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Spray-Foam Insulation Behind Acrylic Stucco

erosin | Posted in Energy Efficiency and Durability on

I have a 1952 house in climate zone 3C, that was recently re-sheathed with OSB plywood, 2 layers of “D” paper, keene driwall rain screen, then 3 coat stucco with an acrylic finish coat.

I have the walls open from the inside, and am considering re-insulating with closed cell spray foam between the studs.  I’m concerned about sandwiching the OSB between 2 vapor barriers. Will the rain screen be sufficient to allow the OSB to dry?

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Replies

  1. Steve Knapp CZ 3A Georgia | | #1

    Erosin,

    Advice is generally climate specific. Where are you located? (And I think you mean oriented strand board was used as the sheathing, correct?)

  2. erosin | | #2

    I'm in northern California. Yes OSB, not OSD.

  3. Steve Knapp CZ 3A Georgia | | #3

    Northern Cali has multiple climate zones, and I really should have asked for your city or ZIP. Or you can look it up here: https://www.energy.gov/sites/prod/files/2015/10/f27/ba_climate_region_guide_7.3.pdf.

    I'll be curious to see what experts suggest. The acrylic finished stucco over OSB makes me think you will want to skip the close cell foam (bad for the environment in any case) and use air permeable insulation to allow drying to the interior. OSB is not as forgiving of moisture as plywood or board sheathing.

    I would inspect the stucco area to make sure that all the openings and penetrations are sealed to prevent bulk water from getting behind the stucco.

  4. Expert Member
    Peter Engle | | #4

    The quality of the stucco installation is first and foremost. The system you described should be very durable if it was done properly. FWIW, 2 layers of D paper are not a vapor barrier. That said, in a marine climate, I always prefer to have walls that dry to both sides. Open cell foam would be fine in your climate, though still not terribly kind to the environment. Any form of "fluffy" insulation would also be fine. There is very little risk of condensation within the wall system in your climate, so drying of any bulk water leakage is your primary concern.

  5. erosin | | #5

    Thanks for you response, Peter.
    Isn't the acrylic stucco itself a vapor barrier?

  6. erosin | | #6

    So is the consensus that closed cell is a bad idea in my situation? I'd like to do closed cell for the additional rigidity it provides (I'm in earthquake country), as well as the R value.

  7. Johngfc | | #7

    You really need one of the experts to weigh in, but in lieu of that here are a few comments.

    First, you seem to be describing EIFS - a "stucco" system that is basically exterior foam with an acrylic finish, rather than a more traditional cement-based stucco. As long as this is undamaged (you'll want to be careful to repair any surface damages) this will effectively be vapor impermeable. A drainage layer is crucial - it'll allow bulk water from e.g. window and other leaks (which _will_ occur sooner or later) to drain. My reading - as very much a non-expert - is that an insulation that buffers water in your climate is a good thing. So dense-pack cellulose would be a good choice in the stud bays. You're in a "moderate" climate - I assume you're mostly heading (vs cooling), so as Peter noted, seems like drying in both directions is a good thing. OSB (vs plywood) doesn't deal well with getting wet. Closed cell foam will restrict drying, but it helps with air sealing and stiffens structures. Without more information it's hard to say whether it's worth the cost in $$ and greenhouse gas emissions. As always, lots to consider and the details matter.

  8. erosin | | #8

    I don't have EIFS, Johngfc. I have 3 coat stucco with an acryllic finish. I'm considering spray foam between the studs. There's no foam outside the sheathing.

    The keen rain screen is (arguably) a drainage layer. Without it I wouldn't consider closed cell. I need to understand if it's a sufficient drainage layer.

    Any idea how I can attract the experts' attention?

    Thanks

  9. Steve Knapp CZ 3A Georgia | | #9

    Erosin,

    Another non-expert comment here. The experts generally advise that foam in the walls is a waste of foam. It's better to focus on the basement (if present) and the attic. Air sealing is really important no matter how you decide to proceed.

    Traditional stucco is normally a reservoir cladding, but I'm not sure if that still true with an acrylic finish. Let's see if this bump yields anything.

  10. Expert Member
    Peter Engle | | #10

    I almost sent a reply this morning, but got sidetracked. Let's try again.

    Your proposed wall system will be durable. Cement stucco is not a vapor barrier. Cement stucco is generally considered a variable class III vapor retarder, with permeance ranging from a little over 1 to nearly 10. permeance depends on cement mix, thickness, additives, etc. Permeance varies with humidity, as with many other traditional products. Permeance increases as humidity increases, and this is a good thing. The acrylic finish is generally more permeable than the stucco, so doesn't reduce the permeability in any material way. Caveat; there are as many different acrylic formulations as manufacturers, so YMMV.

    In my earlier post, I suggested low-density spray foam more for the cost savings and environmental friendliness than its permeance, though that is also an advantage. Closed cell is not very "green," even for HFO blown foam. Low density is better, fiberglass better than that, and cellulose is generally regarded as the "greenest" insulation. In your walls cellulose would work fine.

    Why are you so concerned about adding seismic stiffness? Did you upgrade your connections and shear bracing as a part of your renovations? If you added OSB sheathing, that's a big upgrade in itself. With new OSB properly fastened and reasonable tiedowns, any additional stiffness provided by CCSPF would probably not make much of a difference. If your seismic design is sort of marginal, CCSPF could make the difference, and that might be enough to pay for the upgrade ($$ and environmental costs). If not really necessary, I would prefer going towards greener and cheaper insulations.

  11. erosin | | #11

    Thanks for the responses, everyone.

    Peter, regarding seismic stiffness, I'm not terribly confident with how they fastened the OSB. Upon opening the walls (from inside) I discovered quiet a few nails that were meant for the studs but missed.
    I also discovered that my bottom plates aren't anchored to the slab, but that's a whole separate issue (that I'll have to fix before insulating).

    From what I understand, Seismic design wasn't much of a consideration in 1952, so "marginal" is probably generous.

    I understand the environmental considerations, and plan to use low GWP foam, which I believe to be a net positive to the environment, due to the energy savings.

    My only question is this: Will CCSPF put me at risk of rotting the OSB?

  12. erosin | | #12

    Peter,

    I found the permeability spec for my acrylic finish. It's 13 perms. Is that enough permeability for the OSB to breathe?

    Here's the spec: https://www.stuccosupplyco.com/wp-content/uploads/2020/06/Acrylic-Finish-Spec-Sheet.pdf

    Thanks.

  13. Expert Member
    Peter Engle | | #13

    Since 13 perms is way more than the permeability of the stucco itself, the stucco is the limiting material, not the acrylic finish. And, at 1-10 perms, the stucco itself is relatively permeable. Add the double layer of paper and a drainage layer, and your walls are pretty robust. If you're set on CCSPF, it won't make the walls rot. If the stucco is not installed and flashed properly, your walls will rot whether you foam them or not. Either way, the foam is not going to be your problem.

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