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Slab and foundation insulation for partially earth-bermed home

Mr. Big Fingers | Posted in Green Building Techniques on

Hello GBA forum members! First post here.

We are building a slab-on-grade, two-story home that will have concrete walls on the north and west side of the 1st level. These sides will be earth-bermed and the south and east sides will be framed (sort of like a walkout basement). We will be putting PEX in the slab. We are in Wisconsin.

I am unsure of where to insulate the concrete. I want to try and keep all of it within the envelope.

Please see attached image.

Firstly, I am wondering if using foamboard along the inside of the footings makes any sense (denoted 1 & 2). It seems that no matter how its done, heat will move by conduction from the living space to the earth through the footing via the poured wall (left side). Also, I am already putting foamboard on the outside of the footing and the outside of the concrete wall before we pile earth against it.

Also, is decoupling the slab from the footing necessary by beveling the top of the foamboard?

I am still thinking about this, so I may have more questions.

Thanks in advance for any input!

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  1. user-2310254 | | #1

    Kevin. Are you building this way because of site conditions or in the pursuit of energy savings through thermal mass? If the latter, have you read Martin's article "All About Thermal Mass?"

  2. Mr. Big Fingers | | #2

    Thanks for the link.

    We are earth-berming the walls because we can't dig a basement - the water table is high. Yet, we want some measure of "earth-protection" as tornados can be a threat in the area.

    I suppose that we could insulate the interior on the concrete walls to exclude the mass if it doesn't prove beneficial in this climate. We do have full southern exposure, are elongated east-west, and will be installing more glazing on the south side with 2.5 foot overhangs (more of a solar-tempering design than a passive solar one). The concrete walls won't be in the direct path of the sun (like a trombe-wall).

  3. Mr. Big Fingers | | #3

    I will add that we are not finishing the slab with any floor coverings. I guess I may have to do some calculating. Obviously, we are not as sunny as Colorado, but in the oak savanna where we live we do see a fair amount of winter sun.

  4. Expert Member

    If you are leaving the slab exposed you may want to reduce the width of the top of your foundation walls so that the intersection of the wall and the slab (where your foam is) is covered by the frame wall above.

    You can find a number of variations on the theme in the drawings here on GBA.

  5. user-2310254 | | #5

    It's a good idea to have protection if you are subject to tornadoes. FEMA ( has some guidance on how to create a safe room that might meet your needs without creating a partially above ground basement.

  6. rocket190 | | #6

    On the earth beamed side, I see no advantage to having foundation wall and footing four feet below the slab.

  7. GBA Editor
    Martin Holladay | | #7

    Your detail shows insulation on both sides of the stemwalls. While this approach is possible -- it's the way that ICF walls are insulated -- it's not typical if you aren't using ICFs. Since you have stated a preference for exterior insulation, that's the logical way to proceed. On the interior side of the stemwalls, you can skip the rigid foam.

    For more information on this issue, see Insulating a slab on grade.

  8. Mr. Big Fingers | | #8

    Thanks Martin. Would you suggest using 4" of XPS board under the slab or would it be better to use two 2" boards orienting the two layers at right angles so the seams do not line up?

  9. GBA Editor
    Martin Holladay | | #9

    If you are committed to using 4 inches of rigid foam, then it would be preferable to install two layers of 2-inch foam, installed with staggered seams, than to install a single layer of 4-inch foam.

    That said, the difference in performance between these two options will be vanishingly small.

    I'm not sure that you need 4 inches of rigid foam under your slab (unless you are planning to embed PEX tubing in the slab for a hydronic heating system). My guess is that 2 inches will be adequate, especially considering the fact that you will be installing quite a bit of vertical insulation -- vertical insulation that extends well below the slab -- at the perimeter of the slab.

  10. Mr. Big Fingers | | #10

    Yes, we are embedding PEX tubing in the slab for hydronic heat. We have it currently spec'd for two 1 1/2" layers of rigid foam for a total of 3 inches, but are considering increasing it to 4" total.

    The way I understand it from reading on this site and others, it is preferable to put the vapor barrier above the foamboard to not have any water-pooling issues. The guy who is helping us with the PEX install would prefer to use staples to fasten the tubing to the foamboard, but obviously this would perforate the vapor barrier. We are considering using wire mesh (and maybe dobie blocks) to avoid this. Would you please point me to more info about this if its easily available?

    Thanks to everyone else for your comments.

  11. GBA Editor
    Martin Holladay | | #11

    For a discussion of the pluses and minuses of each approach -- polyethylene above the rigid foam, or polyethylene below the rigid foam -- and how these two approaches affect the installation of PEX tubing, see this article (and the comments below the article): Polyethylene Under Concrete Slabs.

  12. Jon_R | | #12

    I recommend some perspective on tornado risk. At 70 deaths/year vs 21,000 for radon, you might be better off thinking about how to increase isolation between your normal living space and the soil, even in a low radon area.

  13. Expert Member
    Dana Dorsett | | #13

    Regarding the foam, the thermal advantage of using XPS is temporary, and comes with a heavy greenhouse gas hit. Over a handful of decades the performance of XPS falls to that of EPS of equal density. Going ~20% thicker on the foam to achieve the same labeled-R and using 1.5lb density "Type-II" EPS yields constant performance over 5 decades, and is usually quite a bit cheaper too.

    If you are stapling the PEX to the foam rather than tying it to the reinforcement mesh there is an argument for using XPS for the uppermost inch, for better staple retention. But the rest can be cheaper-greener EPS.

    In many locations there are business that reclaim rigid foam from demolition and commercial re-roofing products, and sell EPS & XPS at a steep discount, typically only 25-35% of the cost of virgin stock goods. If you're installing 3-5" of foam under the slab that can add up to quite a bit of cash.

    Tornado Alley covers a pretty large territory (and five US climate zones)- where is this project located?

    (edited to add), never mind- I just re-read the original post- it's Wisconsin.

    In a WI climate you'll want at least R15-R20 under a heated slab, which would be 4-5" of EPS. For virgin-stock goods that would be on the order of $1.50-2.00 per square foot, but with reclaimed EPS it'll be on the order of 50 cents per square foot, sometimes less.

  14. user-6184358 | | #14

    Your back wall is a retaining wall. You need to have a structural engineer design your earth berm wall. You seem to have drawn a typical basement wall. The cantilevered retaining wall will have a large footing to keep the wall from overturning and from sliding.

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