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Zip R-Sheathing (6.6) under floor framing, pier foundation

Redmond2411 | Posted in General Questions on

Hello,
I’m building a 16×20 addition on piers as neither a basement nor a crawlspace are desired. I’d like to fill the 9.5″ floor framing bays with dense-packed cellulose and then apply Zip R-sheathing. I live in NH and don’t use air conditioning in the summer. Do you think the Zip R sheathing with 1″ of foam will be sufficient? I know it will help with thermal bridging, but should I be worried about condensation because it’s only 1″ and not 2 of foam? I appreciate your thoughts.
Chris

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Chris,
    My instinct tells me that you'll be fine. There isn't much outward vapor drive through most floor assemblies, because ordinary subflooring is a vapor retarder.

    That said, you'll be using the Zip-R sheathing in a way that isn't supported by the product manufacturer -- and that, technically speaking, is a code violation. (If that matters to you...)

  2. srenia | | #3

    Using the pier foundation doesn't negate the use of perimeter insulation. The skirt would not be structural, but would solve a bunch of insulation issues at a lower cost. The ground itself maintains a higher temperature. So doing r10 sealed on the perimeter could see better results that trying to insulate against negative temperatures directly under the floor. Using the perimeter insulation you could also insulate between the floor joist as well.

  3. Expert Member
    MALCOLM TAYLOR | | #4

    Stephen,
    In practice what you end up with is a poorly constructed crawlspace. Skirts almost invariably allow pests to get under the house. You now have no ventilation to deal with ground moisture so the soil needs a vapour barrier - and unless you are going to condition the space, some ventilation or heat source. The area is also concealed so you don't see what mischief is occurring..

    For me it's an either or situation. A well-built crawlspace or an open pier foundation.

  4. srenia | | #5

    Properly sloped with use of gravel moister shouldn't be an issue at all. As far as pest, it matters what your skirting is made out of. You can use cement board with foam/roockworol and green board sunk into gravel that is well insulated and resistant to pest. If your about to say this is still weak, then you have to say the outside wall structure is also weak. I'm not saying that this is perfect, but can work for certain price points.

    The best foundation system I've seen is Frank Lloyd Wright's foundation for durability, but for price point pier works the best.

  5. Redmond2411 | | #6

    Guys, thank you so much for your thoughts. Martin, the code does matter to me, but in this case, I don't feel like this would be any different than installing plywood under the framing, so I think I'll take my chances. Stephen and Malcolm, I appreciate the perimeter insulation ideas and the difference this would make, but I'm hoping to store kayaks under this space, so keeping it less closed-in is appealing to me.

  6. Expert Member
    MALCOLM TAYLOR | | #7

    Stephen,
    Practical concerns aside, once you enclose the underside of your building you have created what building codes define as a crawlspace with all the attendant requirements for vapour barriers, natural or mechanical ventilation etc. There is no middle ground for skirts.

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