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Community and Q&A

Rim joist insulation thickness

canadianexpy | Posted in Energy Efficiency and Durability on


Just trying to get opinion on thickness of Rim joist insulation should be? Zone 6
It’s new construction home. With 6″ exterior insulation everywhere but the rim joist because of a full wrap around porch, so there is a ledger board on the other side of the rim with blueskin VP 100 between the two on the OSB. 
2-4″ then fluffy or 4-6″ then fluffy?
Any opinions appreciated

Thanks Dave,

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  1. Expert Member
    Dana Dorsett | | #1

    The thickness is meaningless- code prescriptives are in R-value or U-factor. IRC code min R value for a basement band joist would be R19 where penetrated by joists, or R15 continuous insulation.

    With no insulation on the exterior, in a zone 6 climate at least 35% of the total R needs to be air-impermeable insulation (50% is better) against the rim joist. Going with 2" of closed cell spray polyurethane (or well-sealed 2" cut'n'cobbled polyisocyanurate) and R13-R15 fluff on the interior side snugged up to the foam would work.

    What are the R-values of the above grade wall?

  2. GBA Editor
    Brian Pontolilo | | #2


    I'm with Dana. Focus on R-value and air sealing, not thickness. Here's a article on this topic that you will probably find helpful:

  3. seabornman | | #3

    Why not insulate the porch side of the ledger board?

  4. canadianexpy | | #4

    Thanks for the reply,
    Yes sorry I should have said R- value

    Dana, 2 layers of 3" polyiso outside and 2x4 studs with Roxul, roughly R40-44 depending on how you rate the polyiso.
    I was thinking 4" closed cell which is around R20 - then 2x6 Roxul

    Joel it would be to tough to flash it properly and each piece cut around the joist

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