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Rigid Foam ABOVE Slab – VB Location

Bluegoose68 | Posted in Energy Efficiency and Durability on

I’m building in southern middle TN (zone 4A).  I’m going with a monolithic slab and 2×6 advanced framing.  I’d like to insulate the slab with R10 rigid foam.  I found an article on the GBA website that illustrates how to use rigid foam ABOVE a concrete slab.  (Picture is attached.)   This technique is very appealing because it removes the foam from ground contact (I’m right on the border of what’s classified as “very heavy” termite infestation probability) and it eliminates the thermal conduction around the edge of the slab. I’d like to use a termite barrier / vapor barrier on TOP of the slab; under the rigid foam.  Polyguard makes a product called “Term Wood Floor Underlayment” and Stego makes a product called Pango Wrap.  Both appear to function as both a physical termite barrier and a vapor barrier. Question 1) The GBA article shows the vapor barrier between the slab and the foam.  No vapor barrier beneath the slab.  This is definitely unconventional but it makes sense.  I’d like a second opinion from the experts that this is OK before telling my concrete contractor to “not worry about” the vapor barrier under the slab.  There’s no “undo” button on this decision.  Would a poly vapor barrier beneath AND the termite/vapor barrier above the slab hurt anything?  Would it cause the concrete to take too long to dry? Question 2) Anyone have any experience with either the Polyguard or Stego products?  There websites have info on their products but it’s difficult to find other people reviewing these products (especially the Polyguard). Thank you for your help.

NOTE: I changed the title, hopefully to better reflect the question.

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  1. matthew25 | | #1

    It won’t hurt to have a vapor barrier both below and above the slab. This is the only chance to put that one below so I would do it. If you are willing to tape the seams of your foam board that can double as a vapor barrier without needing another sheet product on top of the slab.

  2. Bluegoose68 | | #2

    I hope to get some consensus on the vapor barrier under the slab. We'll see.
    Regarding taping the foam board, I wasn't planning on doing that because I'd like to use the above slab vapor barrier as a termite barrier too. If I didn't need the termite barrier, I'd probably do as you suggest.
    Thanks for the comments.

    1. Expert Member
      MALCOLM TAYLOR | | #3


      I don't see much benefit from a below slab VB if you are including one above. What you do want is a layer of freely draining fill immediately below the slab to act as a capillary break. Depending on where you are it might also be acting as part of your radon mitigation system.

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