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sub slab insulation for pretty good house in CZ 3a

jon_b | Posted in Green Building Techniques on


In the first stages of designing what I hope to be a pretty good house in climate zone 3A (a bit east of Raleigh NC).   I have 3 questions. 

Background:  roughly 2k square foot 2 story house.  Monolithic slab on grade.  Finished concrete will be flooring downstairs.  

1.  Do you think adding R5 or R7.5 sub-slab insulation be worth it in terms of performance and comfort?  Not required by code but mentioned in several articles (

2.  Detail for covered porches.  Several parts of the house will have covered porches with concrete floor.  I do want to insulate the slab edges with vertical insulation.  Do you have ideas or  details for areas where there is an adjacent slab for a porch.  i.e. how do you thermally break the porch slab from the house slab? 

3. Termites, ants, etc!  Definitely a concern in my area.  For the vertical perimeter foam insulation would you use termite treated foam of mineral wool? I don’t see builders in this area using mineral wool for slab insulation but the article seems like it has the right idea. (

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  1. user-2310254 | | #1

    Here is what Carl Seville did with his recent build (also CZ3a):

    "I installed Owens Corning 1-inch XPS board around the perimeter of the slab and below the slab, 24 inches wide around the perimeter. This should give me all the insulation I need in Climate Zone 3, where it rarely gets very cold."

    On the termites, there are types of EPS that include borate.

    1. jon_b | | #2

      Thanks for pointing to that. Seems like a reasonable approach that wouldn't cost too much.

    2. user-5946022 | | #7

      My recollection is that Carl Seville also then separated the slab and below (including the xps board) from the framing above with a membrane through which termites won't go. I think he used Polywall's Term Barrier.

      The Polywall Term Barrier details are exacting and tricky, and it is almost impossible to get the contractors to properly execute the details. It seems like a good product if properly applied. Finding a detail that provides a continuous barrier and also does not subject the TermBarrier to UV light long term is challenging.

      You are correct to be concerned about thermal transfer. You could pour separate interior and exterior slabs, and isolate them with the Term Isolation joint barrier, which will also add a thermal break.

      1. Expert Member
        CARL SEVILLE | | #11

        See photos of my slab insulation and Term installation. The Term is tricky to get right, you install strips under the walls including sealing around J bolts and any other penetrations, then install sheets on the floor after walls are framed.

  2. dennisgray | | #3

    I am also planning a build in climate zone 3. My area has terrible termite problems and local inspectors do not allow any foam insulation under slab or at slab edge near soil (that includes treated EPS). Check 2018 IRC 408.3.1 for code on this. Also consider if you need termite inspection or bonding, as many termite control companies will not cover (warranty) a house with any foam insulation at slab edge without at least a 6" gap above graded soil (which pretty much eliminates any thermal advantage of the insulation). Check with your inspectors before going down this road too far.

    Too clarify, I would love to install 1.5" XPS from sill plate to about 12" below ground at outside slab edge (monolithic slab on grade). Not so much for the energy savings, but more for winter comfort. I have tried to find a non-foam insulation alternative, but have given up at this point and have figured it is going to be exposed slab edge, as the risks of termite damage are just too great.

  3. voldie997 | | #4

    How about Rockwool? Would that be a better alternative, given that termites are not interested in it.

  4. Expert Member
    ARMANDO COBO | | #5

    Here are my recommendations for CZ3 insulated slab or concrete floors, without any thermal bridging, and no bugs:
    1. Install a conc. footing, then a stem wall with a 6" interior brick ledge, then install 2" rigid foam or rockwool on top of the gravel base, covered with 6 mil. poly, then top it off with your 4" concrete slab.
    2. Install rigid foam on top of the concrete slab and inside of the walls, followed by 2 layers of 3/4" floor sheathing, then flooring.
    These pictures are from a presentation I made at the BS*+Beer Show awhile back.

    1. jon_b | | #6

      Thanks so much for the reply and suggestion. If we go with finished concrete floors, your assembly with the conc. footing/stem wall seems ideal. more steps and time than a monolithic slab but would keep the termites away from the foam entirely.

      I had a workshop that was a converted garage in our last house where I used 1 inch of foam over the slab with single layer of floor sheathing and was surprised by how comfortable this was.

  5. Expert Member


    In light of the problems exterior foam seems to be bringing to your project, have you considered using a foundation that doesn't require it? Something like stem-walls with a separate slab?

    1. dennisgray | | #9

      The foam insulation that would be used to separate the slab from the stem wall is a great termite path. This approach is prohibited in most areas of high termite infestation (I know it is prohibited where I am building).

  6. user-2310254 | | #10

    If you end up using Carl Seville's approach, rockwool might be the best option for mitigating the termite risk while containing cost.

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