Isolating Slab from Interior Basement Footings
Hello,
My ranch home in CZ5/6 with a walkout basement is planned to have multiple interior basement footings to handle load bearing walls. I desire to place 3″ of foam below my basement slab, most importantly for the thermal comfort on my feet when in the finished basement. I have been reading the various articles/discussions on the site around placing foam underneath footings and it still leaves me a bit uneasy…
I was wondering if a reasonable alternative would be to turn up the foam along the edges of these interior footings all the way up just like you do at the edges of the basement walls so there is a thermal break between the footings and the slab. This still leaves a bridge to the framed walls that will sit on those footings, but it at least keeps 95% of the slab fully insulated (and my toes warm).
I haven’t seen this alternative discussed before and wanted to get an expert take on it. Thank you for any input!
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Replies
BuildingAHome,
This is the detail I use. It leaves you with a very small thermal bridge under the wall, but doesn't place the foam under load-bearing elements. The slab gets poured and fills the gap in the foam down to the footing.
I saw-cut the slab directly under the framed wall above to minimize cracks.
Thank you for the response. Interesting approach! Have you noticed any temperature difference of the concrete around these areas? My suggestion implies the foam would essentially go up to the sides of the framed wall there, but that definitely seems like more work than your approach here and probably makes it more difficult to get everything level.
BuildingAHome,
I'm on South Vancouver Island (climate zone 4), so the differences in slab temperatures aren't large enough to notice, but in all but the most northern climates I suspect that would also be the case. Our code calls for sub-slab insulation, so this is what I've come up with, which also allows you to leave the slab as the finished floor without any finicky details. Bringing foam up the sides of the interior stem-walls, and still having enough concrete in between for bearing, would mean a 2"x4" wall above would leave a lot of the foam exposed.
3" of foam will mean your slab should track the temperate of the inside air to within a degree or so, but will still feel cool to stocking feet, due to the conductivity of the concrete.
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