Rim joist strategy for old house with crawl space – GreenBuildingAdvisor

I’d love your opinion on whether or not to use closed cell spray foam at my “rim joists”. Here’s the context:

a) Zone 4A, 1885 house, brick foundation, balloon-framing with fire blocking in the bays, balloon-framing studs attach to mudsill, mudsill directly on brick foundation, no capillary break.

b) Dirt crawl space, ~2 ft clearance, plan to put poly on dirt and rigid foam on foundation interior to make the space unvented and conditioned, hence the need to airseal and insulate the “rim joists”

c) Rim joist area is technically not a rim joist: I see the back of my new plywood sheathing.

d) The exterior wall assembly is plywood, blue skin, 0.5” unfaced rigid foam, cedar breather and then cedar shingles. The foam is (Owne’s corning foamular ngx, an XPS that I understand is more environmentally friendly than typical xps. I went with it because 0.5” was max I could do and it had some vapor permeability at 0.2 perm to allow outward drying of the sheathing if I had condensation issues)

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I’m debating 3” closed cell in that “rim joist” area for sealing, but understand from GBA that there is a risk of the mudsill/sheathing not being able to dry if it gets wet. I’m assuming that my assembly can dry outward if that happens, and I’ll be dehumidifying the crawl space, so I think there’s a case here that I’m ok.

Alternatively, I could cut and cobble with foam (polyiso? or eps for a bit more permeability to allow for some extra inward drying capability?), but I’m concerned about the labor challenge and my ability to really do the sealing well around it given how cramped it is).

Do you see too high a risk with the closed cell “rim joist” sealing and insulation? Is there anything you’d recommend to derisk that approach to bring it to an acceptable level? Or would you advise cut and cobble with material ___?___ ? Or something else?

Thank you,

Luke