what to do with an insulated slab over the winter
I’ve given up on getting any structure up on my workshop slab, so now I need to decide what, if anything I should do to protect it over the winter. I’m in zone 5, close to zone 6. The slab is 36×50, 6″ deep with 12″ thickened edge. There’s R22 of insulation underneath, and R33 of insulation on the edge. Eventually there will be skirt insulation around it, but I have to grade a slope around the slab first. The concrete is 32MPa, 4-5% air entrained.
My primary concern is frost heaving, so getting the skirt insulation done is certainly a good idea, but is a lot of work. My house actually has no skirt insulation, just 12″ of insulation on the slab edge, and it’s been fine for 7 winters. So I’m wondering if I can get away with leaving the area around the slab uninsulated for one winter.
The secondary concern is freeze-thaw cycling on the top of the slab. I think the air entrainment will help with this. Should I even worry about it at all? If the risk is only surface level, aesthetic damage then I’ll just take my chances. I’m planning on putting a floor coating on it, and filling in some areas of spalling and whatnot won’t be the end of the world. The only mitigation method I can think of is buying more foam insulation to cover it, which would cost me about $1000, and I’d have no use for it afterwards. I looked into straw bales, but that’s more expensive than foam. Should I cover it with plastic, to keep it dry? I could put an ice rink on the plastic.
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Replies
I don't see any need to protect the slab from freezing. If you have good drainage underneath with a proper layer of stone, you shouldn't have any issues with frost heave. You shouldn't have issues with spalling, either, that usually takes a while to develop over many freeze/thaw cycles. The only thing I might consider doing is putting a tarp down to make it easier to clean off in the spring (just pull the tarp off and the slab is magically pristine instead of having to get bits of leaves and insect/bird debris cleaned off), but that's a little something extra -- it's not a requirement.
Bill
I agree with Bill as long as you placed your insulation and vapor barrier directly under slab. If you placed rock on top of foam I would cover to reduce risk of water entrapment. Also try to get a bit of slope away from footings. I wouldn't worry about frost heaving unless you have bad drainage and a lot of deep freeze/thaw cycles
This question was first asked a year ago and was the subject of a Q&A Spotlight:
https://www.greenbuildingadvisor.com/question/insulated-shallow-frost-protected-slab-with-no-building-on-it
https://www.greenbuildingadvisor.com/article/protecting-a-concrete-slab-through-a-long-winter
I had forgotten I'd been at the same stage a year before. How depressing.
I found this statement especially painful:
"Forming and placing a concrete FPSF is pretty fast—usually less than a week —so there may be little time-saving advantage to installing the foundation and slab before winter"
My contractor took four and a half months getting it ready to pour, and still wasn't done. I ended up finishing his job for him.
Trevor,
I don't understand what has happened. The typical time-frame from excavation to getting an occupancy permit for the houses I built was four to five months. Now as you say it seems to be a year or more. Everyone works at a good pace when they are on site, but then disappear for weeks on end. I'm baffled.