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Community and Q&A

Insulating rim joists

buildmo | Posted in General Questions on

I’ve read a few articles on insulating the rim joist and the general recommendation seems to be to insulate the rim joist with spray foam or foam board.  That said, I always see a few comments that recommend against this as it caused sill plate rot, mold, etc.

The home is in Missouri and was built in 2006. Regular batt insulation was stuff in the rim joist and the few areas of the basement that were finished they placed batt insulation and framing directly against the concrete wall (three rooms in the basement are “conditioned”). When I removed the framing and insulation I found no evidence of wetness or mold and we do not currently have any signs of water coming in the basement for the past 5 years. We also run a dehumidifier in the basement shooting for 50% humidity.  I do see a foam gasket between the sill plate and concrete wall and to my knowledge the only thing on the outside basement wall is that black water sealer.

My plan is to caulk the rim joist, then use 2″ Owens Corning foam board (r10) and expanding foam to seal/insualte the rim joists. I will also use 2″ foam board on the walls, then frame and drywall.

Has anyone in Missouri had issues with insulating this way where it ended up causing mold to get trapped between the foam board and rim joist? Will I have any issues with water vapor from the concrete wall going up and transferring to the rim to cause rot? Any thoughts or insights would be appreciated.

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  1. GBA Editor
    Brian Pontolilo | | #1

    Hi Buildmo.

    When plate wetting would be a concern is if there is no capillary break under the sill (you have one) and/or if the exterior assembly is not vapor open. Some builders don't worry about the latter unless it is paired with the former, as long as the rim area is not likely to get wet from rain water or snow melt. I'm not from Missouri, but I think you can proceed with your plan to insulate your rim joist with rigid foam insulation.

    When it comes to insulating your foundation walls, be sure to do a good job air sealing the foam edges and seams to keep air from reaching the concrete where it could condense and consider a capillary break under the bottom plate or a pressure treated bottom plate to mitigate problems with any wetting form the concrete floor.

    1. buildmo | | #2

      will do, and thank you Brian. Much appreciated.

  2. Expert Member
    Dana Dorsett | | #3

    >"My plan is to caulk the rim joist, then use 2″ Owens Corning foam board (r10)"

    Can I convince you to change the foam board spec?

    The 2" XPS pink board is really only warranteed to R9 (read the fine print), and as it's climate damaging HFCs bleed out over several decades it'll slowly sink toward R8.4. XPS is by far the most enviromentally UN-friendly insulation in common use today:

    Using 2" of reclaimed fiber faced roofing polyiso would run about R11, costs less, and is environmentally neutral, since it's re-using material where the environmental hit was taken years or decades ago. There are probably 100 acres worth of 2" roofing polyiso stacked up in reclaimer's lots in MO. Run this search (on your more local craiglist) - see if there is any within easy driving distance of you:

    Since band joists are a cut'n'cobble operation a few dinged corners or scars/dents don't really matter- the scrap rate on any cut'n'cobble is pretty high.

    Even brand-new 1lb density polyiso would be substantially greener than XPS, roughly comparable to (slightly better than) new EPS, about half the hit of HFO blown closed cell polyurethane foam. At 1.5" foil-faced polyiso would be labeled R9, but if the foil is facing open air it adds another R0.5-R1 to the actual performance due to it's high infra-red reflectivity/low emissivity At 2" it's usually labeled R12 or R12.

  3. Expert Member
    BILL WICHERS | | #4

    In addition to what Dana said, EPS also provides a little bit higher vapor permeability than XPS which is a plus if you want to allow a little bit of drying ability for the rim joist. I used EPS in my own home for this reason (I have foil faced polyiso on the exterior side, so zero drying ability in that direction).

    I think XPS is as popular as it is primarily because it seems to be the most common rigid foam type found in the box stores, especially in the thicker sizes like 2”. Menards DOES carry 2” EPS in both type I and type II varieties if you have one of their stores in your area. Home Depot carries EPS at least up to 1-1/2”, but with a poly facer which makes it a vapor barrier. Removing the facer from their EPS tends to take chunks out of the EPS sheet.


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