Recommendations on this basement wall & rim joist detail!?
Hi all,
I am located in Ontario, Canada, and currently finishing my basement.
The norm here in Canada has been to put 6-mil poly on the interior ‘warm’ side of basement walls… I understand the building science behind this ‘standard practice’ is flawed (greater water movement through air leakage vs diffusion, heating vs cooling of basement depending on seasons, limited drying potential of wall etc.).
If I could choose to detail the wall assembly again, I would’ve gone continuous rigid foam to act as both the vapor/air barrier, however interior stud furring walls are already in place and poly is up.
What are your recommendations for making this wall/rim joist detail better… or less bad? I was going to switch out to rigid foam for rim joists while maintaining the poly over the stud wall. The exterior 1″ insulated sheathing runs over rim joist, but not doing much thermally.
My biggest concerns are the drying potential for both rim joist and wall cavity, since eliminating moisture within the wall assembly is a near impossibility.
Any recommendations would be greatly appreciated, while limiting the amount of rework.
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Replies
Use fiber faced polyiso, not foil faced. Fiber faced will allow for a little bit of drying for the rim joist. I'm assuming here that your exterior rigid foam is something impermeable, or close to impermeable (foil faced polyiso, XPS, etc.). EPS is a cheaper option than polyiso that will also allow for a little bit of drying (and is what I used in my own home), but it won't get you quite the R value that polyiso will for the same thickness.
Bill
Thanks Bill for your response. The exterior insulated sheathing was detailed as 1" iso board. It is pink, and doesn't appear foil faced, so I'm not sure if its iso or XPS. Felt house wrap over top. House was built early 2000s.
The rigid insulation I'd intended to use for interior was Atlas EnergyShield 2" iso foiled-faced either side.
My concern for drying potential of rim joist was that I don't want to sandwich it between two vapor barriers.
Currently fiberglass batts and loosely stapled poly in rim joist cavity provide drying (and wetting) potential of rim joist.