Cold climate slab on grade
hello all, building a home in climate zone six a, relatively high water table, building slab on grade because we cannot dig a basement. my Builder wants to use precast superior walls for frost walls for the slab, I have not been successful trying to convince him to do his first frost protected cell foundation. it seems like excavating less would be a good thing with the water table only 4 feet down in wet seasons. he wants to use The precast superior walls because he can build up the ground and put the slab a little higher above grade. The gravel that these will sit on will potentially be wet a month or two out of the year.
Anyone here done this before? Is there a good way to insulate the slab perimeter using the superior precast walls?
Any insights or advice as always would be greatly appreciated.
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We built a house in Sunriver, Ore., which has cold winters. We based our foundation design on some "shallow" foundation science being applied in Alaska.
We dug out for an *unvented* crawlspace of close to 40" down (would need to look up the exact depth). Then built Thermomass poured concrete stem walls that are 11" thick, encapsulating 3" of XPS foam. You can get a lot of details, photos, and a cool video here:
https://home4jp.com/systems/foundation/
https://thermomass.com/
We sprayed closed-cell foam across the sill plate from the *inside* of the crawlspace. We placed 2 layers of R-15 rock wool bats on the membrane covering the crawlspace dirt floor (no "rat slab") and lapped over the concrete footing and slightly up the bottom of the stemwall.
The result is that our "thermal envelope" is across the living space lids (under an insulated floor of an unconditioned attic) down the insulated exterior walls, down the stemwalls, across a few feet of the rock wool on the bottom of the crawlspace, and across the uninsulated membrane on the interior of the crawlspace.
There is no insulation under the floors.
This appears to have worked well, although I don't have a comparable "control" to really know the magnitude. Instead of exposing the first-floor level's corner heat loss to the ambient air 10 F to 25F temperature, the thermal envelope's lower corner and the floor's heat loss are exposed to the upper 50's F temperature of the ground a few feet down. We also don't have to deal with trapped vapor and the hassle of putting insulation between the floor joists, and all our HVAC ducts, plumbing, etc. are easily maintained and protected from freezing.
I monitor temps and humidity, and the results have been good. One thing that was an unexpected bonus is that there's a slight benefit from the latency of the soil temperature's rise from summer to winter so the heat loss in winter is against lagged summer ground temps.
I've heard an observation that the only good crawlspace is a "basement," I have to disagree. I agree that's the case if you consider only the typical shallow, vented crawlspace and under-floor insulation. There are a LOT of reasons why that's bad idea.
But, assuming a "basement" means "conditioned" space, and probably even "living space" that's a heck of lot more expensive than a Thermomass, deep crawlspace.
If I were building the house over (same location same design, same budget), I might include a rat slab with some XPS under it. I would have to get some science-based advice because the dirt itself provides pretty good insulation, which I think is indicated because there's not much of a gradient between the surface of the crawlspace floor and a few inches deeper. I would have expected otherwise if heat from the warm floor were being lost into the crawlspace and then lost into the ground at the crawlspace floor. My "guess" is that most of the heat being lost *from* the crawlspace is being lost through the stem walls, more at the top than at the footing.