Spray Foaming a Cantilevered Rim Joist
My home in climate zone 4 was built in 1980 with a CMU foundation. There is a brick facade over the CMUs. At the rim joist, the brick stops, giving way to cedar siding.
The walkout basement is about 800 square feet. On three sides, the 2×6 sill plate is about 7-9 feet above grade, and on a fourth, it is anywhere from 7 feet to 2 feet above grade. The sill plate does not appear to be pressure treated, and there does not appear to be a capillary break. Nevertheless, it is completely dry.
Since the brick facade only rises to the sill plate, the 2×10 floor joists cantilever the rim joist a few inches beyond the sill plate, in order for the siding to sit flush with the brick. This creates an opening that allows a lot of cold winter air into the basement, and rodents, too.
Previously, the rim joist was insulated with fiberglass batts, which were moldy and rodent-infested upon removal. I want to replace that insulation with closed cell spray foam, but due to the cantilever, I need something to hold the spray foam and block rodents. For this reason, I’ve added wood blocking to the rim joist.
Before getting spray foam bids, I wanted to inquire with the GBA brain trust about the wisdom of covering the interior sides of the sill plate in closed cell foam, as opposed to just the rim joist boxes. The blocking I’ve added should prevent the spray foam from encapsulating the exterior side of the sill plate. Will one side of air exposure be enough for the sill plate to dry, if the top and interior sides are covered in foam?
Pic 1: Rim joist with blocking added
Pic 2: The exterior side of the sill plate, at the top of the brick facade
Pic 3: The cantilever beyond the sill plate
Pic 4: Daylight visible between the siding and brick. A lot of air moves through here.
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