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One more question about rainscreens

dcp123 | Posted in Green Building Techniques on

When using a 3D rainscreen material like House Slicker, Delta-Dry Stucco & Stone, or Sure Cavity behind stucco, you need something between the stucco and the rainscreen material to keep the stucco from clogging the channels for drainage and air. Some of these materials come with a pre-bonded fabric to do this, whereas you would otherwise put something like a layer of felt on top of the rainscreen and behind the lath and stucco.

I had thought that the structure that went (from inside to out) sheathing, WRB, rainscreen air gap, WRB stucco would be better because, while the stucco might dry more slowly, the outer WRB would minimize the amount of water making it into the rainscreen gap and thus reduce the amount of vapor in the space and the risk of liquid water finding a path through the inner WRB to the sheathing. My contractor says hell, no. He says that will defeat the purpose of providing a drainage plane. I’m not sure that I see that. The point of the drainage plane is to keep the sheathing and framing dry, not to keep the stucco dry.

I have seen that people have built houses with felt outside of the rainscreen, but maybe that was just to serve as a stucco barrier because they didn’t have a rainscreen material that came with its own stucco barrier. Also, the makers of most of these products sell versions with a non-WRBs stucco barrier on the outside, but I haven’t seen one bundled with a WRB intended for the outside.

So, I think my contractor is probably right, but would like to hear what people here think because I’m sure I got the idea of a WRB on each side of the rainscreen from somewhere and I want to know if anyone thinks it’s best.

Thanks in advance.

Nico

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Nico,
    It's perfectly OK to use more than one WRB with stucco -- in fact, it's necessary. Separating the two WRBs with a 3-D plastic drainage mat will work fine.

    For more information, see To Install Stucco Right, Include an Air Gap.

    -- Martin Holladay

  2. dcp123 | | #2

    Martin,

    Thanks very much for your response. I was just about to post here to say that I had found the source of my belief that a WRB should be on each side of the drainage mat. And I was going to link to that article, which I had just rediscovered. I'm going to pass the article and this thread along to me contractor and see what he says. While his initial reaction was (in more polite words) hell no, he did say that he's open to hear other advice from experts like you.

    To be perfectly clear, he does want to use two WRBs (two-layer building paper), but he wants to put both directly on the sheathing and have no WRB between the stucco and the drainage mat, while I want something more like 15# felt, drainage mat, 15# felt. Is it your opinion (putting aside the different WRB materials that a WRB on each side of the drainage mat is better than two between the drainage mat and the sheathing?

    Thanks again.

    Nico

  3. GBA Editor
    Martin Holladay | | #3

    Nico,
    Q. "Is it your opinion that a WRB on each side of the drainage mat is better than two between the drainage mat and the sheathing?"

    A. Yes, because you need some type of membrane to prevent the stucco from clogging the drainage mat.

    -- Martin Holladay

  4. dcp123 | | #4

    Thanks, Martin. I guess there's not much disagreement then. My contractor is fine with using a filter fabric to block the stucco, like what is included in Delta-Dry Stucco and Stone or one of the variants of Benjamin Obdyke's Slicker Max. I still think there might be an advantage to a WRB in front of the drainage mat, but if you think keeping the stucco out of the mat is good enough and he thinks that having a WRB there is an untested (by him) assembly, I'll probably go with his preference.

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