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Foundation to Sill transition

tp21 | Posted in Expert Exchange Q&A on

Curious if anyone has any comments on the flashing detail provided in the following image from Building America Solution Center.

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Replies

  1. Expert Member
    MALCOLM TAYLOR | | #1

    tp21,

    It works fine. One small point: You don't need to bevel the top of the foam, just make sure the flashing covering it has a 6 degree slope.

    One variant is that if the foam board is thick enough for your climate zone, you can extend it up over you rim-joist, which means you don't need the spray foam, and can use batts on the interior.

    Aesthetically that does expose more of whatever the covering is on your exterior foam though - and it's the need to protect the foam that makes exterior foam more complicated than when the it is on the inside of the stem-walls.

    1. Expert Member
      Michael Maines | | #8

      Malcolm, I'm curious how you came up with 6°? It seems about right to me; I'm just curious. I've found that old windows in New England (100-250 years old) usually have either 11° or 14-15° slopes, but I'm not sure how those slopes were determined either.

      1. Expert Member
        MALCOLM TAYLOR | | #9

        Michael,

        It comes from our code BCBC 9.27.3.8 (4). It's also the recommended slope for window sills on Tyvek's installation instructions.

        I've always just taken it for granted. No idea if it is optimized or not. Being that low I guess there is a chance of ending up with a negative slope on poor installations.

        1. Expert Member
          Michael Maines | | #10

          Interesting. One of these days I'm going to read through your code--it seems like there are a lot of practical elements to it that are not in the IRC.

          1. Expert Member
            MALCOLM TAYLOR | | #11

            Micheal,

            I think it's generally a well thought out code, and is supported by a large appendix that spells out the reasoning behind the provisions.

          2. Expert Member
            Michael Maines | | #12

            We have a version of the IRC that has commentary as well, but in USA fashion, it costs extra! Even I don't have a copy, and I buy ASTM documents for recreational reading.

        2. Expert Member
          MALCOLM TAYLOR | | #13

          Michael,

          Their best practice guides are a great resource too.
          https://www.bcpublications.ca/BCPublications/

  2. Expert Member
    DCcontrarian | | #2

    This is an idea I lifted from Akos in this conversation: https://www.greenbuildingadvisor.com/question/double-stud-redux-detail-on-why-inner-load-bearing-is-more-complicated

    If you offset the rim joist you can keep the exterior insulation flush with the inside of the sheathing. It also allows you to skip the spray foam on the inside, and gives continuous exterior insulation rather than bridged interior insulation. Akos says most codes allow you to offset the bottom plate up to one third of its width, I haven't been able to check it and you'd probably want to check with your local official. But if it's allowed it lets you run the sheathing straight down and not have the flashing at the top of the insulation which is going to get all banged up when they put the siding on. In the drawing I've got 2" insulation on the foundation and 1-1/2" on the rim joist.

    1. Expert Member
      MALCOLM TAYLOR | | #3

      DC,

      That gives you a much cleaner look too.

      1. Expert Member
        DCcontrarian | | #5

        Again, credit goes to Akos.

        One of the things I like is the way the four sides -- top plate, sill, rim joist and sheathing -- create almost like a box girder. It just feels strong. (Not that that area is typically under-engineered, it's just an emotional thing.)

        One of the things I have to figure out is the sequencing. We typically do framing inspection before mechanicals, and then insulation is the last thing before drywall. The inspector is going to want to see that sheathing firmly attached when the framing is inspected.

        1. Expert Member
          MALCOLM TAYLOR | | #6

          DC,

          I did it on the second floor rim joist of my own house. It was the only foam above the level of the slab, so I just did it during framing - much as you would do with insulated headers.

          Ako's point about pests is a good one. Carpenter ants found that ring of foam around my place, and it was almost a year before I managed to dislodge them.

    2. Expert Member
      Akos | | #4

      I like that detail as well, very clean. Even if you are not in termite area, I would still add in a termite shield to keep critters out of the wall.

      The offset is to allow for out of square foundations, so all codes should have it.

      Ours is:

      9.23.11.2. Bottom Wall Plates
      1) A bottom wall plate shall be provided in all cases.
      2) The bottom plate in exterior walls shall not project more than one-third the plate width over the support

  3. tp21 | | #7

    Thanks for the replies, everyone. When originally designing the house, I planned on running 3" exterior insulation all the way up, but then prices went crazy. Then, I mulled over doing something similar to what Akos/DC recommended. However, living in climate zone 6a and having 3 inches of exterior insulation on the foundation, I wasn't comfortable with that amount of offset. That said, I did something similar to the Building America Solution Center picture I attached with the original post. I have attached two pictures of my foundation to wall profile and am curious if the details I have added will help or hurt the overall performance of the wall. I circled an area in green, in the "zoomed-in" picture, which gives me a bit of concern. What would be the best thing to do with this area? Behind the metal flashing, should I connect the foundation to the OSB with some ZIP liquid flashing? Or something similar... Right now, I am still in the rough-in phase and only have the foundation done and the house wrapped, so there is room for change. Thanks again for the help, everyone.

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