Rim joist strategy for old house with crawl space
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 some 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
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Replies
Bumping this as I'm considering crawling and spraying tomorrow, thanks in advance!
Do you have a capillary break between the bottom of the joists/framing and the top of the foundation wall? If you do, you're in much better shape here. If you don't, you have more risk from moisture wicking up and wetting the framing.
My preference in these situations is to foam in ("cut'n'cobble") pieces of EPS (be sure it has no facer), since that's the most vapor open of the commonly available rigid foams and it will provide for a small amount of drying. That's what I'd recommend you do here. It is significantly more labor to insulate this way compared to using spray foam though.
Bill
Thanks!
i) I do not have a capillary break, unfortunately. The mudsill just sits directly on the brick. I see (and have seen before :) ) your argument. The part that confuses me about it: even if it moisture wicks up, if you have outward drying ability, can't the wet framing dry outward if closed cell on the interior stops interior drying, mitigating the risk you describe?
ii) If I go cut and cobble, I was thinking 4'' of EPS (at R value of 4.2 per inch, that's about 16.8) since I have true 2x4s with loctite or big stuff to seal it into place. Is that reasonable?
Luke
Luke,
The risk with no capillary break is that once the mud-sill is encapsulated, and is also much colder, the rate of wetting exceeds the rate of drying to the outside, meaning it accumulates moisture and is at risk of deteriorating. The problem is knowing whether that is the case or not. Often sealing the interior with foam is fine, but sometimes it isn't - and there is no sure fire way of knowing beforehand.
Well put, that makes sense. From what I've read you all post on other threads, if one really wants to go the spray route given these unknowns, you can spray a few rim joists and then inspect after N (6 - 12?) months.
Bill, it looks like from past posts, you've stuck with 2'' of EPS and not more: https://www.greenbuildingadvisor.com/question/wrb-perm-rating-and-rigid-foam
Would you advise against going more than 2''?
What kind of perm number are you targeting? I was considering going with polyiso unfaced because I have a bunch of it leftover from other work, so wondering if I can hit a comparable perm value with that.
Luke
I use 2" in part because it's easy to get 2" thick sheets, thicker stuff is a bit harder to find. Also, you can often use a cheaper material if you need more R value. The Safe'n'sound mineral wool batts (3" thick) are good for this, and give about R13 worth of R value, for a bit over R21 total (including the ~R8.4 or so for the 2" of EPS). The mineral wool then also provides the fire barrier that is sometimes required. Mineral wool doesn't really change the vapor permeance of the assembly since it's very vapor open (i.e. the permeability of the EPS dominates the stackup).
I see no issue going with more than 2" thick EPS here, although it will reduce the drying ability, since EPS is only minimally permeable compared with batts.
Fiber faced polyiso would probably be fine, but I don't have perm numbers handy for comparison purposes.
Bill