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How far should poly run up the edge of a monolithic slab on grade?

ranson | Posted in General Questions on

How far should polyurethane run up the edge of a monolithic slab on grade? If it doesn’t run all the way up (as I’ve seen in some details), should the edge be painted with damp-proofing?


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  1. GBA Editor
    Martin Holladay | | #1

    Although your question concerns "polyurethane," I assume you meant to write "polyethylene."

    In general, the polyethylene is installed under the slab. It isn't necessary to have any vertical polyethylene at the slab perimeter.

    In almost all climate zones, the perimeter of a monolithic slab on grade requires vertical rigid foam insulation that extends from the top of the slab to the bottom of the of the footing. See the detail drawing below.

    For more information, see Insulating a slab on grade.


  2. KJGInMA | | #2

    Mine is a followup question to Martin's answer to John. I have the exact same question, since either dampproofing OR poly would seem to be needed on outside. Martin does not mention the damp-proofing need, but I think he implies it since he says the poly does not need to extend to the surface. Unless I am mistaken and 'no damp-proofing on external edge' of slab is the recommendation, which seems incautious to me.

    Is there any reason why the poly cannot (or should not) be extended all the way up to the top vertical surface, instead of the way shown in the diagram? In that way, all of the concrete will have a good vapor barrier. The reason I ask is that I would like to make my protective surface for the vertical foam be fiber-infused stucco so I am researching whether the wire mesh (to support the stucco), the foam board and the poly can all be placed *inside* the concrete form such that when the pour is complete, then the foam/wire mesh is already firmly adhered to the vapor-protected external surface. I was thinking that long screws with large washers running through the mesh/foam and extending a few inches into the to-be-poured concrete would be advantageous (no drilling, no mastic, no dampproofing), though I fear that the foam placed in this way might pop up during the pour so am looking for a relevant descriptive success story. I feel I saw this as a recommended FPSF practice in a recent paper but don't have the reference at hand.


  3. GBA Editor
    Martin Holladay | | #3

    In the illustration that I posted, the rigid foam acts as a vapor barrier. It performs the same function as polyethylene. Exterior vertical insulation is required at the perimeter of a slab, unless you live somewhere with severe termite problems.

  4. Jon_R | | #4


    "...a continuous polyethylene vapor barrier and capillary break needs to wrap the entire concrete surface that contacts the ground—especially the perimeter"

    Note "continuous" - no joint/gap for ground water to enter/fill and then wick into your slab.

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