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Is a interior formed shelf to support floor joists in a poured concrete stem wall worth the extra effort and cost?

George Wiest | Posted in Energy Efficiency and Durability on

I’m building a single story house with a crawl space in climate 4C, seismic D1. In order to minimize the distance from finished floor to grade, the top of theTJI floor joists will be even with the top of the sill plate. I think there are two ways to do this: (1) pour a 8” thick stem wall, and hang the TJI’s from the sill plate, or (2) pour a 12” stem wall formed with a 6” wide interior shelf supporting an interior sill that the TJI’s would sit on. Option (1) is simpler and cheaper, but I worry about a less a robust floor support and possibly noticeable vibration; and there would be a thermal bridge with no rigid foam between the joists and the concrete stem wall. I’ll insulate with rigid foam between the joists, and have mineral wool on the stem wall exterior. Option (2) uses 8 cu.yd. more concrete, and costs an unknown amount to form the shelf; but eliminates the thermal bridge at each joist, and seems like a more stable floor system. Any suggestions?
George Wiest

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Replies

  1. Expert Member
    Malcolm Taylor | | #1

    George,

    There are a few ways to do this. My preferred variation is to pour a pressure-treated ledger into the interior side of the form. It acts as a screed to level the top of the foundation. The hangers get fastened to the ledger. The bottom plates of the exterior walls sit directly on the anchor bolts, eliminating the sill plate you show on your details.

    One complication which all these details cause it that wiring and plumbing have no path between the walls and the crawlspace below. Whether any of them are worth the effort is an entirely subjective choice based on how much the relationship between grade and the interior matters to you.

  2. Expert Member
    Michael Maines | | #2

    I'd suggest the second option, with a shelf, except keep the wall 8" thick and eliminate the 2 1/2" EPS if you're also using exterior mineral wool. Thermal bridging through I-joists is minimal. I-joists usually need 1 3/4" of bearing so you're covered with a 2" shelf and a 6" concrete wall above. Hangers work fine too; most new construction relies heavily on hangers. I like Malcolm's idea of casting in a ledger. I haven't done that, but do use shelves regularly.

  3. George Wiest | | #3

    Thanks Malcolm and Michael.

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