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Rim joist external insulation and air barrier: How to?

lance_p | Posted in Energy Efficiency and Durability on


I plan to recess my rim joist so that 2″ EPS can be added as a thermal break. This will be flush with the studs so my sheathing can cover the whole area. Pretty standard.

I want to bring my vapor retarder (Intello or Membrain) out and around the rim joist so it attaches with the vapor retarder on the inside walls. This should result in a seamless air seal from foundation to trusses. Again, pretty standard.

My first question is, does the 2″ EPS go on the inside or outside of the vapor retarder and why?

My second question is, do I need to keep a vapor permeable strategy for insulating the inside rim joist area, or is having a somewhat vapor permeable exterior good enough? The stackup of materials would be OSB sheathing – 2″ EPS – vapor retarder – rim joist.

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  1. lance_p | | #1

    Here's a visual in case I wasn't clear in my description.

    Here's the insulation on the inside of the vapor retarder and on the outside:

  2. GBA Editor
    Martin Holladay | | #2

    Will you be supplementing the 2 inches of EPS (about R-8) with fluffy insulation on the interior side of the rim joist?

    An assembly with exterior rigid foam doesn't really need an interior vapor retarder (assuming the rigid foam is thick enough). In your climate zone (Zone 6), if you are putting together a wall assembly that combines exterior rigid foam and interior fluffy insulation, a minimum of 36% of the total R-value needs to come from the rigid foam. So you can safely add R-14 of fluffy insulation on the interior side of this rim joist without worrying about vapor diffusion or condensation.

    Your question about the location of the vapor retarder really has nothing to do with vapor diffusion, however. Really, this membrane is being used as an air barrier, not a vapor retarder. If all you want is an answer to the question, "Where should the air barrier go?", the answer is, "Anywhere you want. It doesn't matter."

    On the other hand, if you want to put more than R-14 of fluffy insulation on the interior side of the rim joist, you would want a vapor retarder on the interior side of the fluffy insulation -- but that's not easy to install.

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