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Under slab insulation

AsimM | Posted in Green Building Techniques on

I am building a house in MN(zone 6A). I have a couple question regarding under slab insulation.
1. I am planning on using 4″ of high density EPS. The question is if it would be overkill compared to using 2″?
2. Secondly, I had bought a couple pallets of reclaimed EPS sheets. The problem is that they are of varying thicknesses ranging between 1.5″ and 2.5″ with odd thicknesses like 1.80″ and 2.3″ etc. I will be making sure to layer them in a way that I achieve close to 4″ of thickness. But my question is that what would be the acceptable tolerance for the foam height? Would it be ok to have the foam 4″ thick in some areas and 3 3/4″ in others and so on?


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  1. Expert Member
    BILL WICHERS | | #1

    I don’t see a problem with slight variation in foam thickness under the slab. The concrete will basically act as a floor leveler and make up for the variation when the installers finish the top surface of the slab. Slabs are poured over rough patches all the time, gravel beds aren’t perfect surfaces either.

    It might be worth it to add a bit of wire mesh reinforcing though. I’d be more concerned with different foam densities than I would with different foam thicknesses as long as your different thicknesses aren’t too different (1/2” shouldn’t be a problem, 2” might be).


  2. RussMill | | #2

    If your variance is less than a couple inches you're fine. More than that you can get telegraphing cracks, unless you 45 angle the sharp edges.

    Im way down in zone 4, i still use 4 inches under slabs in conditioned space, AND SURROUNDING CONDITIONED SPACE, porches, etc. It makes a difference.

  3. GBA Editor
    Martin Holladay | | #3

    Q. "I am planning on using 4 inches of high density EPS. The question is if it would be overkill compared to using 2 inches?"

    A. I don't think it would be overkill. But if you want to learn more about my recommendations on this issue, see "Determining Sub-Slab Rigid Foam Thickness."

  4. AsimM | | #4

    Thanks everyone!

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