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Insulating a rim joist in a tight space

WBowling | Posted in General Questions on

Hi all,

I just acquired a house in KY (zone 4 climate). The house is built in a crawl space, and I’m planning to encapsulate the area. That’ll also entail moving the insulation from the bottom of the subfloor to the crawl walls (planning on using rigid foam on the walls).

Unfortunately, one entire side of the rim joist is inaccessible due placement of a floor joist. The rim joist is completely covered due to joist placement. I drilled a small test hole, and it doesn’t appear that there’s any insulation inside that joist bay (i.e., the rim joist currently has zero insulation on it).

I’ve been able to come up with two possible ways I could tackle this…

Option 1. Drill holes (~1″) into the offending joist, and try to hit the rim joist with a two-component spray foam kit.

Option 2. Put rigid foam on the outside of the rim joist. This wall is under a porch, so I could put rigid foam into the porch floor joist bays and get pretty good coverage on the exterior of the rim joist.

I’m not crazy about Option 1 due to joist integrity and not being sure what sort of coverage I’d be getting with the spray foam. I’m not sure whether Option 2 would really work, however. There may be other options as well, and I’d love any advice you all could provide.  

Thanks for any help,

GBA Prime

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  1. Expert Member
    BILL WICHERS | | #1

    A series of 1” holes isn’t really a problem structurally as long as they’re in the center part of the joist and you don’t string them all together (space them out). The load is primarily carried by the edges of the joist, like the chords of a truss. The middle part of the joist is like the web of an I beam, serving to keep the spacing between the flanges.

    I would be concerned about getting even coverage though. You’re likely to have a lot of voids that you can’t see. Can you show a drawing of the rim joist arrangement so it’s possible to see where the rigid foam would go if installed on the exterior?


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