Rigid insulation between interior bearing wall footings and slab?
我们建立一个新的事件位于爱荷华州中部的家中。It will have a below grade basement with 9′ foundation walls. The slab will be about 8 feet below grade.
The foundation walls will be insulated with 2″ EPS foam on the interior. The foam will extend from the top of the footing to the top of the foundation wall. The original detail for the slab insulation was for 2″ of Foamular 250 XPS over the top of the exterior footing, butting up against the 2″ EPS foam at the interior side of the foundation wall, and extending continuously over the top of the interior footings. We intend to lay plastic sheeting over the foam as a vapor retarder, between the foam and the poured slab.
Local contractors have cautioned us against this approach, citing that the interior footings should be in direct contact with the slab above. They say the 2″ foam between the footing and the slab will allow settling and will cause structural issues.
I have read on this blog that foam, with a 25 PSI compressive strength, has more compressive strength than many soils over which footings are typically poured. Should we continue with our original specs or raise the interior footings 2″ so the concrete slab will lay directly over the plastic sheeting and rest on the interior footings?
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Replies
Silas,
Is your proposed detail similar to the detail shown in the illustration below? If so, this detail works fine (although my preferred wall insulation detail omits the fibrous insulation between the studs).
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Thanks, Martin!
Silas,
The danger of differential settlement is the reverse of what the builders have been telling you. Pouring the slab directly on the footings is a good way to encourage cracking. There should always be a break, whether it is foam or several inches of compacted fill.
Resurrecting this old thread... as I am trying to find the correct answer to this same question that doesn't look like it was addressed in this 5 year old thread. When using sub slab insulation, how are the internal footers handled? My contractor is planning to pour the internal footers (internal load bearing walls) and the basement slab all at the same time as a monolithic footing/slab. Do we just stop the XPS or EPS at the footer trenches? Thank You!
cdgatti,
While this is only my amateur understanding, your situation sounds like it would be no different than an insulated monolithic pour where the exterior 'beams' are insulated under.https://www.finehomebuilding.com/2016/01/06/heat-loss-through-footings#ixzz410TN0SxD&i
And also this://m.etiketa4.com/article/foam-under-footings
I suppose if the exterior footing lacks insulation beneath there could be some concern with differential settling due to the 'creepage' referenced by Straube.
The OP's question is a bit different since its not monolithic, and I too am curious if the foam would be better between the footing and the slab or between the gravel and the footing. Both seem to have precedent in principle as applied to other details. Harder to detail the foam/plastic beneath the footings in that case.
Lol, I too am resurrecting this. What did you find, cdgatti? I just figure it would be easier to put that interior bearing wall on top of the slab, or I could thicken it and deal with leaving an insulation gap there. I also know that a PWF would just spec an 8 or 10" plate on gravel there. Please help!
Shawn, the builder and I were not comfortable putting the XPS foam underneath the interior load bearing walls. So the footers are about 12” of concrete over the gravel with the foam under all other areas of the 4” slab. We are very happy we did the sub slab insulation even though it is not typical for our area, E. TN. We have LVP in the finished area and it and the unfinished concrete slab are never cold to the feet.
Thanks cdgatti. I'll probably just put the picture from the code book in there and make the slab "thickened."