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Building structural load-bearing walls on high-density rigid foam

Matt Berges | Posted in Green Building Techniques on

Building structural load-bearing walls on high-density rigid foam. Besides making sure that the psi is strong enough, is there any reason not to?

Looking for examples this approach: Trench footer, CMU block (solid), infilled with compacted gravel making inside foundation flush with top of 8″ block wall, then 4″ of rigid foam over the entire surface, with perimeter bolted down by way of the sill plate, then 3″ concrete slab poured inside as finished floor (prior to outside walls). Below slab foam would then be neatly connected to outside wall foam, providing a seemingly simple insulated slab. I would share a sketch if I could.

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  1. kevin_in_denver | | #1

    The manufacturers and structural engineers and the City of Denver have all signed off on my project that uses a cast footing on foam detail.

  2. Expert Member

    Matt, here is a link to a JLC article on building foundations on foam. The only reason not to is if you have doubts as to the durability of the foam over the lifespan of the house. There is another thread here dealing with possible problems caused by insect infestations. Beyond that I don't know of any real threats to buried foam.

  3. jklingel | | #3

    Depending on who is doing the work, it may be cheapest to pour an 8" thick slab over XX" of foam and build on it. Done. Simple. It's easy to insulate, as opposed to an edge beam. I am sticking w/ the edge beam (barely) because the 8" slab will cost and extra $3K. (A ranch house has a large footprint.) So, I'll have to fiddle around insulating around the "bump".

  4. GBA Editor
    Martin Holladay | | #4

    For more information on this topic, see Foam Under Footings.

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