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Community and Q&A

Below grade polyethylene and slab

Matthew Michaud | Posted in GBA Pro Help on

I am questioning as to the order of polyethylene vapor barrier to foam when placed under a slab. The reason I ask is because I am currently having my house built and today I have discovered that the contractor placed the poly directly on the ground (good drainable material) with the blue board foam boards on top of it. I have read in the past that the poly should be placed on top. I am worried that it has been done incorrectly. Any thoughts to put my mind at ease as I am rather stressed out right now. Thanks in advance.

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Replies

  1. User avatar GBA Editor
    Martin Holladay | | #1

    Matthew,
    The best approach is to put the polyethylene on top of the rigid foam (directly under the concrete). That way they poly won't hold "bleed water" or rain water that sits between the foam cracks or trickles down to form puddles under the foam.

    For more information on this topic, see Concrete Floor Problems.

    In your case, the solution is fairly simple. Get a 4-foot length of rebar and use it to poke about 50 holes in the poly by jamming it between the seams of the rigid foam. Then install new poly on top of the rigid foam. Remember, polyethylene is cheap.

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