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Community and Q&A

Wing Insulation for Frost-Protected Shallow Foundation

DavidDrake | Posted in Green Building Techniques on

Beginning construction of a two story garage/home office, with 600 SF apartment above. (Plans for permitting go to the city early next week).

I’m going with a shallow frost protected foundation. For my area (air freezing index=960) horizontal wing insulation isn’t required, but I’m wondering if it’s a good idea anyway. Maybe 2″ of Type II EPS, extending 12″ at perimeter and 24″ at corners?

Vertical and under-slab insulation is 4″ of Type II EPS. Building is planned to be net-zero energy, with r-40 walls, r-60 roof, and 3000-4000 W of south-facing PV panels. Site is flat.

Given the high cost of building materials, I’d like to save money where I can, but obviously not at the expense of performance or durability.

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  1. user-2310254 | | #1


    I suspect the EPS will be one of the few "bargains" on your project. You also might be able to source reclaimed foam to help with costs.

  2. tvrgeek | | #2

    Not an insulation contractor, so just thinking out loud raising questions:

    With under slab, and vertical, not sure how much more the wing would do. Would it do more to have that much foam added to the vertical perimeter?

    Jealous you can insulate the slab. I lived in a Levitt house for 30 years. No slab insulation at all and worse the cement( asbestos) heat ducts ran only about an inch from the perimeter. Snow melted within a foot of the house. I am sure I was cooling the ground just as much.

    What is your frost depth? Might be a hind how deep to put the vertical insulation.

    These days, I bet foam is cheaper than a 2 x 4!

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