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Community and Q&A

Vapor Barrier at Floor Joist and Rim Board

arnoldk | Posted in General Questions on

Hi,

What is the best approach to have a continuous vapour barrier around the floor joist/rim board from the first to the second floor?

Thank you,
Arnold

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Replies

  1. Expert Member
    AKOS TOTH | | #1

    Vapor barriers are not air barriers so they don't need to be continuous. The poly can stop at the top plate and restart at the bottom plate of the next floor.

    If you want to use the poly layer as an air barrier, you can install 3' or 4' strip of house wrap stapled to the top plate on the inside and flapped over the top plate, build the floor over it, wrap the house wrap over the rim joist and over the new subfloor, build the 2nd story walls and staple the house wrap to the bottom plate. The poly is installed over the house wrap on both floors and sealed with acoustic caulk.

    If you are not in a seismic area you can improve the assembly R value a bit by insetting the rim joists a bit. This will give you space for a strip of rigid insulation along the rim joist on the outside to somewhat break the thermal bridge of the floor joists. This works well with a 2x6 wall, a 2x4 might not have enough bearing area left.

    1. arnoldk | | #2

      Hi Akos,

      My primary air barrier is the exterior plywood sheathing with the WRB as a secondary.

      As for the vapour barrier, I always thought it needed to be continuous to avoid moisture from migrating into the wall assembly such as the floor joist and around electrical boxes.

      Thanks,
      Arnold

      1. Expert Member
        MALCOLM TAYLOR | | #3

        Arnold,

        If you are going to include a vapour-barrier (or retarder), it needs to be sufficiently close to the warm side of the wall that it stops moisture from entering it and condensing. Having one that close to the exterior will make things worse.

        1. arnoldk | | #4

          Is there anything that I should I be doing around the floor joist/rim board?

          Note that am using Pro Clima - Intello Plus as my vapour barrier.

          Thanks,
          Arnold

          1. Expert Member
            BILL WICHERS | | #5

            I would put rigid foam blocks against the inside of the rim joist and seal them in place with canned foam around the perimeter. If you’re careful, you can tuck your vapor barrier in on the edges and the canned foam would seal the rigid foam, framing, and vapor barrier all together. Just makes sure that the vapor barrier does not prevent the canned foam from completely sealing the rigid foam to the framing here.

            Note that I like to use EPS for this since it allow for a small amount of inward drying of the rim joist.

            Bill

          2. Expert Member
            MALCOLM TAYLOR | | #7

            Arnold,

            Edit: I miss-read the section and thought there was exterior insulation. See Bill's reply below.

            Edit again: Now you have posted a new section that shows exterior insulation. I'm lost.

  2. arnoldk | | #6

    Bill,

    Is there another method for the rim joist that does not use foam?

    Thank you,
    Arnold

    1. Expert Member
      BILL WICHERS | | #8

      The two most common ways to go are spray foam and rigid foam blocks. Spray foam has the advantage of extream "sealification" -- it goes everywhere so you can fill every gap. The downside is it's messy and expensive, and the closed cell spray foam you would use here doesn't allow any drying at all, which is sometimes an issue. Rigid foam is a lot more labor intensive to install, and trickier to completely air seal, but it doesn't have the issues spray foam has, and if you use EPS (or kraft faced polyiso), it allows for small amount of inward drying which is an advantage in some cases.

      It's not usually recommended to use any air permeable insulation against the rim joist due to the risk of moisture problems. If you want to minimize the amount of rigid foam you use, you could use enough rigid foam to satisfy the tables for continous insulation and then use batts for the rest (or other fluffy insulation). I would recommend mineral wool for the extra fluffy stuff if you try this method. In my own home, I used 2" EPS. I have foil faced polyiso on the exterior, so zero external drying potential. The EPS allows for some minimal inward drying which I think is a little safer.

      If you want to avoid rigid foam completely, I'm not sure what would really be safe long term. That's not to say there isn't an option out there, I'm just not familiar with one. The risk you take with most insulation is that most is air permeable, so moisture laden air can migrate back to the cold rim joist and condense there. This can happen in wall assemblies too, but the rim joist area is usually considered to be an area of extra concern, and it's very difficult to put an air barrier in the rim joist on the interior side of batts (for example) if you used something like that. The important part is to keep moist air from being able to circulate through the insulating material to the rim joist itself. The rim joist area is going to be very tricky to air seal well with a membrane material over fluffy insulation.

      Bill

      1. arnoldk | | #9

        Hi Bill,

        Thank you for your detailed response. The drawing I got (see attached) doesn't show details on how to install the vapour barrier around the rim joist and the framing company I was going with were not longer interested.
        The reason for all of my question is because I am essentially building the entire house mostly on my own and it's only there that I realised the gap in my knowledge.

        You'll see that we have 2" of Comfortboard on the exterior wall of our double stud wall but that's not enough on it's own to prevent condensation. We are using the Pro Clima products (Intello Plus, Solitex...) to allow our wall assembly to "breath" in either direction but even with that I suspect I may end up having to use EPS as your recommended unless there is another solution.

        Thank you,
        Arnold

        1. Expert Member
          AKOS TOTH | | #10

          Arnold,

          If you decided to self build, you have to pick your battles. It takes a lot to build a house, good part of that is the motivation to finish it off. The worst situation you can end up in an unfinished or partially finished house. I know people that lived in a partially renovated house for a decade without a running sink in the bathroom and no trim before selling the place as is.

          So, I would step back a bit and see what you can do to simplify your life. Details cost money and most importantly time, which seems free, none the less it is the most valuable commodity if you DIY.

          In your case, I would pick either a double stud wall or one with exterior rigid. Having both simply adds complexity, cost and time and saves no energy or operating costs.

          In terms of your rim joist area, with a 2x6 load bearing wall, the simplest is to inset the rim joist and have extra rigid insulation on the exterior there. OBC requires 1.5" of bearing for joists and bottom plates can be overhang by 1/3 of width so you have some space to work with. With this extra insulation you won't get enough thickness for full condensation control but it will make a big difference. There is a lot of drying potential there, a bit of extra seasonal moisture will not be an issue. As with most house details vapor barriers are not as critical as air barriers.

          1. arnoldk | | #11

            Thanks Bill. As for the house build, I've already got the detached garage building minus the siding and the first floor of the house is already framed. I'll be installing the flood joist next week and get started on the exterior walls.

            All to say that I am already neck deep in this project but I am enjoying the process so far. It's been something I have been wanting to do since I was 13.

            Thanks,
            Arnold

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