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Insulated rim joist with exterior Zip R

jimgove30 | Posted in Energy Efficiency and Durability on

Hi all,

Looking for a sanity check on my rim joist insulation. Climate zone 6 (NH). Have zip R9 exterior, over an engineered rim joist with I-joist floor framing over my basement. (Basement will be conditioned spae, but I’m tackling that on my own with rigid foam after the rest of the build).

Since the zip R9 is essentially vapor impermeable, do I need something permeable inside the rim? At the R9 exterior, I’m not quite at the ideal number for condensation control, right?

Insulators coming soon so wanted to be sure what my options were.


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  1. rileyo | | #1

    I came to see what the responses where to your question as I am working though the same issue at my build. I had the same thought in that I believe that the interior would need to be an air barrier and not a vapor barrier (or a smart vapor barrier potentially Like Intello). Have you made a decision on how to move forward with your project?

  2. Expert Member
    Michael Maines | | #2

    In CZ6, for walls, the exterior insulation needs to be at least 35% of the total R-value if you want to eliminate the interior vapor control membrane and rely only on painted drywall, a class 3 vapor retarder, to reduce the risk of moisture accumulation. If you use a variable permeance membrane on the interior you'll be code-compliant and the assembly should be safe.

    Somewhere in your wall assembly you need one really good air barrier. With Zip sheathing (or Zip-R) that's usually the outer face of the sheathing--just be sure to seal all edges and penetrations, not just the field joints.

    Zip R--9 isn't vapor impermeable; when dry, the OSB is actually the least-permeable component. But in moist conditions, the polyiso becomes the lowest-perm part, at roughly 1 perm for the glass fiber faced polyiso they use for Zip-R9.

    1. jimgove30 | | #3

      Hi Michael, thanks for chiming in. So, to be clear, on the rim joist in the basement, assuming I am conditioning, and eventually finishing that space, should be treated as any other wall assembly component. And, I should avoid closed cell spray foam against the interior of the rim with the exterior Zip r9 taped to the foundation as the air barrier. Open cell however, could be used at the interior at the correct ratio for condensation control? 3 inches or so?

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