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Community and Q&A

Wall Assembly Sanity Check

Luke906 | Posted in Energy Efficiency and Durability on

Hey, all! Long time reader, first time poster. I’d purchased my first home here about 2 years ago in central Wisconsin (climate zone 6B) and I have found myself with quite the interesting structure. The whole house is made of concrete block and during renovations I’d found there’s zero insulation in the walls. As long as I’m opening up walls to update electrical and repair damaged drywall I thought it best to add some insulation. I make my living managing an engineering laboratory so it’s in my nature to want to do this “optimally” (as in a robust design that will keep my family warm and safe). I’d had a few insulation contractors out and when I pressed them with pointed questions based on what I’d learned here and from BSC, I wasn’t inspired with confidence that they really knew what to do with this type of structure (a few admitted as much). So in the interest of being my own advocate, over the last 2 years I’ve basically made a hobby of digging through this site and reading the articles and whitepapers on BSC (printed copies and went after them with a highlighter even). After all of that, I still lack the confidence to implement my plan without “sanity checking” myself against some of you clearly more knowledgeable folks (every time I think I finally truly understand vapor profiles, I find a new wall assembly diagram that makes me question everything).

At one point I did pay to have a PE in the house to look over some structural items and to help me come up with a plan for insulating the walls. I’m looking to check against his advice as well as, based on conversations with him, I wasn’t entirely convinced he’d been keeping up with latest building science practices.

I’ve attached two pictures here. The first is the design the PE recommended. One thing to note (and not represented in my drawing) is that the masonry block has a lot of variation in how plumb it is. Varying by about +/- 1/4″. For that reason I have concerns about how well I could get the foam board he recommended to stick to the wall. I don’t understand the purpose of the poly sheeting with the ccSPF. I thought ccSPF had such low permeability that poly wouldn’t really add anything?

The second is the design I’d dreamt up. Major difference is forgoing the foam board in place of full ccSPF which would negate the block being so out of plumb (I know, not the “greenest” design, but it seemed it would address the air and vapor leakage characteristics inherent in the concrete block). I’d also thought a fluid applied, vapor permeable, air & water barrier might be a more appropriate WRB. One thing not shown was that I’d also toyed with the idea of flipping the studs “on the flat”, allowing a full 2″ of foam and creating even more distace to prevent thermal bridging.

The wall cavity depth is 3.5″ as that’s where the bucks for door and windows landed. I could get creative with that if needed but I’m really hoping to avoid changing that dimension. Exterior insulation is a no-go as I’ve got maybe 4″ for roof overhangs. A local contractor (who is on the forum actually) had suggested a “chainsaw retrofit” but the roof was replaced a couple years before we bought the house so I’m hesitant to go that route.

To anyone willing to take a look and offer their thoughts, I’d very much appreciate it! We’re going on two years without drywall and paying an absurd amount for heat in the winter. My fiancee has been patient with my pace at solving this problem but I think I have a responsibility to get this done sooner rather than later (I promise she’s not making me type that). Thank you!

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  1. Expert Member
    Akos | | #1

    I don't have much experience with these, but it is very common in commercial construction. There it is almost always stud walls with fluffy insulation and interior vapor barrier. Either of your assemblies is much better than this (the poly behind SPF is not needed, but sometimes inspectors will still want it).

    You can probably save some money by going with a hybrid type assembly with rigid insulation glued to the CMU and a 2x3 stud wall filled with fluffy. 1" of foil faced polyiso with 2.5" fiberglass batts sold for sound insulation should work. 1/4" of unevenness doesn't make much difference here.

    Although the center of cavity insulation of the all SPF will be higher, the thermal bridging of the studs reduces this to about the same as batts+rigid. You can also go with 2x4 on flat with more rigid on the CMU and no fluffy but this will make your wiring harder.

    One item to watch is around your floor joists. Most likely these are pocketed into the CMU and are usually big sources of air leaks. These should be sealed up with SPF.

    1. Luke906 | | #3


      Thank you for the reply! I appreciate feedback from a prolific poster on here like yourself.

      I'm surprised to hear that "fluffy" insulation is sometimes used in masonry wall commercial construction. I know that BSC Digest 114 "Interior Insulation Retrofits of Load-Bearing Masonry Walls in Cold Climates" specifically says that solution is "risky" and "cannot be recommended". Although, as I'm finding out, just because something ought not be done, doesn't mean that's not how places are being built.

      I'm interested in your suggestion of board and batt, although as I understand, the air-sealing details of the foam board will be critical for this approach, yes? You mentioned sound insulation with this approach and I was curious, does this mean batts perform better than SPF for sound insulation?

      You are absolutely correct about the joists being embedded into the masonry. Everything I read around this lines up with your suggestion to SPF around those. Specifically I'd read to not apply the SPF heavily there so as to allow some drying to the interior to prevent joist rot at the tradeoff of some heat loss.

      1. Expert Member
        Akos | | #6

        My suggestion of sound insulation was because of the 2.5" thickness. There are only a handful of batts that thick, the rest you have to compress to fit in there. Regular FG batts can also be squished a fair bit and would work. It wasn't for sound proofing reasons. CMU makes for a pretty quiet wall already.

        Air sealing is important here, so make sure to tape all the seams and foam around the perimeter.

        The above assembly is pretty close to how you would insulate a concrete basement wall, as long as you have enough R value of rigid insulation for condensation control (35% in zone 6) you should be fine.

        P.S. SPF is great for air sealing which is one of the important parts of soundproofing, there are certainly cheaper ways to air seal. Any more than the 1" to air seal does nothing for sound though. The cavity should still be filled with fluffy.

        1. Luke906 | | #7

          Akos, thank you again for the reply. Do you believe the all SPF design would suffice and is sound? I understand I could save money with the some of the alternate designs you've proposed but I'd also be introducing more risk with the number of air sealing details I'd have to be diligent. This might be one of those instances where I'd spend the extra money because I wouldn't trust my own ability to do a new thing well the first time I attempt it. As well as what I imagine to be time savings.

          I'd read quite a bit about the insulation R value for condensation control but must admit it was one of the concepts I didn't find to be obvious in the forms I'd found it written. When you say 35%, that's 35% of what? I thought in previous items I'd read it was a percentage of interior vs exterior insulation, yet I've no intention of doing any exterior insulation on this so perhaps I've misunderstood.

          As always, thank you!

          1. Expert Member
            Akos | | #9

            The all SPF you suggested is pretty common around me when insulating old brick homes. So far, it doesn't seem like there are any issues with it. CMU would hold up to moisture much better anyways.

            I would still go for a version of flash and batt, this way your wiring won't be embedded in SPF, much easier to change things down the road. 1.5" of SPF with some R8 FG rolls can be squished in there even with the SPF is uneven.

            The 35% comes from here:

            This is the ratio of impermeable insulation to overall assembly R value. The impermeable insulation doesn't have to be outside of the wall, it can be also on the inside. Putting it on the outside generally is better as it keeps the wall warmer and less chance of moisture issues.

            If you are looking to apply a liquid WRB, it should be on the outside of the CMU not the inside. The SPF will seal this up quite well already, making any coating on the inside redundant. To do this, it would mean taking off your siding, with is probably more work than it is worth. Making sure your flashing details, roof overhangs and downspouts are in good shape goes a long way in keeping moisture out of the walls.

  2. Expert Member
    Peter Engle | | #2

    I would not discount the chainsaw retrofit quite yet. The best way to insulate a CMU house is on the outside. You could use reclaimed EPS and get quite a bit of insulation at a low cost. If you go with an EIFS system, you would start with the fluid-applied WRB, adhesive applied EPS foam, and then the synthetic stucco coating system. You can use up to 4" of insulation, giving a continuous R-value of around R-16 - better than code for your climate. 3" of foam would meet code if you count all the other wall materials. If you don't like stucco, you could also strap the foam with furring and hang siding as you show in your sketches. If you want to go much better than code, you can still fill the interior walls with fluffy stuff to your heart's content.

    You might not even have to do much violence to the roof when the overhangs are extended, depending on its configuration. Your local contractor will be able to talk about that with more knowledge. Shingles can be woven in, especially on newer roofs, and you might find that a color match isn't too hard to do. The benefits of exterior insulation so outweigh the cost, this approach should still be on the table.

    1. Luke906 | | #4


      Thank you for the feedback! Unfortunately my 1st floor ceiling is quite low already and roof line goes with that. If I extend the roof out, it will also go down and I've already only got maybe 6" until the roof would be into the window trim. And all of my windows have big poured in place concrete sills that only stick out from the masonry ~4" so there would be the additional complexity of extending those as well for a chainsaw retrofit. I'm sure it could be done, I just have concerns that it would be a much longer path than I'm prepared for. I really wish this house would play nice with some of these ideas because, agreed, I'd have much preferred the exterior insulation approach.

  3. Luke906 | | #5

    Out of curiosity, does anyone have any recommendations on a fluid-applied, vapor-permeable, air and water barrier? I've seen this mentioned many times in the documents I've read, and I'm certain there's restrictions for some places in naming particular brands, but any recommendations to brands or suppliers might potentially be very helpful to me. Thank you!

    Similarly I remembering reading a BSC article about attaching framing members to a masonry wall using non-metallic masonry brackets to prevent thermal bridging yet I've found no evidence that such a thing exists. Perhaps an opportunity for a new product if anyone is enterprising enough...

    1. Pmulls870 | | #8

      Prosoco R Guard is great stuff for sprayed barrier. I also live in WI up by lake winnebago, i ran into a similar issue in my house and after pricing things out I ended up going with PolyIso insulation sheets salvaged from reroofs, theres actually a guy down just west of madison that sells the stuff. I was able to put in 6" of polyiso for 1/2 the cost of 2" of spray. I was using great stuff foam between the sheets and their canned adhesive. I can almost guarantee you they will make you put on an interior vapor barrier inside the drywall.

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