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Is there a benefit to insulating only the exposed part of concrete foundation above grade?

Drew Goldsack | Posted in General Questions on

I am redoing siding and adding continuous exterior insulation on my early 70’s home in a cold (Zone 7) climate area and am wondering what to do about the above grade foundation.

I have approx 30-35″” of exposed concrete foundation above grade and am wondering if there’s energy efficiency benefit to attaching approx 3″ thick rigid insulation to the portion of the foundation that’s exposed? I know ideally the entire foundation would be insulated but the sort of excavation required isn’t feasible in this case. I can probably extend the insulation about 6-8″ below the grade level for a little added insulation as well as support for the insulation with back-fill.

I have read that concrete foundations act like giant heat-sinks and am curious if insulating the exposed portion would have much benefit?

The basement is finished and the interior of the walls are insulated with 2×4 framing and batt insulation.

The one functional benefit of adding rigid insulation to the foundation exterior is that it will even out the wall thickness to match the thickness of the exterior insulation on the walls themselves however there are certainly cheaper ways to do this if there’s minimal benefit insulation-wise.

Please let me know your thoughts!

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Replies

  1. Expert Member
    Zephyr7 | | #1

    Yes, it's definitely worth insulating that wall. The above-ground portion of the wall is actually the part that loses the most total thermal energy, as you go deeper into the ground, the temperature delta reduces and the energy loss is reduced as well.

    It would be best to insulate the entire wall, or at least down to the frost line, but if you can only insulate the above-grade portion along with a little bit near the surface, you've still added insulation where it will help the most so there is a big benefit from doing so.

    Bill

    1. Drew Goldsack | | #2

      Thanks for the feedback, I figured it should make a difference but didn't want to be spending the money if the benefit was minimal.

      Sounds like 3" of rigid insulation should definitely be worth the cost and effort.

  2. Expert Member
    Dana Dorsett | | #3

    Anything that's above grade is worth insulating for sure!

    But in zone 7 it's also worth insulating to as much as 4' below grade if & where possible. While 6-8" is good, deeper is definitely better.

    With the 2x4 batt insulation on the interior it would take at least R10 to have adequate dew point control at the cold side of the batts, but with an interior side poly vapor barrier that's less of an issue. While 3" of EPS would still be "worth it", 2" of EPS that extended 2' below grade would be better. IRC code minimums for zone 7 call for R10 to a depth of 4' even for slab edges:

    https://up.codes/viewer/connecticut/irc-2015/chapter/11/re-energy-efficiency#N1102.1.2

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