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Community and Q&A

Rim joist with exterior EPS

ryryry | Posted in Energy Efficiency and Durability on

My home has 1″ of EPS on the exterior walls and covering the rim joist. The rim joists are uninsulated on the inside. If there was no exterior rigid I’d use a combo of rigid foam board and batts on this inside, but because of the exterior insulation, I’m concerned about creating a vapour sandwich.

Is the best approach here to air seal from the inside with caulk or canned foam, then add FG or roxul batts to allow inward drying?


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  1. GBA Editor
    Martin Holladay | | #1

    In this situation, adding rigid foam to the interior side of the rim joist shouldn't cause any problems. Researchers have found that this kind of foam sandwich, in this part of your house, doesn't lead to damp rim joists.

    That said, we still need to know your climate zone or geographical location to give you good advice.

    -- Martin Holladay

  2. ryryry | | #2

    Thanks, Martin.

    I'm in Nova Scotia (Zone 6).

  3. Expert Member
    Dana Dorsett | | #3

    At 1" unfaced EPS (no plastic or foil facers) will have a vapor permeance greater than 2 US perms- it's not a vapor barrier, or even a Class-II vapor retarder. As long as there is at least some amount of outdoor air access behind the siding the band joist can dry at reasonable rates toward the exterior.

    If you add a mere 1" of EPS foam tight to the interior side of the band joist you will have enough total foam in the foam|joist|foam sandwich for up to R15 of interior side fiber insulation without needing more than latex paint on wallboard for the interior side vapor retarder. But the vapor retardency may still be too high to eliminate risk of moisture accumulation without a separate vapor retarder. (A WUFI simulation could suss that out.) But a bit more is better. At 2" (R8.4) unfaced Type-II EPS you would have nearly as much R exterior to the R15 fiber insulation (say 3.5" of rock wool batt), and still be in the Class-III vapor retardency range, but at the very low end of that range, between 1-2 perms, with at least some capacity for the band joist to dry toward the interior. That would almost certainly be sufficient, independently of the drying capacity toward the exterior.

  4. GBA Editor
    Martin Holladay | | #4

    Dana gave you good advice. Here is the short version:

    1. Don't worry about the foam sandwich issue.

    2. In a cold climate like yours, rigid foam on the interior is better than fiberglass batts on the interior. So adding interior rigid foam is a good idea.

    -- Martin Holladay

  5. ryryry | | #5

    Thank you both!

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