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Community and Q&A

Sub-Slab Vapor Barrier: Above or Below Insulation

airfix | Posted in General Questions on

My contractor was thinking of putting the 6mil polyethylene vapor barrier below the 3″ of slab insulation.  I assumed you want it above the insulation?  He was concerned about the wire mesh used for tying my in slab radiant floor tubing t0o would damage the polyethylene.

Is it necessary to seal the polyethylene to the foundation wall before the slab is poured?


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  1. Expert Member
    Dana Dorsett | | #1

    The damage to the polyethylene is minimal- it's still a vapor barrier even if not air tight.

    Putting the polyethylene above the foam minimizes the risk of "iceberging", where if concrete gets under a corner of the foam during the pour it floats up, creating a thin spot in the concrete or even sticking out of the top surface. It also guarantees that no pockets of water get stored/trapped under the foam that could take weeks or months to dry through the foam + concrete, potentially damaging the finish floor.

  2. GBA Editor
    Martin Holladay | | #2

    I advise you to read this article: "Polyethylene Under Concrete Slabs." Check out the comments on that page as well.

    The polyethylene is a vapor barrier, not an air barrier, so there is no need to seal the poly to the foundation wall before placing the concrete.

  3. Aedi | | #3

    Always place the polyethylene barrier above the insulation. Otherwise you will trap water in the insulation layer, which is bad.

    The polyethylene will likely get damaged, but it doesn't matter too much. Joe Lstiburek explains it well in this article:
    "The polyethylene does not need protecting. You can poke holes in it, you can puncture it, you can tear it, you can leave gaps in it, and pretty much have your way with it as long as it is in direct contact with the concrete. Huh? But, but, vapor barriers have to be continuous and free from any holes. Actually, no. Air barriers need to be continuous and free from holes, but vapor barriers do not need to be. Lots of vapor moves by air movement, not a heck of a lot of vapor moves by vapor diffusion. The concrete slab is the air barrier, and the ripped and torn and punctured polyethylene sheet is the vapor barrier. It’s that Fick’s Law thing. Diffusion is a direct function of surface area—if I get 95 percent of the surface covered I am pretty much 95 percent effective—and the parts that are left I have filled with concrete which is also pretty good as a vapor barrier. I could wear golf shoes and march around the plastic vapor barrier and not do much damage."

    As for sealing the polyethylene to the foundation wall beforehand, I do not believe it is strictly necessary. I'm not as confident on this detail though.

    Edit: Martin and Dana beat me to it.

  4. airfix | | #4

    You guys are great. Thanks for the links.


  5. stuartd | | #5

    Too late now, but despite instructions to my builder, my basement floor in climate zone 6 was poured directly on top of 2" blue XPS, with poly vapor barrier below, and crushed stone below that.

    top to bottom:
    2" xps
    vapor barrier
    crushed stone

    From what I've read, it will take a lot longer to dry. I was not present at the pour, so do not know if there was any iceberging.

    Based on advice from concrete co., he would not wrap the vapor barrier up the sides of the basement walls, so it is simply cut off where it meets the wall (and is now not visible since foam is above it)

  6. blamus20 | | #6

    If holes are ok Isnt the foam (even without taped seams) already a vapour barrier? Why need the poly at all?

    1. Expert Member
      MALCOLM TAYLOR | | #7


      You may find this related discussion useful:

  7. AndyKosick | | #8

    I just wanted to note that practically every section, detail, or drawing I can find of this assembly online shows it with the poly on the bottom of the foam. Including one on this website.

    Not that we can change everything but this appears to still be an uphill battle.

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