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Stem Wall Foundation and Super-Insulated Slab

user-7002058 | Posted in Energy Efficiency and Durability on

Can the concrete slab be eliminated in a super-insulated slab? The Passive House Build designed by Steve Baczek has a 4″ concrete slab sandwiched between 10″ of foam below and 1 1/2″ of foam and p.t. 2 x 4 sleepers above. Featured in the Nov/2016 issue of JLC was a home designed by Mr. Baczek that had a basement floor that consisted of 2 layers of 3/4″ Advantech plywood subfloor over 6″ of foam. The 6″ of foam was above a 2″ layer of pea stone, which was above a 6″ layer of 3/4″ stone. Can a plywood/foam/compacted stone floor similar to this basement floor be used in a home with a concrete stem wall foundation that would typically have a floating slab on grade?

-RickJT

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Replies

  1. Expert Member
    Malcolm Taylor | | #1

    Rick,

    The short answer is yes. I suspect we will see more houses built that way in the next few years. During that time the system may well be refined. Perhaps just one layer of plywood on sleepers, or some other variation? The problem with being an early adaptor is you end up working out the kinks for the builders who come after.

    If a client asked me to build one, I would be concerned with getting the foam layer flat enough so that the plywood subfloor was as level as those I lay on framing. I'd also be concerned that it would probably preclude framing in bad weather - something we do all the time here in the PNW. Apart from that I don't see any real difficulty.

    When I first saw the articles on building basements that way I was temperamentally opposed to it, but if pressed couldn't give a convincing reason why. It Just feels wrong I guess - but that's a poor reason to reject something.

  2. GBA Editor
    Martin Holladay | | #2

    Rick,
    Depending on the soil conditions and the height of the foundation walls, some basement slabs serve a structural purpose, I think -- helping prevent the bottom of the foundation wall from moving inward.

    With most foundations, the footings serve that purpose adequately (assuming the bottom of the wall is keyed to the footing) -- but if you are going to omit the basement slab, it never hurts to let an engineer know what you're doing to verify that the design will work.

  3. Stephen Sheehy | | #3

    Is the cost of a plywood subfloor and finished floor significantly lower, than a finished slab?

  4. Expert Member
    Malcolm Taylor | | #4

    Stephen,

    No. Two layers of Advantech will be significantly more expense than a slab. There might be savings if the slab had to be prepped depending on what finished floors the owner wanted. Your polished slab is hard to beat from a cost standpoint.

  5. Expert Member
    Malcolm Taylor | | #5

    Martin,

    I think Rick is suggesting the sandwich to replace a slab on grade, not a basement slab.

  6. Expert Member
    Michael Maines | | #6

    I designed an owner-built project that was built last year using a version of the Steves' (Baczek, architect and Demetric, builder) project. A professional engineer checked my design--an ICF frost protected grade beam, with 6" of EPS, 2x4 sleepers and a single layer of Advantech under a wood floor. If cost is the highest concern, Malcolm's right (except polishing can add quite a bit--often just troweling for longer than normal is good enough, and less expensive). My clients required a wood floor, so the concrete-free slab was actually less expensive. The owner/builder is a former mason so he was up for the task of getting the foam laid perfectly. Demetric presented at the recent NESEA conference and said that the double Advantech floor (with wood on top) was roughly the same cost as a slab would have been, fitted out with a wood floor.

    The other cost is environmental--Advantech has a much smaller carbon footprint than concrete. Time is running out to slow the pending climate disaster, and every little bit of effort helps.

  7. Paul Kuenn | | #7

    Anyone using 2x4 sleepers with one layer of Advantech? I'm assuming 16" on center. Any bounce? Thanks!

  8. Peter L | | #8

    How does one deal with termites? Having a wood floor at grade seems like a disaster when it comes to termites.

    What about fires and flooding?

    I'm sticking with the time tested concrete.

  9. Mark Nagel | | #9

    Gotta ask it...

    How do you tie in the walls to the "foundation?" I'd think that this could be a particularly tricky issue in tornado/hurricane prone locations.

    1. GBA Editor
      Kiley Jacques | | #10

      Some things have changed since this question originally posted. Mark, check out this design from contributor Randy Williams: Another Take on a Concrete-Free Slab.

  10. Paul Kuenn | | #11

    How about keeping the expense down and using treated plywood as the first layer and then the Advantech on top? Seems to be a waste having two layers of expensive tongue and groove.

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