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Insulate Above or Below Concrete Slab

Debra | Posted in General Questions on

I have a friend planning an inexpensive home in North Carolina (CZ4).  It will have a slab foundation, in a relatively high termite risk location. She does not want to use foam boards below ground, but termite-resistant mineral wool boards are more expensive and not always easily available.

She’s considering installing foam board insulation and a floating subfloor on top of the slab instead, after installing Pango wrap termite/vapor barrier or Polyguard Term termite barrier.

Is that a reasonable option?  Any other issues about this that she should be aware of?

I also wondered if having the concrete slab within the conditioned enclosure (above the insulation) would help to moderate interior temperatures, when days get too hot or nights too cold.  Though that might not have much effect if the slab is under click flooring that has a thin layer of attached foam for padding. Not sure. Thanks for any feedback.

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  1. Expert Member


    Something I didn't know (not being that familiar with US codes) - and wouldn't have guessed based on their ubiquity in the building details posted here - is that sub-slab foam seems to be largely prohibited by the IRC in areas with termite problems.

    "318.4 Foam plastic protection.

    In areas where the probability of termite infestation is “very heavy” as indicated in Figure 301.2(7), extruded and expanded polystyrene, polyisocyanurate and other foam plastics shall not be installed on the exterior face or under interior or exterior foundation walls or slab foundations located below grade. The clearance between foam plastics installed above grade and exposed earth shall be not less than 6 inches (152 mm).

    1. Buildings where the structural members of walls, floors, ceilings and roofs are entirely of noncombustible materials or pressure-preservative-treated wood.
    2. Where in addition to the requirements of Section 318.1, an approved method of protecting the foam plastic and structure from subterranean termite damage is used.
    3. On the interior side of basement walls.

  2. Patrick_OSullivan | | #2

    Nothing wrong with above slab insulation and a subfloor, given appropriate details. I'm sitting on one right now.

    Having lived in spaces with flooring directly over a slab vs. where I currently have foam + 2 layers of 1/2" ply over the slab, I can say that the latter is way more comfortable under foot for long periods.

  3. bpontolilo | | #3

    Hey Debra,

    If you (or your friend) haven't read it yet, Martin has a detailed article on installing insulation above slabs ( This is a common detail in retrofits, and can work just fine in new homes too, as Insaneirish already said.

  4. Debra | | #4

    Thanks, folks!

  5. maxwell_mcgee | | #5

    Has your friend considered a foam glass product like Glavel? This may be a good situation to consider it. These would provide insulation as well as drainage, and should be of zero interest to termites or other bugs.

    1. Patrick_OSullivan | | #6

      This strikes right at the heart of Martin's recent article:

      How the heck does one get Glavel?

      I've come to learn something. If a manufacturer's website doesn't have a section that says "here's where you buy this product near you", it's probably really darn difficult to source.

      I don't want to "contact" you. I don't want to "request a quote". I don't want to "speak to a product specialist". I want to *buy* a product.

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