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Rigid foam retrofit over stucco

aibaxter | Posted in Green Building Techniques on

Hi all,
I live in Columbus, Ohio and I’m getting ready to start a new project. It’s a 1930 craftsman house that’s been abandoned for about 12 years.  We just bought it from the city.

The exterior of the house is stucco over brick (it’s original as far as i can tell), and the interior is plaster right over the brick. The stucco is not in great shape and I want to insulate the exterior anyway (rather than the interior–for a whole host of reasons: space, issues with insulating brick on the interior–plus I’m hoping to keep all that plaster right where it is). So to insulate, I plan to put 4-6 inches of reclaimed polyiso on the exterior, then a ventilated rain screen and (probably) concrete board siding. (Same with the roof, but then a metal roof).

My question is about a WRB. Do I need one? The original final exterior was the stucco, but it’s got a lot of cracks, and I know stucco takes on a lot of water anyway. Plus I really want an air tight house.
If I do put one on, should it go over the stucco or the polyiso? Do I just paint over the stucco with a fluid applied? If I do that, what about polyiso getting wet? And do I need an expensive LAWRB or could I just use drylock or something? I keep going back and forth, and just need to talk it out with some people in the know.

Oh, yea! And what about flashing the windows? (I’m putting all new ones in).

HALP!

Thanks!!!

Austin

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Austin,
    Your last question was the key to this whole issue. It's all about the windows and doors.

    You need a water-management plan, and the key to keeping everything dry is good window flashing and door flashing. That means that every window rough opening and door rough opening needs to be flashed before you install your new windows and doors. And the flashing you install in your window rough openings has to convey water to the exterior of your WRB.

    To do this, you need to know where your WRB is.

    Rigid foam can act as a WRB (although reclaimed fiber-faced polyiso wouldn't work very well as a WRB), but it's probably simpler to use housewrap. It doesn't really matter where you install the housewrap, as long as you can integrate your window flashing with the housewrap.

    For more information, see these two articles:

    "Where Does the Housewrap Go?"

    "Installing Windows In a Foam-Sheathed Wall"

  2. aibaxter | | #2

    Martin,
    Thanks for your reply.

    I have read both of those articles. But since I'm going over stucco, I didn't know if I even needed a house wrap.

    How would you attach house wrap to stucco? Should I just go with a fluid applied? Also, all the window sills are concrete, can you tape to that?

    Thanks

  3. GBA Editor
    Martin Holladay | | #3

    Austin,
    Stucco is not a WRB. For more information on WRBs, see "All About Water-Resistive Barriers."

    Q. "How would you attach housewrap to stucco?"

    A. With cap nails.

    Q. "Should I just go with a fluid-applied?"

    A. If you want. For more information, see "Housewrap in a Can: Liquid-Applied WRBs."

    Q. "All the window sills are concrete -- can you tape to that?"

    A. Yes. I suggest you use Tescon Vana tape from Pro Clima.

  4. aibaxter | | #4

    So, it seems like the most cost effective approach would be to use, say, a tyvek wrap (probably stuccowrap to let any water that gets behind the foam board to drain easily) and use flashing tape on the windows. is the Tescon Vana tape compatible with most house wraps?

    My only concern about tape is that the windows are going to be innies and there could be some tape 'origami' as they say. --but if you do a liquid applied flashing, then how do you deal with covering the wrap? so, it seems like i need to stick with one system or the other. seems like the fluid applied would be a better air barrier, but cost about 3x as much.

    so if i went with the wrap and tape approach, would i spray foam where the foam meets the rake overhang & soffits,then tape the joints of the foam to air seal?

    i plan to excavate and use a waterproof membrane and xps on the foundation walls down to the footer.

    and i plan to use full adhered membrane and polyiso on the roof deck (under a ventilated metal roof).

    so i feel like i should be able to get a pretty good air seal from the exterior with all that--short of cutting off the rafter tails and rake overhang and overlapping the roof membrane over the side.

    what am i missing air wise? seems like i'll need something at the rafters/soffits on the inside...

  5. Trevor Chadwick | | #5

    Why not just cut off the rafter tails, and make the wrb continuous from the peak to the footer?
    It might even be less work than trying to tape, seal, and fit foam

  6. GBA Editor
    Martin Holladay | | #6

    Austin,
    Q. "Is the Tescon Vana tape compatible with most housewraps?"

    A. Yes, as far as I know.

    Q. "My only concern about tape is that the windows are going to be innies and there could be some tape 'origami' as they say. --but if you do a liquid applied flashing, then how do you deal with covering the wrap?"

    A. It's possible to flash an innie window with peel-and-stick flashing. What's important is conscientious attention to detail.

    Q. "If I went with the wrap and tape approach, would I spray foam where the foam meets the rake overhang & soffits, then tape the joints of the foam to air seal?"

    A. That sounds like a sensible approach to air sealing the exterior.

    Q. "I plan to excavate and use a waterproof membrane and XPS on the foundation walls down to the footer."

    A. Green builders generally avoid the use of XPS, which is manufactured with a blowing agent that has a high global warming potential. You should use EPS. For more information, see "Choosing Rigid Foam."

  7. Expert Member
    Peter Engle | | #7

    You're already halfway to an EIFS finish, why not consider that? Change the exterior polyiso to EPS and use fiberglass mesh and finish. This way, you can keep the Arts & Crafts styling for the most part. 4" of exterior is the maximum EPS thickness for fire code reasons, but that's still better than nothing right now.

    A FAWRB is certainly more expensive, but I think it's going to make your life much easier. If you use grooved foam, you can use adhesive basecoat to apply it to the WRB. No fasteners involved in the entire finish system. Waterproofing the windows becomes simple. The WRB is also your air barrier, though the EIFS skin is an excellent second air barrier. As long as you provide a good drainage break between the above grade walls and the foundation, this is pretty much a bulletproof system. This is a commercial EIFS and you want to find a contractor with primarily commercial EIFS experience. You will have a much better chance of getting the job done right.

    The only real downside is termites. They can still get into the below grade brick and come up through the walls. How is the termite pressure in your area? If anything more than "none", you should talk to a bug company about soil pretreatment or bait/monitoring systems.

  8. aibaxter | | #8

    The house definitely has termites now. So, I'm planning on bait treating for that as soon as I'm done with demo.

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