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Any concerns with 6 mil plastic over spray foam, under slab on grade?

daniel_kidney | Posted in Energy Efficiency and Durability on

The subcontractor pouring our slab on grade foundation put 6 mil plastic on top of the closed cell spray foam we had sprayed over the entire rock surface below the slab. I noticed there’s a little moisture sandwiched between the two. Any concerns with this?

I realize it’s atypical to put plastic over foam, but there was some question as to whether the inspector would allow foam only at the time the installation was done. We’ve since learned from the inspector that they are okay without the plastic, but barring any issues with the moisture (I think) I’m glad it’s there. I’ve noticed a few places where it looks like condensation is making its way up through the foam, plus the plastic is an actual vapor barrier not just a vapor retarder (Class II in this case).

What do you think? Could there be any long term negative issues associated with putting plastic on top of the foam? Or is any moisture likely to dissipate through the foam in time anyways?

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  1. Expert Member


    I'm not sure it does much good, but I can't see how it does any harm. It's basically an extraneous second vapour-barrier - (well third if you count the other layer of poly you will need directly under the slab). I don't see any worries about a bit of moisture being sandwiched between the two. It doesn't really matter if it ever dissipates.

    The only potential - and I think very remote - problem that could occur isn't related to that, but rather that there is no path for any moisture to drain which might make its way into the fill between the foam and the slab above.

    I wouldn't lose sleep over any of this.

  2. maine_tyler | | #2

    Out of curiosity, what is this spray foam on rock situation? It has fill between the spray foam and more poly, then slab?

    1. Expert Member
      MALCOLM TAYLOR | | #4


      I'm assuming it's something like the one Randy Williams did, but with an intermediate layer of fill.
      If the slab is poured directly over the foam, that decreases the worry even more.

  3. mr_reference_Hugh | | #3

    Hello Daniel, it is actually not atypical to put a vapour barrier over the foam with an insulated slab.

    If you read this GBA article you will see 2 assemblies showing the vapour barrier (class 1 retarder I expect) over the foam below the concrete.

    Here is a FHB article that explains insulated slabs. I attached an image about them explaining that the last layer before concrete is poly.

    There is another GBAarticle on this link but I will paste the relevant text below. They explain that putting the vapour barrier below the insulation would create a swimming pool for the insulation.

    “Does the StegoWrap vapor barrier go above or below the rigid foam?

    An important concern that arose during the coordination process was the location and detailing of the subslab vapor barrier. The vapor barrier was not clearly indicated in the architect’s details, although a vapor barrier had been specified. In the slab on grade assembly drawing, the vapor barrier was indicated to be installed below the slab insulation. The Walsh team questioned this location, given our concern that a large amount of water could collect in the slab insulation layer if it should rain prior to a slab pour. The configuration of insulation and vapor barrier essentially created a sealed “bathtub” that could hold a lot of water. Not a good scenario!

    Even though we were in the dry summer months in Portland, there is always a chance of rain. When we pointed this out, the architect understood the concern and agreed with relocating the vapor barrier to the top of the slab insulation. Furthermore, the detailing of the vapor barrier at the foundation perimeter was not clear in the design drawings. We discussed this with the architect and sorted out the termination details as part of the coordination process, working with the vapor barrier manufacturer’s standard details and sealing products. With these details resolved, construction on the building foundation began.”

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