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Rim joist foam sandwich between impermeable layers

Andy6572 | Posted in General Questions on

I apologize I am re-posting a similar question that I asked a few days ago but I want to make sure this question gets answered as I am worrying sick over it! Basically, I cut and cobbled 2” XPS into all my rim joists without realizing I already had exterior 1/2” foam board under my siding and against the wall. The issue I am worried about is that, after recently examining the outside more closely, the exterior foam board is wrapped with facings, which as I understand are very vapor impermeable. Am I asking for trouble with this assembly, or will there somehow be sufficient drying to the interior? 

A slightly similar question has been asked before, and I know i’ve seen martin say that it’s OK to sandwich the rim joists between foam boards, but I’ve never seen an example where both foam boards are very impermeable.

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  1. GBA Editor
    Brian Pontolilo | | #1

    Hi Andy,

    Where are you located?

    In general, moisture-rich air is a greater contributor to condensation issues than vapor diffusion. Your cut-and-cobble-style air sealing at the rim joist will be a benefit by keeping warm indoor air from getting to the sheathing.

  2. GBA Editor
    Martin Holladay | | #2

    I think everything will be fine (assuming that your house doesn't have flashing defects that funnel rainwater to your rim joist). Relax.

  3. Andy6572 | | #3

    I am in Idaho zone 5a. Luckily we get about 10" of precipitation annually here so based on your advice I will simply leave the boards in and hope that there aren't any flashing defects around my windows. Out of curiosity, is there a reason why the rim assembly is more safe than a wall cavity trapped between impermeable layers? Because based on the GBA article about that topic, I'm under the assumption that this assembly is quite risky. Does the vapor diffuse above and below?

    1. GBA Editor
      Martin Holladay | | #4

      Sandwiching sheathing between layers of rigid foam is not ideal -- in general, if a builder can avoid it by planning ahead, it's best if sheathing can dry in at least one direction. That said, I'm not aware of any GBA articles that describe that type of sandwiching as "quite risky." (Correct me if I'm wrong -- there are hundreds of articles on GBA, and I haven't memorized them all.)

      1. Andy6572 | | #5

        I suppose what I meant by "risky" is that having wall sheathing not dry in at least one direction, is like you said, not ideal. I suppose I am wondering why the rim joists are not subjected to the same rules.

        1. GBA Editor
          Martin Holladay | | #6

          Q. "I am wondering why the rim joists are not subjected to the same rules."

          A. They are. But in your case, it was too late. You already broke the rules -- and I was assuaging your anxiety.

          Rim joists represent a relatively small percentage of a home's wall sheathing, and some hygric resdistribution (sideways) is possible if the sheathing gets a little damp near the rim joist. But the basic answer is: If the sheathing is dry on the day that it is encapsulated, and the details of the house are otherwise good, the risk is low.

          Generalizations are tough to make, though. If you have stingy roof overhangs, and not much height clearance between grade and the lowest wooden components of your house, you can have rot due to splashback. And bad flashing (or a poorly detailed deck ledger) can certainly result in sheathing rot near a rim joist.

          1. Andy6572 | | #7

            OK thank you for your expertise Martin. I think my anxiety over this is relieved a bit. I suppose if anything bad does happen, you can consider me a good case study.

            Another note that I have been thinking about... I feel like you should update the fine homebuilding and GBA articles about insulating rim joists to include info about risk factors before starting that project. I didn't understand much about insulating when I first started my project, and as most articles touted it as being essential and an easy DYI, I went ahead because of my desire to be as green as possible. I wish I had somehow known the risk factors before, without digging into comments/discussion boards, then worrying after realizing I made a mistake because I jumped the gun.


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