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Community and Q&A

Beefing Up Rim Joist Insulation

the74impala | Posted in General Questions on

Cljmate zone 7, Northern Wisconsin

I am contemplating my rim joist insulation. I have 2″ of XPS, some is ant eaten, on the outside of my basement. On the inside, I have had a number of scenarios in my head. In a small section I have placed 2″ of EPS with can foam against the inner surface of the rim joist. Then 1.5″ XPS to cover the open CMS block below to make up for the 2×6 not fully covering the 10″ block. Can foam on edge of ledger board to XPS.

I want more insulation, and I have some rockwool for that.

My concern is the wood is Sandwiched between two not so vapor open products.

Should I use a hole saw, cut a 4 inch opening in the center of the EPS, fill with rockwool, then cover with scraps of Intello and tape it?  Seems like it would keep moisture levels down without much worry of condensation.

Overkill? Dumb? Missing something?

Check my awesome artist’s rendition of my idea.


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  1. Expert Member
    BILL WICHERS | | #1

    No, don't cut a hole. You only need a teensy little bit of drying ability to be "safe". You do need to ensure you don't have bulk water problems. Bulk water problems are problems (obviously :-), but they are different from moisture control problems which is what you're really thinking about here. Moisture is very small in terms of total volume in most cases, and your idea to cut a hole risks a lot more getting INTO the assembly, so that's why it's not a good idea. I would leave the 2" EPS foamed in place (which is what I did in my own home, BTW, and I have completely impermeable foil faced polyiso on the exterior).

    You can put mineral wool over the EPS on the interior to bring your R value up a bit, but don't go overboard -- you don't want to get that EPS so cold that moisture might migrate through the mineral wool and condense on the interior-facing surface of the EPS. I would just use an a Safe'n'sound batt here, which would be about R12 or so. 2" of EPS is about R8.4 or so. The ratio ove the assembly (which includes your exterior R value too), is what determines the temperature of that first condensing surface which is what you care about.

    The short answer is leave your EPS as is, don't cut a hole, try to us R15 (2x4 size) mineral batts or the slightly thinner Safe'n'sound batts for extra R value. There is no problem sticking some extra XPS in the window to bring it out flush either -- I did the same thing in my own home.


  2. the74impala | | #2

    Thanks Bill

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