Detailing Rim Joists on Up-Step Foundation
Updated Jul 15: Condensed the background info and reduce to 2 questions – any additional help greatly appreciated.
In late 2020 we purchased 4 Bedroom, 1968 side split home in Ottawa, Ontario. Climate zone 5/6A(US) or 6(Canada).
The top of the foundation wall was formed in an ‘L‘ shape with two mud/sill plates. The lower one supports the floor joists and the upper one the 2×4 wall framing. (Photos and scans of construction plans from the city archives attached).
I am told that this known as a ‘curb’ or ‘up-step’ foundation detail and was used in our area in the late 1960s and early 1970s.
How best to insulate the rim-joist areas from the inside given that there is:
* no capillary break between the concrete and the two mud/sill plates
* at joist ends, there is a rim board which has an ~1/8″ air gap behind (confirmed in a few spots by selective examination)
* ~55′ of the exterior grade is within ~3-6″ of the top of foundation and upper sill plate. From what I can feel with my fingers below the siding it is soft but not rotten.
How best do I insulate and air seal the rim joist area?
I worry (perhaps unnecessarily) about trapping moisture here.
I have given thought to trying to lift the house and install a capillary break under each sill plate but this requires access from the outside to get to the hold down anchor nuts and I don’t plan to remove the siding yet.
The rim joists and foundation exhibit no visible signs of moisture or bulk water damage and according to the previous owner’s children the basement has never flooded. Generally the humidity is not overly high in the basement but I don’t have data (yet) to defend that statement.
The house tested at 5.57 ACH/50. Day to day there is noticeable air movement through the rim-joist areas which has probably contributed to seasonal moisture management of the sill plates and rim joists. In the future with the siding removed I plan to seal this transition properly.
I’ve come up with the following plan: Feedback greatly appreciated.
Existing Basement Floor Slab:
Walls – goal of R-23:
1) R10 (~2 1/8″) of Rigid GPS foam board
2) 2×4 @ 24″ OC framed wall, bottom plate in PT
3) Rockwool ComfortBatt R14
4) 1/2″ mold resistant drywall, taped and mudded, 1 or 2 coats of latex paint
Rim Joists – goal here is to allow drying to the interior for the sill plates but be airtight:
1) Pro Clima Viscon applied to the two sill plates and rim/floor joist transition areas for exterior to interior air infiltration sealing. This might also unintentionally prevent drying to the interior if applied too liberally.
2) Sufficient Rockwool ComfortBatt or snug fitting ComfortBoard 80 as appropriate to fill the rim joist cavities and bring it flush to the GPS foam board.
3) Intello Plus smart barrier cut, caulked (Contega HF or similar) & taped with Tescon Vana to the floor joists, underside of the plywood floor and the interior face of the GPS foam board.
4) Another layer of Rockwool ComfortBoard 80 left exposed in the joist bay above the top-plate of the newly built 2×4 wall to increase the total R-value
The application of carefully detailed Intello Plus is intended to act as an air barrier to keep moist inside air from reaching the rim joist area during the winter and allow drying to the interior when/if needed to remove moisture wicked up through capillary action at the sill/mud plates and concrete foundation transition.
To summarize the points I’d like help on:
Q1) Am I over thinking the risks of the curb/upstep foundation with respect to trapping moisture?
Q2) Does the proposed basement insulation detail make sense? How could it be improved or simplified?
I did investigate insulating the foundation from the exterior as part of adding continuous insulation to the framing but the quotes were 60K+ CAD. I could however do just the top of foundation to 2 feet below grade and use that as a transition to interior rigid foam board insulation.
Any and all suggestions/feedback most welcome.
-Floor plans from the city with foundation details -Annotated photos of a floor joist cavity with the drawing of a foundation cross section
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