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How do I insulate an already framed concrete wall?

gi4ever | Posted in Energy Efficiency and Durability on

Thanks for the amazing posts. I’ve learned a lot from this site. Here’s the scoop. I live in Wisconsin, Zone 6. New construction and a walkout basement with full, 3/4 and 1/2 concrete walls. Builder had fiberglass and poly over studded wood walls and fiberglass only over concrete walls. We had a wet fall and at the end of it, there was just a mess downstairs. I pulled it all out to dry, and there it sits.

How would you recommend I Insulate an already studded concrete wall system, that has the 1-2″ gap from the concrete? Also, was thinking of using fiberglass and poly again on wood walls, but just before drywalling and painting. Thoughts? Thanks for the help!

ST

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Scott,
    You may want to contact your builder and point out that the installation was defective.

    A 1- to 2-inch gap isn't much. One approach is to insulate with closed-cell spray foam, taking care to ensure that the foam fills the gaps behind each stud.

    Another approach is to knock out all of the studs with a sledgehammer -- most will be salvageable -- to provide access for a continuous layer of rigid foam. Then you can reinstall the studs, a little proud of the plates.

    -- Martin Holladay

  2. gi4ever | | #2

    Martin - I was afraid you would mention something about "demolition" or "sledgehammers."

    Couple of follow-up questions.
    1. The wood stud walls sit such that there's a shelf. Part of it is concrete, and the other part would be the wood stud wall flush against concrete top. How do I insulate the top of the concrete wall portion?
    2. Would you recommend I can still go with unfaced fiberglass in wood stud walls just before drywalling?

  3. gi4ever | | #3

    Another possible solution to add to yours perhaps? Can I slide 1" rigid foam insulation behind the studs, tape the seams, then unfaced fiberglass on top with no poly?

    If we knock out the stud wall, and put 2" foam, can I also put the unfaced fiberglass over that, then drywall?

  4. GBA Editor
    Martin Holladay | | #4

    Scott,
    Q. "The wood stud walls sit such that there's a shelf. Part of it is concrete, and the other part would be the wood stud wall flush against concrete top. How do I insulate the top of the concrete wall portion?"

    A. You probably want to install some horizontal rigid foam -- between 2 and 4 inches thick -- followed by a wooden shelf (attached with TapCon fasteners or glue).

    Q. "Would you recommend I can still go with unfaced fiberglass in wood stud walls just before drywalling?"

    A. In this case, yes. For more information, see How to Insulate a Basement Wall.

    Q. "Can I slide 1 inch rigid foam insulation behind the studs, tape the seams, then unfaced fiberglass on top with no poly?"

    A. Only 1 inch of rigid foam insulation is a little thin for your climate zone. You could maybe do 1 inch of continuous foam, followed by 1 or 2 inches of cut-and-cobble foam between the studs -- then fiberglass. But I think knocking out the studs is easier and faster.

    Q. "If we knock out the stud wall, and put up 2 inches of rigid foam, can I also put the unfaced fiberglass over that, then drywall?"

    A. Yes.

    -- Martin Holladay

  5. gi4ever | | #5

    Martin,

    I've thought quite a bit about your answers and have read many of your articles. Thank you for doing this for people so faithfully. You're truly a credit to your profession to help like this without a fee in these sites. I do want to post a few more questions and begin work. I intend to follow your direction to the letter. I've attached three pics.

    1. Would you recommend that I use rigid foam (EPS right?) on both concrete and wood walls, then unfaced FG, no poly, then drywall? Given that I have plenty of time to accomplish this insulation job, do you think this is the best option (SPF isn't for me due to prohibitive cost)? Or would you suggest that unfaced FG with poly is just as good in the wood walls?

    2. You mentioned in another article to cut the foam so it's loosely in there, so the SPF nozzle can fit. Can I cut basically to size, with slightly tight fit, and then Great Stuff around the edges (so that it's basically along the top and not between the foam and OSB)? In other words, I would just be sealing the 90-degree corner because the foam would be snug in the cavity.

    3. Are the siding and cladding nails coming through the OSB a problem? Some extend a couple inches into the cavity.

    4. If foam is pushed to back of wood stud cavities, will this create a 'cold sheathing' effect you mentioned in other posts? My exterior is vinyl 2/3 of the way down, rock exterior 1/3 of the way down, with DOW wrap, then OSB.

    5. I've drawn, in red in the pic, what I think I could caulk for air sealing. Realizing this is a LOT of caulk, do you recommend this course? I'll be using Big Stretch. Do you recommend another type of caulk over Big Stretch? If pic doesn't upload, basically I'm asking if you would recommend 'picture framing' each wood stud cavity with caulk, then installing either the FG/poly or rigid foam/FG then drywall.

    6. I also have a concrete slab I'd like to carpet over nearly the whole space. Can you recommend the stack for that? I'm thinking either 6mm poly tight to bottom plates all around, then padding, then carpet. Or perhaps the dimpled poly, then padding, then carpet? Any advice here is welcome. Afraid that raising the level of the floor too much will mess up stairs height at bottom, walkout door, etc.

    7. Take a look at the concrete shelf. You mentioned a solution above, but with this pic, will that still work? Placing 2" of foam along the top?

    8. How far up do I add the rigid foam on the concrete foundation? The foundation ends, then there's a gap, then the I-joists. There's a shelf up there of several inches before it meets the plate. Should I spray foam all along the top there when I do the box sills (I intend to have all the sills SPF soon)?

    9. There's also a pic of the electrical box, radon tube, etc. This is the most complicated area of the basement. You mentioned to sledgehammer out the framing, but might be difficult with this setup? What to do with the elec box?

    10. Lastly, you mentioned above to sledgehammer out the studs, then continuous foam, then reinstall studs a "little proud of the plates." Are you suggesting I only sledgehammer the verticals, leaving the PT plate on floor, and top plate intact? Then installing foam, then reinstalling the verticals, which will extend beyond the top and bottom plate?

    After this I intend to get to work! Thank you for taking the time to answer an amateur's questions.

  6. GBA Editor
    Martin Holladay | | #6

    Q. "Would you recommend that I use rigid foam (EPS right?) on both concrete and wood walls, then unfaced FG, no poly, then drywall?"

    A. That's one approach. EPS is a good choice if you need to use rigid foam. The above-grade framed walls can be insulated like any above-grade framed wall -- in other words, there are lots of choices.

    Q. "Given that I have plenty of time to accomplish this insulation job, do you think this is the best option (SPF isn't for me due to prohibitive cost)? Or would you suggest that unfaced FG with poly is just as good in the wood walls?"

    A. Given the choices you list, I would say that cut-and-cobble plus fiberglass batts is better than just fiberglass batts.

    Q. "You mentioned in another article to cut the foam so it's loosely in there, so the SPF nozzle can fit. Can I cut basically to size, with slightly tight fit, and then Great Stuff around the edges (so that it's basically along the top and not between the foam and OSB)?"

    A. You can do that if you want -- ignoring my advice -- but you won't reduce air leakage as much as you would if you followed my advice.

    Q. "Are the siding and cladding nails coming through the OSB a problem?"

    A. No.

    Q. "If foam is pushed to back of wood stud cavities, will this create a 'cold sheathing' effect you mentioned in other posts?"

    A. Yes. If you want warm sheathing, you need to install a continuous layer of rigid foam or mineral wool on the exterior side of the wall sheathing.

    Q. "I've drawn, in red in the pic, what I think I could caulk for air sealing. Realizing this is a LOT of caulk, do you recommend this course?"

    A. It would be a good idea if you care about air sealing. Another approach, if you want to save money and time, would be to feel for air leakage with your hand (or an incense stick) on a very cold day. That way, you'll identify the worst areas for air leakage.

    Q. "I'll be using Big Stretch. Do you recommend another type of caulk over Big Stretch?"

    A. Not really. Lots of brands of caulk work well, as long as you avoid the cheap latex "painter's caulk."

    Q. "I also have a concrete slab I'd like to carpet over nearly the whole space. Can you recommend the stack for that?"

    A. Carpeting is the worst flooring choice for an uninsulated slab. Do you know whether your slab has a continuous layer of rigid foam under the slab? If there is insulation under the slab, carpeting is OK. If the slab is uninsulated, then you'll get mold or wet spots or bad smells under the carpeting.

    You can install a different type of flooring, like tile. Or you can install a continuous layer of horizontal rigid foam above the concrete slab, followed by plywood or OSB, and then carpet.

    Q. "I'm thinking either 6 mm poly tight to bottom plates all around, then padding, then carpet."

    A. Don't do it.

    Q. "Or perhaps the dimpled poly, then padding, then carpet?"

    A. Nope.

    Q. "Take a look at the concrete shelf. You mentioned a solution above, but with this pic, will that still work? Placing 2 inches of foam along the top?"

    A. Yes, the rigid foam is a good idea.

    Q. "How far up do I add the rigid foam on the concrete foundation? The foundation ends, then there's a gap, then the I-joists. There's a shelf up there of several inches before it meets the plate. Should I spray foam all along the top there when I do the box sills (I intend to have all the sills SPF soon)?"

    A. Either spray foam or carefully installed rigid foam (installed with attention to airtightness) will work in the rim joist area. Here is a link to an article with more information: Insulating rim joists.

    Q. "There's also a pic of the electrical box, radon tube, etc. This is the most complicated area of the basement. You mentioned to sledgehammer out the framing, but might be difficult with this setup? What to do with the elec box?"

    A. Needless to say, it would have been better to have selected a builder who understood insulation issues and addressed these problems at the construction stage. If you don't feel competent to remove the framing in this area, you can try to solve your problems with closed-cell spray foam. It's hard to teach all carpentry and demolition tricks over the internet. If you're in over your head, hire a contractor.

    Q. "Lastly, you mentioned above to sledgehammer out the studs, then continuous foam, then reinstall studs a 'little proud of the plates.' Are you suggesting I only sledgehammer the verticals, leaving the PT plate on floor, and top plate intact?"

    A. Yes. Then you would add shims to the plates so that the shimmed plates were co-planar with the studs.

    -- Martin Holladay

  7. gi4ever | | #7

    Thanks again Martin, for assisting a novice! This is a wealth of information I'll be acting on all Summer.

    Oh, what about the storage area of my basement? Can I leave that uninsulated, and insulate the interior wall along the perimeter? Or do it differently somehow? It's about 25% of the space, so I could save some money by doing nothing or an alternate approach.

  8. GBA Editor
    Martin Holladay | | #8

    The best approach is to insulate the foundation wall of your storage area, in a way that is similar to the way you insulated the rest of the foundation wall. That brings the storage area into your home's conditioned space.

    But if you want to insulate a partition wall instead, that approach may save money and time. It won't be as good, however, because the concrete walls of your foundation and your slab will act as thermal bridges, reducing the thermal performance of your basement.

    -- Martin Holladay

  9. gi4ever | | #9

    Martin,

    I've done what you suggested in prepping the site and rigid foam. Now all that needs to be done is the contractor to install insulation. I've decided to go with either "wet" blown in cellulose or dense packed fiberglass in exterior wood walls. Any preference? Use poly for one or both? MemBrain for one or both? Thank you again sir. Oh, and a related question. I noticed sider (vinyl) cut the exterior window trim a little short on the top on a couple windows. So that rain could drip down and along the housewrap/exterior window tape. Is that a problem and should I expect him to redo the windows?

  10. GBA Editor
    Martin Holladay | | #10

    ST / GI4ever,
    Q. "I've decided to go with either wet blown in cellulose or dense packed fiberglass in exterior wood walls. Any preference?"

    A. Are these walls adjacent to concrete, or are you talking about above-grade walls? For above-grade walls, either material can work. If you are talking about below-grade walls, I would probably choose the fiberglass over the cellulose -- assuming, of course, that you have a layer of rigid foam between the studs and the concrete.

    Q. "Use poly for one or both? MemBrain for one or both?"

    A. MemBrain is preferable to polyethylene for above-grade walls. For below-grade walls, you want neither polyethylene nor MemBrain -- just attention to airtightness.

    Q. "I noticed [that the] sider (vinyl) cut the exterior window trim a little short on the top on a couple windows. So that rain could drip down and along the housewrap/exterior window tape. Is that a problem and should I expect him to redo the windows?"

    A. It's hard to tell what type of trim you are talking about from your description. But if your siding contractor made an error and cut some pieces too short, of course you should tell the contractor to remove the pieces that are too short and do the job correctly.

  11. gi4ever | | #11

    Thanks Martin. The walls I'm referring to are as in the pic attached. The concrete has portions that are mostly below grade - wood walls are completely above grade. Either cellulose or dense packed fiberglass with MemBrain would work for the wood walls? Equally well?

    For the below grade concrete, I'm installing rigid foam or spray foam.

  12. gi4ever | | #12

    Pic of walls

  13. GBA Editor
    Martin Holladay | | #13

    ST / GI4ever,
    Q. "Either cellulose or dense-packed fiberglass with MemBrain would work for the [above-grade] wood walls? Equally well?"

    A. Yes. The skill of the installer matters more than the choice between cellulose and fiberglass.

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