Rim joist insulation: will it jeopardize the sill plate?
I live in East Central Illinois, climate zone 5. Our home is 1941 on a poured concrete foundation. The 2×6 sill plate seems mostly dry and solid though I’ve found a few soft spots on the exterior side (likely caused by outside water.) There is no capillary break between the concrete and the wood. The amount of above grade foundation exposed to the exterior ranges from as little as 10″ to as much as 22″ but most of it is 18″ to 20″ above ground. The interior basement walls have at least 2 coats of mystery paint and I have no way of telling if the exterior has ever had an application of sealer or not. Generally the basement seems quite dry, although there are a couple hairline cracks in the foundation through which I’ve observed a small amount of moisture weeping through.
My question is whether or not it will be safe to spray foam the rim joist. I’m worried things won’t be able to dry sufficiently and worried about sill plate rot from capillary wicking. As far as I can tell the sill plate is not fastened to the foundation at all and along some of walls the sill plate has “kicked up” under the weight of the rim joist leaving a fairly sizable gap under the plate. Would it be prudent to try to slip some type of capillary break beneath the plate in the areas where I can? I’m not really up to the task of jacking up the house, even though I’ve read it’s not as hard as one might think. Would closed cell spray foam fill the gap in those areas and serve as a capillary break? Something about encapsulating that area with foam makes me nervous, although it’s pretty standard practice around here. Then again so are a number of things which as I learn more about this stuff don’t seem to be best practice. I also have concerns about the environmental impact of the foam.
I’ve thought of asking them to keep the foam in the “box,” not totally cover the sill plate and then I can seal the gap beneath the plate using a different strategy. I’ve also thought of asking them to keep it thinner in general.
It looks like my family will likely be eligible for the federal weatherization grant and I’m sure that area will be one thing they want to address and probably with spray foam. Beggars can’t be choosers, right? But if there’s any concern about rot I was thinking maybe I should fill/caulk gaps and try to do the rigid foam (polyiso) cut & cobble method myself before they come do the energy audit. This way, also, the money could possibly go toward things that I really can’t afford and don’t feel as comfortable doing myself.
Many thanks for all of the awesome information on this forum.
GBA Detail Library
A collection of one thousand construction details organized by climate and house part