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Pouring a Concrete Slab Over Rigid Foam Insulation

Anna_Ohara | Posted in Green Products and Materials on

Hi there,

Asking for a friend (really!) who’s installing a stego vapor barrier under his slab. The plan was to put the neopor graphite polystyrene insulation on top of the vapor barrier, then pour.

He’s using that method at the suggestion of a builder he’s consulting. Now it’s time to lay down the barrier and pour in a couple days, but the concrete people are nervous/unhappy about having the insulation on top of the vapor barrier. From what I understand they’re unsure about the stability of it/the possibility that it will shift around and cause issues.

Any advice on how to proceed will be much appreciated
We’re in climate zone 5A—coastal NH.


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  1. GBA Editor
    Kiley Jacques | | #1

    Hi Anna,

    Martin Holladay addresses this very situation in this article: Determining Sub-Slab Rigid Foam Thickness. He says, “The answer to this question depends on at least four factors: (1) your climate zone, (2) the height of the slab with respect to grade, (3) whether the room above the slab is used for living space; and (4) whether the slab has hydronic heat.” (Providing this information here may help GBA experts determine how they will weigh in.)

    1. Anna_Ohara | | #4

      Thanks for the redirect! I'll take a look! He does seem to have the answer to everything....

  2. creativedestruction | | #2

    Best to have the concrete in direct contact with the vapor barrier. The foam has a better chance of drying and staying dry underneath the sealed vapor barrier on a bed of crushed rock, acting as both capillary break and drainage plane. Dry is good--wet foam will not perform to the rated R value.

    This applies in all climates, wherever foam is desired/necessary under a slab.

  3. GBA Editor
    Martin Holladay | | #3

    Here's a link to an article that may be more relevant to your situation than the article suggested by Kiley Jacques: "Polyethylene Under Concrete Slabs."

    1. Anna_Ohara | | #5

      Thanks very much!

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