Rim Joist Insulation
Hi,
I have a house in NJ built in the 1960s and am getting ready to finish the basement. The basement walls are made from concrete block and mortar. The double 2×6 sill plate does not appear to have a gasket between the sill and concrete.
I think I will need to insulate the inside of rim joists to pass code inspection. I’d like to use 2”XPS and spray foam around the edges. However I am concerned that by doing so, I will eliminate the ability to dry to the inside, and will potentially trap in moisture and cause rot.
The exterior of my house is brick, and there is a ~1-2” gap between the outside of the rim joist and the inside of the brick. There does not appear to be any vapor barrier or house wrap between them.
Does this assembly allow for adequate drying to the outside, or would I be at risk for trapping moisture if I was to insulate?
Any advice would be much appreciated. I want to insulate the rim, but not at the cost of long term durability. Thanks!
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Replies
Vic8,
It's not just the foam stopping the rim-joist and sill-plate's ability to dry to the inside, it's also that they stay colder, encouraging moisture accumulation.
The problem with houses with this condition is that there isn't any good way to assess in advance whether they fall into the majority which do just fine once insulated, or the few that will experience moisture issues.
This question comes up a lot. The best advice is perhaps to insulate the rim-joist but leave the sill-plate uncovered. Definitely air-seal both as well as you can.
Thanks Malcolm! To clarify, when you say air seal but not insulate the sill plate, how would I do that? Just spray foam or caulk the edge between the sill and concrete?
Do you think since I have no house wrap, and an air gap between the rim and the brick that will help promote outward drying?
Thanks!
Vic8,
Yes, caulk every seam you can get at.
House-wrap is so vapour-open (Tyvek is 77 perms, building paper up to 60 perms when damp) it really doesn't impeded drying. Having an air gap is useful, but brick is a reservoir cladding, so it can add moisture to the wall too.
Understood, thanks! Are there any other alternative, safer insulation methods you’d recommend for the rim?
I feel a bit stuck: My code office is going to require me to insulate them, but given the age of my house I feel as if there are no good options that don’t put the rims at risk of trapped moisture and future rot. Thanks!