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Under slab

Tom Sloss | Posted in General Questions on

I have seen 2 different “absolute” sequences for slab radiant floor. Some say (from bottom up) gravel, foam (EPS, XPS,??), poly vapor barrier, then concrete. Others say the plastic goes below the foam. This seems like it should be an easy question. Maybe the radiant part makes it different? What is the proper order, and is EPS the best choice (cost, green, moisture resistance, benefit, etc.). Thanks guys.

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  1. Expert Member
    Malcolm Taylor | | #1

    Whether the slab has radiant heat or not, the poly must go in direct contact with the bottom of the slab. Otherwise you will get bleed-water pooling below the foam during the concrete cure, and it will take literally years to dissipate.

  2. Jon R | | #2

    The search button in the upper right, then "Polyethylene Under Concrete Slabs".

  3. GBA Editor
    Peter Yost | | #3

    What can make this confusing is the real function of this plastic vapor "barrier."

    Concrete at 4 inches has a vapor permeance of about 1 or so. 2 inches of EPS has a vapor permeance of somewhere between 0.5 and 3 perms and 2 inches of XPS a vapor permeance of about 0.5. So does the need for a layer of Class I vapor retarder between the sub-slab insulation and the concrete depend on what type of rigid insulation you use? I think so; I think this layer is a bit of a "belt and suspenders" approach to vapor control at the slab.

    IF this assembly lacks free-draining gravel or non-porous rigid insulation, then the polyethylene "vapor barrier" becomes your capillary break and it is definitely needed for both vapor and capillary moisture control.

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