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Community and Q&A

Are EPS seconds under slab a problem?

joge2468 | Posted in GBA Pro Help on

I’m in the middle of installing 2” of GPS ‘seconds’ on top of 1” of recycled XPS under a 4” basement slab. The GPS boards are not of uniform density. Now I’m wondering if my slabre is going to settle unevenly, which would super suck… Thoughts? 4” gravel sub-base has been compacted btw. Thank you for your help.

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  1. user-1072251 | | #1

    Frankly, I would not worry about that in the least. Concrete doesn't "settle" after it drys, as long as the base stays intact.

    1. joge2468 | | #2

      Hi Bob. You’ve hit on the crux of the issue. Some areas of the insulation boards are ‘softer’ than others. If those areas compress when the slab is poured, all should be fine. But if they slowly compress over time, the slab will crack and settle in those areas.

  2. user-1072251 | | #3

    We use 4"-6"of type 1 EPS, which is the least dense, under slabs. If you walk on the foam, you can indent the foam, so it's slightly soft. We also pour the floor over interior concrete footings which is solid. And we put 2" EPS between the concrete wall & footing and the floor, so except for the interior footings, the floor is mostly on foam. We've seen far less cracking initially and long term than ever, and none in many houses. After the floor cures, any loads on the floor will be spread out over a much larger area, and the chances for a high load on any point on the floor are greatly diminished. Also, concrete is heaviest when it's wet and full of water; it will probably be the heaviest per square foot it will ever be.

  3. dickrussell | | #4

    Run the numbers. The density of concrete varies with the mix, but a decent working number is 145-150 lb/cu.ft. Call it 144 for convenience, and divide that by 144 sq.inch per sq.ft. to get 1 lb/sq.inch of load for a foot thick slab, or just one third of 1 psi for a 4" slab. EPS compressive strength is 15 psi or more (, 45 times the actual load of the slab.

    1. Trevor_Lambert | | #8

      EPS compressive strength varies from 5-33psi. One of the most common types is type 1, which is 10-14psi. The issue at hand however is not the absolute strength, but the varying strength under different parts of the slab.

  4. joge2468 | | #5

    All of those calcs assume the foam board is has a UNIFORM density. I am dealing with factory ‘seconds’ that vary in density - from rock hard to spongey - and thickness within a single board. I’m not concerned with the EPS being able to handle the load. I’m concerned about the spongey sections condensing over time.

  5. user-1072251 | | #6

    Varying thickness are not an issue unless you're leaving voids under the top surface. If the top of the foam is not "flat", not an issue - concrete is a liquid when installed and will end up flat with no voids underneath. I might have concern about installing "sponges", but I'd think the concrete would flow into the voids and solidify even them. Do you have pictures?

    The density of foam does not vary with what is next to, above or below it. Each piece has a specific density, and whether a denser or less dense piece is on top does not matter with the loads we're discussing. Sponges are another matter.

  6. Expert Member


    I understand what you are worried about and agree it may be a problem. Having some foam that may compress more than the rest is equivalent to having the fill underneath compacted to different bearing strengths. Short of running this past an engineer I don't think you are going to get any reassurance it's not an issue.

  7. joge2468 | | #9

    Photo 1 shows edge where board is most dense but starting to loosen. Photo 2 shows the center of the same board where it is least dense. Walking weight does compress those areas.

  8. joge2468 | | #10

    Why can’t I post photos?

  9. joge2468 | | #11

    These may be in reverse order from mentioned above. Sorry.

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