Retrofit interior insulation on Wisconsin basement walls
Hi,
We want to retrofit the wall insulation in our WI basement. Currently, in the unfinished section, there is 2×4 framing spaced roughly 0.75” from the concrete wall with R13 fiberglass batts stuffed in the stud bays and poly sheeting over all of it. The finished section, fortunately much smaller in area, is the same as the unfinished section only over the poly sheeting is drywall. The rim joist bays are stuffed with fiberglass batts. The basement is dry from water infiltration as there has never been water in the sump pump pit. We do run a dehumidifier, and it runs a fair amount during the fall months. The basement floor is not insulated but does have a layer of poly sheeting under the slab. It is a daylight basement open to the south.
Our concern is mold. We plan to remove the poly sheeting and fiberglass batts and install foam board. Against the concrete walls, we plan to slip 0.5” foil-faced polyiso rigid foam behind the existing studs and seal with tape. We will use additional rigid foam, either EPS or polyiso, to fill up the stud bays and then cover with basement board drywall. The rim joist is engineered wood (OSB-like) with a 1” of XPS foam board, house wrap, and vinyl siding covering it on the exterior. Very little basement wall concrete (4-5”) is exposed on the exterior. We plan to cover the rim joist on the interior with 4” of un-faced EPS foam board followed by drywall to meet fire code.
We are leery of spray foam. We have already air sealed the rim joist/mud seal areas in the unfinished section. Is this a reasonable plan?
Thanks, David
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Replies
Can you tell us where in Wisconsin (Zones 6-8)? With 2x4 studs, I think you will be a little low on the R-value (15/19) that you want on the walls. Maybe Dana will chime in.
If you haven't already, be sure to read this article: https://www.greenbuildingadvisor.com/blogs/dept/musings/how-insulate-basement-wall
Also read this article on cut and cobble (it may give you second thoughts): https://www.greenbuildingadvisor.com/blogs/dept/musings/cut-and-cobble-insulation
Wisconsin spans US climate zones 6 & 7. Where are you located?
How much of the foundation wall has above-grade exposure?
Filling the stud bays with rigid foam is expensive and not necessary if there has been no flooding history, and it impedes the ability of the studs to dry toward the interior. Even in zone 7 you don't need more than R10 of foam at the above-grade section for dew point control on 3.5" of fiber insulation (up to R15), In zone 6 it only takes R7.5, or about 1/3 of the total-R, in zone 7 it's more like 2/5 of the total R. See:
https://up.codes/viewer/utah/irc-2015/chapter/7/wall-covering#R702.7.1
You have ~4.25" of space between the sheet rock and concrete, the 3.5" of stud + 0.75" between the stud edge and concrete. Cutting 3/4" foil faced polyiso edge strips to insert behind the studs might need to be hammered into place with a block to compress it into the tighter spaces, but would put a tigth ~R4 -R5 between the stud and the cold concrete, doubling the R value of the framing fraction. If you then put 2" thick polyiso in the remaining space between the edge strips and can-foamed the small gaps between the foam & stud to seal it you'd have 2.25" of space for some fiber insulation, and only 1.25" of the stud would be encased in foam, with ample surface area on the rest of the stud to allow drying toward the interior. An R11 batt compressed to 2.25" would deliver about R8, a compressed R13 would deliver about R9. (See: https://www.greenbuildingadvisor.com/sites/default/files/Compressing%20fiberglass.JPG )
With 2"/R12+ polyiso and R8 of fiber you would be R20+ at center cavity, code-minimum R19 with some margin, and even the framing fraction's R would be higher than a 2x6 due to the R4+ edge strips.
https://up.codes/viewer/utah/irc-2015/chapter/7/wall-covering#R702.7.1
In zone 6 you can use 1.5" polyiso for the field between your edge-strips, with 2.75" of space for fiber and still have PLENTY of dew point control. A compressed R13 would deliver about R10, the foam would be R9+, for a total R19 meeting code for R value, but again outperforming a code-minimum assembly due to the higher performing framing fraction.
If there is ANY concern about flooding, stop the fiber insulation above the anticipated high tide mark, and fill in with cut'n' cobbled EPS. You may have to shim out the foam with 1/4" or 3/8" fan-fold siding underlayment to eliminate a void/gap between the wallboard & foam in that section.
@ Steve - We are in climate zone 6. Thanks for your response and I will read those articles.
@ Dana - There is only around 4-5" of foundation wall exposed to the exterior. We are in climate zone 6 and there is no flooding history nor have we found any mold in the unfinished section. Thanks for your detailed comments. Certainly like the idea of eliminating expense and utilizing the existing R13 batts (assuming we find no mold). As to the rim joist, I thought I should allow it to dry to the interior and can't seem to find unfaced polyiso so I was considering 4" of EPS. The rim joist is engineered wood with 1" of XPS, house wrap and vinyl siding on the exterior. Other ideas?
David,
I am less concerned about drying to the interior (either at the wall studs or the rim joist) than Dana. Either Dana's approach or your original plan will work, in my opinion.
There is plenty of research on the question of whether adding interior rigid foam to rim joists that are already insulated on the exterior is problematic -- and the answer is no, this approach is not problematic. I think you can feel free to insulate the rim joists on the interior with any type of rigid foam you want.
Why do you need unfaced polyiso? So the concrete can dry toward the interior? Concrete doesn't need to dry- wood does. The difference between a true vapor barrier (less than 0.1 perms and 4" of EPS (a class-II vapor retarder, about 0.5 perms if Type-II, which would qualify as a vapor barrier under Canadian code definitions) is somewhat academic- there is very extremely limited drying toward the interior.
With only 4-5" of exposure on the exterior there's very little drying to the exterior, and if there isn't a good capillary break between the foundation & foundation sill there is at least a theoretical risk, but given the very dry history of that basement it's not much of a risk.
If (against all evidence of a real risk) you feel it needs to dry toward the interior, stop the insulation about a foot above the slab. Any moisture wicking up from the footing or through the soil into the concrete can dry through that 1 foot strip of uninsulated foundation wall. That's generally true even when there is sufficient moisture migration to exhibit some efflorescence several inches above the slab.
Before insulating the rim joist with rigid foam (any type) it's prudent to spot-check it's moisture content in several places. Ideally it would be under 20% everywhere (and probably is.)
@ Martin – Thanks for commenting and addressing the rim joist. And thanks for a very helpful site.
@ Dana – Not trying to dry the concrete foundation to the interior, but allow the rim joist to dry to the interior (concern it may get wet via the siding cladding, etc.). The mudsill capillary breaks looks to be 1/4" foam. Perhaps, with only a 1” of XPS insulation covering the rim joist on the exterior, it can dry just fine to the exterior and the foam board I use to cover it on the inside is not relevant (I think this is what Martin was suggesting?). I could always go with less thick EPS on the rim joist and make up the R-value balance with fiberglass batts? Good idea on checking with a moisture meter.
It's fine to go with thin foam and fiber on the interior. With an inch of XPS on the exterior and an inch of EPS on the interior you'll have sufficeint dew point control for R13 o R15 batts at the foam/fiber boundary. If going fatter on that with the fiber (like R19s) give it 2" of interior side foam.
It's difficult to impossible to make reliable truly air tight interior side air barriers the rim joist batts between the floor joists, so it's prudent to give it a bit more than the IRC prescriptive exterior-R for air tight walls with class-III vapor retarders. In zone 6 that would be R10 for 5.5" deep fluff, so give it at least R12+ if going with 5.5" deep batts that are all but guranteed to leak some air.
On the foundation walls you could use 2" EPS and compressed R19s. From a theramal performance point of view putting fairly tight fitting foam between the stud edges and concrete will do as much or more than another R3-R4 at center cavity. Foil faced 3/4 polyiso or 1.75 lb FrothPak foam filling that space would deliver the best performance, followed by 1-part can foam.
@ Dana - Thank! I feel like I can move ahead now.
Dana - It's clear to me that I will not be able to spray foam air seal every nook and cranny in the rim joist area. However, with effort, I can air seal these areas with caulk. What caulk do you recommend for a flexible bond from EPS & foil-faced polyiso to wood? Thanks!
Just about any polyurethane caulk would work well enough to bond the dissimilar materials with soem long-term flexibility. Acoustic sealant caulks will work too (they're a bit messy though.)
Thanks Dana!