Rigid insulation between interior bearing wall footings and slab?
We’re building a new 2-story home in central Iowa. 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: https://www.greenbuildingadvisor.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."
Have this same question… what are people doing? I’m getting ready to pour our footers/foundation in the next week or so and I hadn’t thought of this detail and what best to do. I was planning on have a few load bearing walls in our full 9’ basement to save on beam costs. From reading several threads, it seems the choices are either to 1.)pour the interior footers along with the exterior foundation footers, then put the 2” of XPS over everything with no breaks/gaps, or 2.) pour monolithic slab with thickened interior concrete for the footers and either a.) stop the XPS at the trenches for the thickened areas, or b.) create trenches lined with XPS for the them. Or of course ditch the load bearing wall idea and just run beams. But if I do that, I’d still have to figure out how to solve the same problem at the footers under the support posts…
jvidamins,
A third alternative is to pour the interior footings with a short stem-wall above, and run the foam over the footings, but stop it it a the stem-wall. This leaves you with a continuous un-insulated stem-wall, which isn't that big an area. The approach works better with posts on pads where you only end up with a 6"x6" area of un-insulated concrete at each one.
Before deciding on a solution, check what your code says. Under ours, going from separate footings and stem-walls to thickened slabs, moves it from the prescriptive solutions offered in Part Nine, to Part Four where it requires an engineer's approval.
For interior bearing walls, I typically do 2.b, trenches lined with foam. But I've always done that when working with an engineer, so I don't know what issues you may or may not have with inspectors if you don't have a stamped detail to show them.
If you can talk to your inspector about your proposed detail before they show up on site, that will likely save you a lot of heartache.
I’m leaning toward option 1 above using the more dense 60psi XPS under the load bearing walls. Does anyone see a problem with this?