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Community and Q&A

Slab edge insulation and termites in Climate Zone 3A

NormanWB | Posted in Energy Efficiency and Durability on

My basement walls should be poured by the end of the month and I am wanting to get the details right in CZ 3A.

While slab insulation is not “required” for my climate zone, I am considering it for comfort reasons and to insure against the future (better safe than sorry). However, termites are common in my area and the house is on a wooded lot that will have plenty of food for them, so I am concerned about them coming up the edge insulation into the stud wall noted on the attached diagram.

Assuming I can properly seal this area from termite intrusion via a physical barrier (perhaps in concert with a chemical one), what thickness of insulation should I use on the edge of and under the slab in CZ 3A?

I did find this article on the site, but mostly it was dealing with exterior insulation:

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  1. GBA Editor
    Martin Holladay | | #1

    Your title refers to slab edge insulation, but the body of your question refers to a basement. Are you talking about insulation at the edge of a basement slab, or insulation at the edge of a slab on grade?

    The reason I'm asking: When it comes to termite worries, these worries usually apply to a slab on grade, but not so much to a basement slab.

    Assuming you are talking about a slab on grade, and assuming you come up with a termite solution that gives you confidence (and meets local code requirements), here are links to two articles you might want to read:

    Insulating a Slab on Grade

    R-Value Advice from Building Science Corporation

    The second of these two articles notes that the Building Science Corporation recommends the use of R-7.5 slab edge insulation in Climate Zone 3.

    If you are talking about a basement slab, you probably don't need any slab insulation in Climate Zone 3.

  2. NormanWB | | #2

    It's a basement slab. I am pretty sure I read recommendations for insulating under slab as well as the edge for comfort reasons even when not recommended for energy efficiency somewhere on this site (or maybe somewhere else). Perhaps that was just for slab on grade, though I am unsure why that would not apply to a basement slab.

  3. GBA Editor
    Martin Holladay | | #3

    For your basement slab, are you asking about slab-edge insulation, or a continuous horizontal layer of rigid foam under the entire slab?

  4. NormanWB | | #4

    Edge first and possibly some along the edge underneath, say 2' in to create a thermal boundary and, maybe, a "heatsink" with Mean Earth Temp of 64 F.

  5. user-2310254 | | #5


    Not sure if this product is still available, but I used Cellofoam PermaBG. It is an EPS with borate. At the time of my build, there was a manufacturing plant in Conyers, Ga.

  6. NormanWB | | #6


    I used to live in Conyers! Small world.

  7. GBA Editor
    Martin Holladay | | #7

    If you want to install rigid foam under your basement slab for the reasons you state ("to insure against the future" and "better safe than sorry"), go right ahead. Rigid foam under a basement slab doesn't raise any termite concerns, as far as I know. What you want is an inspection strip between the exterior grade and the top of your concrete foundation.

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