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Community and Q&A

Rim Joist Must Be Allowed to Dry to Inside?

Lindaloowho | Posted in General Questions on

Hi All,

Just looking at my above grade crawl space assembly in Southern Ontario.

There are no sill plates, rim joist only. I have cobbled rigid EPS foam, used foam adhesive to keep in place in rim joist spaces, and spray foamed around the perimeter of rigid foam on 3 sides so far. I will be spray foaming the bottom perimeter of the foam pieces and the underside of the rim joist soon. 

The walls of the crawl are rigid and spray foamed too. Vapour barrier sits on the interior of the insulation on the walls, a few inches below the rim. 

I’m wondering about the space between the wall and the rim, as well as the rim itself. I DO NOT vapour barrier this area, correct? Although concrete does not need to dry to the inside, the rim joist must have to opportunity to do so, right? I’m assuming that the rim joist can only dry through the EPS foam and not the spray foam? 

The rim joist is 2 layers of 2x10s on 2 sides of my rectangular cottage, and 1 layer of 2×10 on 2 sides. The cottage is clad with wood ship lap siding. 

I don’t have to worry about spray foam on the underside of the rim joist blocking the drying potential of the rim (in particular the outer layer), do I? 


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  1. Expert Member
    BILL WICHERS | | #1

    You typically want the rim joist to be able to dry to at least one side, so if you can dry to the outside you're OK, OR if you can dry to the inside you're OK. If you can dry to BOTH sides that's a plus, but not a requirement. What you don't want is to have vapor impermeable material on BOTH sides of the rim joist so that NO drying can take place. With NO drying, you can get into trouble with rot.

    Concrete does not need to dry. Concrete is not prone to degredation if it stays moist.

    EPS allows some drying (if it's unfaced or has a perforated facer), but not very much if you have a really thick layer (2+ inches). If you're siding is wood ship lap, and doesn't have a vapor barrier material (polyethylene sheet, foil faced polyiso, etc.), then your rim joist can dry to the exterior through the siding.

    Spray foam, assuming this is canned foam, is probably not going to be thick enough to really act as much of a vapor barrier and should be OK too. It's usually closed cell two part spray foam that gets into vapor barrier territory, but it still depends on the thickness of the insulation layer to know how vapor permeable it will be.

    Ideally you want a capillary break between the top of the concrete wall and the bottom of the rim joist. The purpose of the capillary break is to protect the wood structure from moisture in the masonry. With a capillary break in place, there is less risk of moisture getting to the rim joist in the first place, so less of an issue with the need for drying.

    A pic or a drawing of your particular assembly would make it easier to give you more specific info.


  2. Lindaloowho | | #2

    Thanks Bill,

    I’m away from the house today, so no pictures, sorry.

    I used some thin polypropylene foam with a vapour barrier on it. I butted that material up to place where the rim and pier meet, then wrapped the top of the piers “gift wrap style”. Jacking up the cottage to place a capillary break isn’t possible at this point.

    There is definitely no vapour barrier under the house sheathing...only wood ship lap. In that case, if the rim joist can dry to the outside, would you throw up a traditional vapour barrier from the top of the joist all the way down to the ground? Or is that asking for trouble? I just figure, the more moisture I can keep out, the better, as long as I’m not promoting rim joist rot.

    1. Expert Member
      BILL WICHERS | | #4

      I would just insulate the interior and rely on exterior drying in your case. The general building science rule is "you can let moisture out, and you can LIMIT moisture ingress, but you can't STOP moisture ingress". Basically the thinking is that a LITTLE mositure can almost always get IN to ANYTHING, so it's best to always allow a little bit of drying ability. It's difficult, if not impossible, to completely seal anything -- especially in a way that holds up over time -- so allowing some drying ability is safer than trying to make something completely mositure proof.


      1. Lindaloowho | | #5

        Okay great,
        Thanks again!

  3. Jon_R | | #3

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