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Raised Floor on Pier Foundation

cassarch | Posted in General Questions on

Hi All,
I have a tough +/- 10’x10′ house addition (no plumbing) to deal with here. Zone 6.

It would need to be very close to the ground. I researched the articles and previous posts and I think I incorporated a lot of the suggestions for the best detail possible, given the constraints. We cannot excavate for a FPSF slab.

Assuming the building inspector is ok w/ the different foundation types, the contractor and I have been discussing a detail like the attached image. Would build the 1.5″ insulated zip board upside down, in order to tape the seams.

The whole assembly is a work in progress, but this is a start for the conversation.

Any feedback? Go ahead, let me have it! Thanks.

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Replies

  1. Expert Member
    MALCOLM TAYLOR | | #1

    cassarch,

    I've done a few very similar structures, building and flipping the floor. A couple of suggestions:

    - Make sure your code doesn't require the underside sheathing or framing to be PT being that close to grade.
    - You may find it just as easy to use foam and plywood, as opposed to Zip-R.
    - Rather than use 2"x10"s as beams, I would frame the whole thing with 2"x8"s (or just use 2"x10"s as the rim joists). That gives you a continuous layer of sheathing on the underside, and eliminates the thermal bridging. Where the beams bear on the sono-tubes, replace the foam with a square of wood blocking.
    - Think through what you will embed in the top of the piers to connect to the framing above. And how you will access it to add fasteners.
    - You may want to start with a layer of 2"x4" framing on the underside as shown in this link: https://www.southmountain.com/wp-content/uploads/2019/02/19-Small-Home-Harmony-Article-PDF.pdf.
    The advantage is being able to work entirely from the top, and set it closer to grade. It also means you can build the 2"x4" under-framing and sheathing from PT material, and the rest from regular lumber.

    1. cassarch | | #3

      Malcom, thanks. It was largely your previous posts that help guide me already on this.
      I was using the ZIP-R because it acts as the vapor barrier, I thought.
      If I switch to rigid insulation and PT plywood, where does vapor barrier go and how is it attached?

      1. Expert Member
        MALCOLM TAYLOR | | #4

        cassarch,

        You don't need a vapour-barrier. The foam will act as a vapour-retarder. Installing a vapour-barrier at on just below grade as Michael suggested will help keep moisture from migrating up into the floor system. Just make sure it is graded so water doesn't pool on it.

  2. Expert Member
    Michael Maines | | #2

    I would add a vapor barrier at or just below grade. I've used EPDM for similar situations.

    1. cassarch | | #5

      Thank you. Will do.

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