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Wall/foundation for ADU in cold climate

Anders_Bostrom | Posted in Green Building Techniques on

Hi all,
I plan to build a small studio/ADU (384 SF, 1 floor with 1/2 loft) this spring. We are in zone 6B with 7250 HDD. I would like to make the build to pretty good house “standards”.
I’m planning the wall and foundation per attached sketch. The soil is pure clay (60′ deep) but I had good luck with the same kind of foundation on another smaller building on the same property. 
The 12″ EPS below the slab might be seen as excessive (a common practice in Northern Europe) but the alternative would be to raise the gravel bed and have less EPS for the center parts but that might be more work.

Feedback appreciated.

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  1. GBA Editor
    Brian Pontolilo | | #1

    Hi Anders,

    For the most part, this all looks like a smart and elegant assembly. As you mentioned, that's a lot of insulation below the slab. You may find this article helpful, with a cost analysis of the thick EPS vs raising the grade another way: Determining Sub Slab Rigid Foam Thickness.

  2. Expert Member
    Peter Engle | | #2

    This seems like a great application for Glavel. Are they shipping yet? I know nothing about the cost difference, but it is very green and would make installation of an insulated slab way easier. Use the perimeter foam as the formwork, pile and shape the Glavel inside, right around your rough utilities, and cast the slab/footing.

    1. AndersB | | #4

      Glavel would be a great product to use. However, we are in the wild west where most lumberyards never heard about zip sheeting! Tough luck to find any "high tech" stuff here.

  3. walta100 | | #3

    The thing I like about pretty good house is that it is whatever you want it to be but to my ear R84 sub slab insulation is well beyond the point of economic return on investment.

    Zone 6 PGH is generally R10 sub slab, R40 walls, R60 roof.

    If in your heart you truly want to build a passive house admit it to yourself and embrace it.


    1. AndersB | | #5

      You are probably right. I might build up the center portion of the slab with crushed drain rock and keep one layer of R20 EPS. Build to PH standards is intriguing but doesn't make sense in our situation.

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