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Community and Q&A

Double Vapor Barrier Under Concrete-Free Slab

timgodshall | Posted in General Questions on

I am installing a concrete-free slab for a client who read about these in Fine Homebuilding.  (very simply put: two layers of Advantech screwed together on top of EPS rigid foam on top of gravel)

He is concerned that the 6-mil poly vapor barrier between the rigid foam and the gravel will get torn by the gravel, so he would like an additional layer of 6-mil poly on top of the foam as well. The two layers of overlapping subfloor will be installed on top of that.

Is there any reason sandwiching the subslab rigid foam between two layers of poly would pose a problem other than using more plastic than necessary?

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  1. Expert Member
    PETER G ENGLE PE | | #1

    Though it probably won't cause any trouble, I still don't think it would be the best solution. To address the damage issue, you/he could just go with 10 mil poly or even a fabric-strengthened poly. Or, you could move the poly up to the top of the insulation. From a building science standpoint, I'd prefer it on top anyhow. In that position, it is closer to room temperature and that reduces the risk of condensation on top of the poly. Condensation on the bottom of the poly would only happen if room temperature was significantly lower than ground temperature and that should rarely happen. Even if it did, EPS/XPS insulation doesn't much care if it gets a little bit wet.

    1. timgodshall | | #2

      Thanks - that makes sense. After posting my question I found the "Polyethylene under Concrete Slabs" article on GBA that goes into some good details on this design question.

    2. don_christensen | | #4

      What about sealing penetrations? Also easier with the poly on top? If you have 2 layers of EPS, would you still put the poly on top, or between the 2 insulation layers? Again, this is specifically for a concrete-free slab. Thanks.

  2. Expert Member
    BILL WICHERS | | #3

    I agree with Peter -- just beef up the first layer of poly. Thicker poly will be more resistant to punctures, reinforced poly (the kind with the web of threads inside) will be more resistant to tearing. A few small pin holes doesn't make much difference with a vapor barrier. I don't really see a problem with two vapor barriers here since the material in between doesn't really care too much if it's wet, but I think one "better" barrier would be better.

    If you wanted more moisture protection, XPS is more resistant to moisture than EPS, but is less green. There is always a tradeoff, unfortunately.


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