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Vapor barrier underlayment + vinyl plank over XPS + plywood subfloor

onejeremias | Posted in General Questions on

We are finishing our basement with a lot of info from GBA. The slab is not insulated underneath but does have a vapor barrier (built 2020). We have 1″ XPS foam + 1/2 OSB tapcon’d to the concrete. We are about to purchase vinyl plank flooring, and the installer is strongly recommending an underlayment that acts as a vapor barrier under the vinyl planks. I’m wondering if this presents a problem with trapping moisture between the XPS and the underlayment. Some comments I read today indicated that even the vinyl plank itself may present a problem, and that we should go with something a little more permeable, but I wasn’t sure if that applies given our particular situation.

We are in NH, I believe climate zone 6A.

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  1. onejeremias | | #1

    Normally I wouldn't bump after just a day, but this question arose in the middle of negotiation with the installer, so we only have a limited time to decide whether to 1) go with vapor barrier and 2) skip vinyl plank altogether if it's not a good idea. So...bump :-)

    1. Expert Member
      MALCOLM TAYLOR | | #2


      If you were going to introduce another vapour-barrier, the place for that was under either the foam or plywood, as it's the plywood which is the only part of the floor stack-up that is vulnerable to moisture damage. I don't see what a VB flooring underlay helps.

      The vinyl flooring is itself a pretty effective VB, which does make the floor less resilient to moisture concerns that something more permeable. Whether that is a real problem depends on how effective your existing sub-slab VB is.

  2. onejeremias | | #3

    Thanks Malcom. I guess I'll just assume the VB under the slab is ok. I did have several boxes that had been sitting directly on the concrete for several months, and found them to be completely clear of moisture when I checked them, so hopefully that applies across the entire floor.

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