Insulating Rim Joist with Mixed Exterior Cladding

AJKing
| Posted in General Questions on
Hello,
I am attempting to properly insulate the rim joists in our basement prior to finishing the walls, however I am unsure of how to proceed due to the mixture of cladding and exterior detailing. I am in zone 5a, Columbus, Ohio and the house was built in 1992 with a full concrete block foundation. The exterior is brick and stucco. The brick walls are brick, OSB, 2×4 framing, fiberglass insulation, polyethylene and drywall. The stucco portion is on 3 sides of the house and consists of traditional 3 coat stucco, 1” thick blue Dow foam board (I think R-3), 2×4 framing, fiberglass insulation, polyethylene and drywall.
The current rim joists have unfaced fiberglass insulation in all bays. My original plan was to do 2” XPS, spray foam the perimeter, followed with R-15 Rockwool. due to the 1” XPS on the exterior stucco portion, I am concerned I will make a foam sandwich. I know the 1” exterior foam isn’t enough to properly insulate the Rim joists, but due to the type of cladding it is not feasible to remove it and add more exterior insulation. I wasn’t sure if I should do one of the following or something else:
- 1” EPS foam followed with the R-15 Rockwool in all the stucco bays and do the 2” XPS or EPS with R-15 in the brick bays or
- R-15 Rockwool in all bays with Smart Membrain sealed and taped in those bays
- Cut the Rockwool in half for the stucco bays yielding R 7.5. I know that’s silly but I don’t think I would run into moisture issues.
I had planned to use 1.5” XPS in two layers with seams taped for the walls, 2×4 framing and 1/2” drywall. There is a sill gasket between the concrete blocks and the sill plate. I have already caulked the perimeter of all of the joist bays.
Thank you in advance for your help.
Amanda
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Replies
Amanda,
You don't need to worry about a "foam sandwich" as, unless it is faced, the 1" EPS is between 2 and 6 perms, so you will still have adequate drying to the outside.
Thank you, Malcolm for your quick reply. Just to clarify, the 1” Blue board foam is faced and is on the exterior. Provided I use unfaced EPS foam on the interior of the rim joists, I should have adequate drying potential to the interior of the basement if something happened that the rim joists got wet? We have not had any water intrusion issues behind the stucco, nor am I planning for them to be wet, it’s a “just in case” situation.
Amanda,
Unfortunately, you can't rely on drying to the interior, as any permeable materials will allow more wetting that drying during cold weather when the vapour-drive is to the exterior.
With no viable drying path, this is one of those situations where I don't see any alternative to relying on the exterior cladding and foam to exclude water intrusion and keep the rim-joist safe.