation( but it's more complicated) is to do 2 pours. First pour the "grade beam" reinforced with rebar, and insulated underneath and extending out 3-4 feet horizontally. Then foam inside vertically and foam under the separately poured slab. Now the slab is in the heated space and the foundation is "cold" but protected from frost heaves by the foam under it. The walls are built on the cold part with the inside vertical insulation covered by your new wall.I wouldn't go to the expense of an additional floor on sleepers. Wear cushioned shoes. Concrete got a bad rap for being uncomfortable to stand on. A solid wood floor on sleepers isn't exactly flexing while you're standing on it, though it will hurt less to kneel on it or if you fall down. Martin's solution is much simpler and allows the simplicity of one pour.If you don't like the idea of P.T.plywood covering the foam on the outside you could use cement boards and cement or one of those acrylic cement coatings that can go onto the foam. Get help with these details if you're unsure. There's lots of smart builders on the Maine coast, but many very good builders who know nothing of slabs and good insulation .It's pretty easy to figure out the difference between what people know and what they think they know.This site is a good resource. Best of luck to you on your project.

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