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Community and Q&A

Slab insulation with exterior foundation insulation

frankschmitt | Posted in Energy Efficiency and Durability on

I have a 1914 bungalow in Climate Zone 4C. About 15 years ago I had the foundation under most of the house replaced. It has a footing (15” wide IIRC), and an 18”-tall, 8”-thick stem wall topped with an 18” 2×6 cripple wall up to the original floor joists.

I’m planning to add an ADU under the current main floor by lifting the house around 5 feet, and (to drastically oversimplify) replacing the cripple wall studs with 6-foot studs to make the current crawl space 8 feet tall, and then pour a slab whose edges will rest on top of the footings (ending up 12–16″ below grade, aiming for as low as practically possible). 

If I insulate the exterior of the stem walls with R-10 continuous insulation, is it also advisable to insulate between the slab and the stem wall? What about between the slab and the footing? What if the slab is heated?

I’m also going to end up with a little concrete shelf around the perimeter based on where the 2×6 wall is. Can you all point me to some discussions of how to hide/insulate this?

The obvious thing would be to fur out the 2×6 wall with, say, 2×4 lumber and just run drywall down to the floor with some nailing strips.

But that’s almost as much lumber as building a double-stud wall, so I’m wondering if I should give up an extra 3” of floor space to get to around R-33. I’ve also seen the inner wall used in a clever way to allow sistering floor joists after the house-lifting apparatus is removed (which I might well have to do). 

Lastly, what do you think of building the inner wall on the top of the footing and then lowering the slab to be flush with the footings? That buys an extra 4–6″ of (lower) height, but it probably adds complications that I’m overlooking. 

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Replies

  1. Expert Member
    MALCOLM TAYLOR | | #1
  2. jollygreenshortguy | | #2

    "What if the slab is heated?"
    Check your local code. If you use floor heat you may be required to insulate under the entire slab, regardless of what you do elsewhere.

    1. frankschmitt | | #3

      I’ll definitely need to have R-10 under the slab, but I’m mostly wondering if a thermal break is important at the slab edge with exterior insulation (which a strict reading of “under the entire slab” suggests it is). I could see the footing being a somewhat significant heat path to the ground, and presumably it’s not practical to insulate under the footing once it’s poured.

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