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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?

Asked by Silas Hoeppner
Posted Sep 1, 2014 2:15 PM ET

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3 Answers

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1.

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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Foundation detail 14.jpg
Answered by Martin Holladay
Posted Sep 2, 2014 7:30 AM ET
Edited Sep 2, 2014 7:32 AM ET.

2.

Thanks, Martin!

Answered by Silas Hoeppner
Posted Sep 2, 2014 4:18 PM ET

3.

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.

Answered by Malcolm Taylor
Posted Sep 2, 2014 5:51 PM ET

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