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Community and Q&A

Mineral Wool for Rim Joist

cmcgrath09 | Posted in Energy Efficiency and Durability on

House has 2in poly-iso on the exterior. The 1st and 2nd floor I am using Kraft face fiberglass next gen due to the price. For the walk out portion of the basement I am using mineral wool due to potential humidly in the basement,. . I planned on using mineral wool in the rim joist a well, but recently read I should use kraft face fiberglass instead? What is better? I got plenty of both.

Thank you,

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  1. Expert Member
    Deleted | | #1


  2. Expert Member


    Just like the rest of the wall, as long as you stick to the correct ratio of exterior foam to interior batts, it doesn't really matter what you use.

    If you are using less foam that recommended, you should include an interior side vapour-retarder, so maybe the kraft faced batts would be better.

  3. Expert Member
    BILL WICHERS | | #3

    You should never use air-permeable insulation against the inside of a rim joist, regardless of if the insulation has a facing or not, IF the rim joist is "cold" (uninsulated) on the EXterior. In your case, since you have 2" of polyiso on the exterior, which is about R13, you're safe using mineral wool or fiberglass on the interior side for some extra R value, as long as you stick with the interior to exterior R value ratios that Malcolm mentioned above.

    Personally, I would use mineral wool here since it's going to stay in place better and be much easier to install well in an area that is likely not particularly easy to work in.


    1. derekr | | #4

      Is this the case for rim joists in a crawl space as well or only for basements?

      1. Expert Member
        BILL WICHERS | | #5

        It doesn't make any difference if the rim joist is in a crawl space or a basement -- if you're going to be insulating it, all the same rules apply for either location.


        1. derekr | | #6

          Thanks I’ll keep that mind, I have a feeling when the people put insulation in my crawl space they would have just put it any place they could, including against the rim joist, just leave a small gap between insulation and the rim joist or should it be even further away like a foot or more?

          1. Expert Member
            BILL WICHERS | | #7

            Crawl spaces are often insulated poorly, likely due to the difficulty of working in crawl spaces in general. Ideally, you want your insulation pressed right against the rim joist with no air gap.


        2. derekr | | #8

          Oh ok I thought you said do not put insulation against the rim joist if the rim joist wasn’t insulated on the outside which mine isnt

          1. Expert Member
            BILL WICHERS | | #9

            If the rim joist was NOT insulated on the exterior, you would need to use a non-vapor permeable insulation material on the inside face, which would limit you to rigid foam (or closed cell spray foam). That also applies if you only have a little exterior insulation, with "a little" meaning not enough to hit the inside/outside insulation ratios that were mentioned earlier. That's actually the case for my own home, since I have only the original 1/2" polyiso on the exterior (I haven't done a reside project yet), so I used EPS on the interior to insulate the rim joist. I put the EPS blocks directly against the inside face of the rimjoist and sealed them in place with canned foam.

            Since you have ~R13 worth of exterior side insulation already, you should be OK to just use something like an R15 mineral wool batt on the inside, unless maybe if you're in a very far north climate zone. That exterior side insulation will keep the rim joist warmer, so you don't have the condensation issues using a batt on the interior that you would if the rim joist did not have that exterior insulation to keep it warm. A warmer rim joist means you're more likely to be above the dew point temperature, so that's what's important here. It's all about keeping the rim joist from being a condensing surface and getting wet.


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