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Retrofit rigid foam outside CMU walls

James Howison | Posted in Green Building Techniques on

I’m looking for resources to plan a retrofit of rigid foam insulation outside our walls. The vast majority of resources are about wood framed walls, but we have walls built from concrete masonry units (CMUs).

Relevant background: House is in Austin, TX (Zone 2; mild winters, but some heating load, very hot summers, continual AC for 6 months). We’re working on the south facing wall first. It has some deciduous shading from oaks trees (which is great). We’re replacing 4 5×3′ windows with 3 2×3 windows (fiberglass, low-e). The inside of the walls is going to be covered with American Clay finish (no drywall).

We’re keen on insulating, but concerned about the overall thickness of the walls. The CMUs are covered with very hard, thin stucco and painted. There is some ‘rot’ in that from damp siding that we removed. The walls are already 8″, so we are thinking max 2″ polyiso (staggered 1″ sheets), foil on the outermost, then 1×4 furring strips (thus creating a rain screen gap and getting some benefit from the foil. Final outside finish will probably be metal lathe and stucco (although in other areas we might do metal or wood siding). Total width at that point is probably 10 3/4″.

Main questions in my mind (although all comments welcomed) are about air barriers/water barriers. My thinking is that we don’t really have to worry about this, since the current walls are adequately waterproof. To protect the foam we’ll tape/caulk the foam seams, but I’m not sure if the rain screen gap is sufficient to allow drying so that we don’t have to worry too much about that.

Final question is about core-filling the CMU walls. I think not, they are older, heavier three cell units which I judge to be about 65% concrete with lots of concrete all the way through parts (i.e. thermal bridges. We’ve heard, however, that filling is effective for noise and for avoiding air circulation and thus convection (although we tend to think that’s a just a question of sealing the tops and any penetrations.

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  1. GBA Editor
    Martin Holladay | | #1

    Q. "Main questions in my mind (although all comments welcomed) are about air barriers/water barriers."

    A. I'm not sure what your questions are. Obviously CMUs are rot resistant. It sounds like you are planning to use taped foil-faced polyiso are your water-resistant barrier; if you want, you can install plastic housewrap on top of the polyiso instead. More information here: Using Rigid Foam As a Water-Resistive Barrier.

    A CMU wall finished with interior plaster is a pretty good air barrier. Your plan sounds fine.

    Q. "Final question is about core-filling the CMU walls."

    A. I agree with your conclusion. You won't get much benefit from filling the cores.

  2. James Howison | | #2

    Thanks Martin. If I can restate my question a bit better: how important is a water-resistive barrier in my situation? The article you linked to seems to come down against using the foam in that way (especially given the possible shrinkage of the foam). If I had wooden walls and I read that I'd add a house-wrap layer on top of the foam. But given that the walls behind are concrete it seems that the water-resistive barrier is not so important. Given that is it even worth taping the seams? Or is the middle ground of taping the seams (and other elements of using foam as a water-resistive barrier mentioned in that article) the way to go?

    Still trying to figure out what to do with the gable roof section above. Current thinking is that we'll continue the lathe up onto that and stucco it. Probably easier than "popping out" the existing wood panels up there and then trying to flash the top of the insulation.

    Thanks again.

  3. GBA Editor
    Martin Holladay | | #3

    The building code and common sense both require a WRB. I don't think a WRB is optional.

    After all, you don't want water leaking in at your windows and penetrations. You need to manage water. That means you need flashings, and that means that the flashings have to be integrated with your WRB.

    In your case, I see nothing wrong with using rigid foam as your WRB if that's what you want to use. But you have to tape the seams and integrate your window flashing with the WRB.

  4. James Howison | | #4

    Thanks Martin, that makes sense. Now I have to figure out where to order polyiso (1x1" foil-faced and 1x1" unfazed) and compatible tape.

  5. TJ Elder | | #5

    James, if you're only installing 2" then you could save some effort (and reduce the number screws that you are driving into concrete) and use one layer. Now is your big chance to consider increasing that to 3", in which case you might want two layers. Normally people use all foil face boards if that's what is readily available.

  6. Steven Medlin | | #6

    LeeCor 1-3/4" split panels could be a decent option for your project. They are a eps sip w/ studs on 16" centers and do not require any additional sheathing. The split panels are not a structural component like the full panels and I am using them on a block wall for a church rehab. Very light-weight (20 lbs) and easy to handle.

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