Risk reduction when insulating rim/sill under suboptimal conditions
We had an energy audit done back about 5 years ago and I’ve become a bit of an energy/building nerd since then. I’ve read hundreds of hours of articles on fhb, gba, nrel.gov, etc.
Last year there was a question posed about the risk involved with interior insulation on the sill/rim joist areas.
When asked about insulating these areas “even when a capillary break is lacking” Martin replied: “How risky is this? Somewhat — because the interior insulation reduces the ability of the sill or rim joist to dry inward, and these wooden components may be damp due to capillarity”.
My house has poor original construction choices related to this that lead me here before I stop thinking/obsessing about, and finally begin this next project.
The house in question is an aluminum sided 1989 2500sf 2 story located W of Chicago, zone 5A with clay soil. Basement is poured concrete, has an interior perimeter drain and no water issues for ten years. 1,000sf is basement and 440sf is a partial basement crawl of sharp gravel covered with torn polyethylene sheeting. The basement has a large Sanidry dehumidifier, a two-pipe condensing furnace and a NG powervent water heater.
In three spots, the builder placed concrete that appears to be directly against the rim joist. Two of these are stoops under doors, the other being a 32×5 NW facing concrete front porch with a roof. One of the stoop areas was completely rotted thru when we purchased the house and was replaced before closing- meaning in just 16 years the wood was destroyed. Unknown if there were contributing factors.
The last pieces of the unfortunate puzzle are that between the NON-pressure treated sill and the foundation is only maybe 1/8” compressed fiberglass, and a grading that has soil up to within 4 inches of the sill.
I’m getting ready to insulate the entire basement/crawl and then later create a finished basement. With all I’ve learned here, I’m worried about the drying potential for the wood if I cover the rim with XPS like cut-n-cobble OR XPS and spray like Martin’s ‘peanut brittle’ method.
My questions are 1) How does one mitigate risk in situations such as these? Especially in those areas that seem high risk? Different insulation types? Redo the grading?- even if it would reduce slope? Replace soil at the top foot of grade with gravel? 2) When it’s time to finish the basement, would different construction types change this risk? Meaning air-tight drywall vs. dropped ceiling vs. no ceiling.
The rest of the plan:
3/4 XPS R-4 with taped seams on the floor topped with 3/4 ply screwed down. Followed on walls by double layer 1” foil faced polyiso R-11ish w/offset taped seams ‘glued’ to the concrete, with 2×4 wall butt against the polyiso and covered with drywall. (IL code is 10/13)
The crawl plan: Cover the sharp rocks with geotextile fabric, followed by polyethylene sheeting that will be adhered to the concrete. Overlapped seams, taped, etc. Followed by the double layer polyiso on the wall like the basement.
It’s entirely possible I’m overthinking this and just need to jump in. But I also don’t want to create huge problems where they don’t currently exist.
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