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Community and Q&A

Vapor barrier on top of slab

Gloria Curtis | Posted in General Questions on

I just had a flooded bathroom and bedroom from a plugged septic system. Toilet stuck open and ran over into both rooms. I cleaned up all the water on the surface of tile bathroom and laminate floor in bedroom. The tile is fine and the laminate floor is curling so i know there is water under it. The house is on a cement slab with radiant heat. We put a plastic vapor barrier and a foam pad under the laminate flooring. My question is when i take the flooring up and remove the foam and plastic and dry the cement do I put plastic back in and also the foam before replacing the laminate? Is the plastic necessary? I feel without the plastic I could have turned up the heat and let the floor dry on the cement so no mold or mildew could grow. Help please a job in limbo


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  1. GBA Editor
    Martin Holladay | | #1

    Most residential slabs have a layer of 6-mil poly (or a similar, thicker vapor barrier membrane) directly under the slab. That's why builders rarely install 6-mil poly on top of a slab.

    Who installed your radiant-floor slab? Did the builder forget to install poly under the slab? Even if the builder forgot to install poly under the slab, there should by a continuous layer of XPS insulation under your slab. This XPS is itself a fairly decent vapor barrier. In short, it's hard to imagine that you need poly on TOP of your slab -- unless something unusual is going on.

  2. Michael Chandler | | #2

    My guess is that the laminate floor was installed on a new slab that hadn't thoroughly dried and the poly was laid under the foam cushioning to slow the loss of humidity from the curing cement into the floating laminate which might potentially cause it to curl at the edges or pucker at the joints.

    I doubt that the poly under the laminate was installed to make up for a lack of poly or foam under the slab or any issue with that, there may be a crack membrane under the tile but it's equally likely that the tile was laid directly on the new concrete,

    It may be that the floating floor is curling due to water pooling in the foam on top of the poly but it's more than likely that the water is in the concrete as well.

    Best practice at this point is to remove the laminate, foam and poly and leave them off for a month or so with the radiant heat on to allow the slab to dry thoroughly before replacing it. You will want to test the slab for moisture content before starting again with the laminate. Generally this is done by taping a 2'x2' piece of clear poly to the floor with duct tape and checking to see if moisture accumulates on the bottom of the poly after it has been there for a day or two. Rather than laying 6 mil poly under the cushioning foam you may just want to have the installer tape the seams but the poly is cheap insurance and the laminate installer would be within his rights to insist on it to maintain the warranty on his work.

  3. Riversong | | #3


    There should be neither foam nor vapor barrier under the wood laminate flooring. A radiant slab must have both a vapor barrier and foam insulation underneath, otherwise you risk a constantly damp floor and will be heating the ground more than the house.

    For good thermal transfer from the concrete to the wood flooring, there should be no insulating layer between them. The flooring can be floating on the slab or secured by mastic to the concrete, once the concrete is cured enough so that it is not releasing moisture. The maximum moisture release for applying any kind of flooring other than tile is 3 lbs water per 1000 SF per 24 hours. This typically takes three months of curing time if the slab is sealed below.

    As you discovered, a vapor barrier under a wooden floor on concrete creates a swimming pool in the event of a leak or even wet mopping of the floor.

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