Retrofit basement wall insulation
Hello all. I’m in the midst of a major remodel on my ~1910 house in Portland OR (Marine 4). Original construction is a concrete basement/foundation wall extending about 4′ below grade and 3′ above. Above that, 2×4 walls with no insulation. The sheathing of the stud walls is flush with the outside of the concrete. I’m adding a second floor to the house. Upstairs is 2×6 framing with batt insulation. I’ll add batt insulation to the downstairs. Outside both is 1″ polyiso continuous.
I plan to also insulate the first-floor rim joist and the basement walls. Thus my question is this: Should I continue the exterior foam down to the ground, covering the rim and the upper 3′ of concrete? I’d then continue my rainscreen and siding down to near-grade. (Note: I’d switch to XPS near-grade to avoid polyiso in contact with the ground.)
Of course that R6 polyiso is not sufficient on its own, so I’ll have to also insulate on the interior of the basement:
(a) This article indicates, albeit very briefly, that I’d be ok to use fiberglass on the interior rim joist. I presume I wouldn’t want foam in the inside that would make an impermeable rim joist sandwich [ Insulating Rim Joists – GreenBuildingAdvisor ].
(b) What about the interior of the concrete in that scenario? It seems like I still need foam on the interior to prevent condensation. Am I safe to use foam for the full height, such that the upper 3′ would have foam on both sides? Since concrete doesn’t need to dry, I’d think so.
Or is there a reason not to continue the exterior polyiso on the concrete? In which case I’d just do all the basement insulation on the interior.
Thank you!
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Replies
I would extend the exterior insulation down as you describe. It's typically better to insulate the exterior of the wall, it's just difficult to do as a retrofit, which is why you usually see the interior side insulated instead.
Regarding (A), if you have enough rigid foam on the exterior for your climate zone (and you do here with R6), then you can use batts on the interior. I would put some effort into air sealing first though. I'd also recommend you use mineral wool here instead of fiberglass, because mineral wool will support itself and make the installation easier as a result. I'd probably use mineral wool batts for 2x4 walls here, cut to fit, which would give you R15 on the interior, giving you about R21 total when including the exterior rigid foam. Note that this is somewhat riskier than a rigid foam or spray foam interior side insulation, since moisture can migrate through the batt. My own preference is to foam in EPS panels in situations like this.
Regarding (B), you don't need insulation to avoid condensation as long as the surface is kept above the dew point for the interior air. I would add additional insulation on the interior here to get you up to at least code minimum (typically at least R10 total here for CZ4). I'd use polyiso for this unless you have bulk water problems, in which case I'd use EPS. Note that you'll need an ignition barrier over that, which can be drywall, plywood, or any material suitable per code. You can also use a product like Dow Thermax that is rated to be left exposed.
Bill
Is there any kind of capillary break between the concrete and the sill plates? Without one, there is some danger that covering the concrete wall on both sides, limiting its ability to dry, may increase the risk of the sills rotting. One inch of XPS is somewhat vapor permeable though, so it would still allow some drying to the exterior, especially with 3' above grade. However, it would be safer to use mineral wool board or just put all the foam on the inside.