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Exterior foam foundation detail

kszach | Posted in Green Building Techniques on

I am installing two 1″ layers of exterior foam. My house is built into a hill. My question has to do with the upper part that is built into the hill. I cannot dig down to the footings. I was planning on digging down about 6″. I planned to do one continuous layer of foam from the point below the soil to the roof truss. I am a bit concerned with frost heave, insect intrusion, and god knows what else I may encounter as i live in the woods, since I am not going all the way down to the footings. will I be alright to run foam from 6″ below the surface to the roof truss. There is about 9″ of foundation above grade right now.

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Replies

  1. jklingel | | #1

    Is 2" enough to keep your exterior sheathing warm? Martin has an article here about how thick exterior foam should be for various zones (climates). If no one links it, it may be advisable to search here for it. It may be helpful to elaborate on your foundation system (stem wall to footer? basement?) and your location, too.

  2. GBA Editor
    Martin Holladay | | #2

    Kacey,
    Q. "Will I be all right to run foam from 6 inches below the surface to the roof truss?"

    A. Yes, as long as you use XPS or high-density EPS below grade (no polyiso), and as long as the R-value of the foam meets the minimum requirements discussed in this article: Calculating the Minimum Thickness of Rigid Foam Sheathing.

    Concerning ants: it's always possible to get ants in your foam, but ants are usually associated with damp foam, not dry foam. So be sure to have good drainage away from your foundation (properly graded soil, with a slope), as well as roof gutters and well-planned conductor pipes to channel your roof water away from your house.

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