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Emercor insulated rim board: “Do not insulate”?

k3tch | Posted in General Questions on

I’m finishing my walkout basement, so I’ve gone the route of 2″ XPS direct on the poured concrete walls, framed off an inch, and then filled my stud bays with Roxul Comfortbatt. I’ve got some leftover bags of Roxul and was going to throw some up in the rim joist, but as I made my way up thereI noticed a stamp on the rim board that says “DO NOT INSULATE!”. They’re insulated rim boards made by Emercor and I believe they provide an R-20 rating. So my question – would it really be bad to place Roxul against the rim board? I will not be covering the Roxul with OSB or drywall, but I will be putting in a drywall ceiling.

It’s about 20 degrees F out right now and the rim board is cool to the touch but not cold. I don’t feel any drafting, etc and all seams are foam sealed already.


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  1. Dana1 | | #1

    Emercor is really just a narrow polyurethane SIP, and comes in both R14 and R20 thicknesses. The R20 is a center-cavity value, and does not include the thermal bridging of the top & bottom 2x splines, so it's actual "whole-plank" R value is going to be substantially less than R20.

    To advise further requires more detail about your climate/location, and the R-value of the Roxul, and the sheathing & siding layup.

  2. k3tch | | #2

    I'm in Zone 5 and the Roxul has an R-15 rating. The house is vinyl sided, and as far as I know, is OSB with house wrap. I have never looked under the siding and I was not the original homeowner who built the house, but I've seen the builder using this method on newer construction in the neighborhood.

  3. GBA Editor
    Martin Holladay | | #3

    Go ahead and install the Roxul mineral wool on the interior side of your insulated rim joist. There is no need to worry.

  4. Expert Member
    Dana Dorsett | | #4

    What he said. With only R15 on the interior side of an R20 SIP you won't even need an interior side air barrier on the Roxul to be moisture-safe at fiber/OSB in climate zone 5.

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