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Window detail: innie window with Rockwool, rainscreen, and stucco

pbout | Posted in Green Building Techniques on

I am almost ready to install windows (Marvin Integrity Wood Ultrex w nailing fins) on my remodel in CZ 3c. I’m still struggling with the window detail. Wall plan is 2×4, plywood sheathing, Blueskin, 1.5″ Rockwool, 1×4 vertical furring strips to create a rain screen, Cor-A-Vent horizontally on the top and bottom, and stucco cladding (on paper backed lathe).

Our original high-level plan was to have stucco returns on the head and jambs, and figure out something wood on the sill. I’m stuck on how to detail the head and sill so that the rainscreen cavity is still ventilated, and not draining any water back towards the window.

One idea was to add a z-flashing on the head, and the best detail I have found for that is in Chapter 5 of the The Guide for Designing Energy-Efficient Building Enclosures for Wood-Frame Multi-Unit Residential Buildings in Marine to Cold Climate Zones in North America, excerpt attached (and full guide available at this link:

If we go with a detail like that, then I have wood at the head, metal at the sill, and stucco on the jambs. I’m worried about the consistency of multiple trim finishes; and another option would be to trim the jambs with wood as well.

Our preference is stucco on three sides of the window. Is it possible to have a rain screen under stucco and still have stucco returns on the head (and/or sill)? Alternate ways to detail innie windows with stucco over rainscreens? Any ideas appreciated, thanks!

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  1. Expert Member
    Peter Engle | | #1

    Stucco returns are difficult to detail well, and it may not be worth the effort. You don't want stucco sills because water sitting on horizontal stucco is not a good idea. You need drainage at the head, so there must be some sort of gap between the wall stucco and the return stucco. You could install a thin stucco panel behind the head flashing, but that would be fussy. That leaves just the jambs, and the somewhat complicated material transitions, as you have discovered. Also, at only 1.5", there's not much of a return visually and that's why I suggest it may not be worth the effort.

    That said, the details you attached are very well thought-out and would likely be durable. And, while somewhat complicated, they should not be too difficult to install if the builder pays attention to the details.

    1. pbout | | #3

      Thanks Peter, that reinforces my gut feeling. We'll do a mock up this week to see where we land.

  2. Expert Member
    ARMANDO COBO | | #2

    FWIW, I've designed hundreds of stucco homes in the SW CZ3-5, and I've found that using 1x4 rainscreens gives too much bounce to the cladding, which IMO, allows for more cracking and damage. I prefer to use mesh rainscreens, like Delta Dry Stucco & Stone or Slicker MAX rainscreens.

    1. pbout | | #4

      Thanks Armando for the feedback. I looked at using Delta Dry or an equivalent mesh rainscreen, but over the semi-rigid Rockwool Comfortboard, I couldn't find a product that was rigid enough to hold the weight of the stucco and also accept fasteners to transfer that load. Have you installed the mesh directly on top of Rockwool, then a 3-part stucco on the mesh? How did it go?

      1. Deleted | | #9


  3. Expert Member
    ARMANDO COBO | | #5

    No, and I didn't think about the semi-rigid Rockwool; I've always used rigid foam. Having said that, I still think that I would rather deal with the negatives of installing rigid foam (GWP) than having to repair stucco or thin-veneer stone from time to time.

    1. pbout | | #7

      My Rockwool gets delivered tomorrow... no turning back now!

  4. Yupster | | #6
    1. pbout | | #8

      Yupster, thanks for the link! Interesting approaches. I'm not going to put my insulation on the exterior of the drainage plane. The moisture diverter could be an option though.

  5. JC72 | | #10

    Stucco with Rockwool compliments of Matt Risinger

    "In this video, I'll show you a traditional 3 coat stucco installs with a drainage plane PLUS Rockwool exterior insulation! I'm using Rockwool's Comforboard 2" thick R-8 rigid insulation panels"

    Of course Roxul basically requires "backerboard" on purlins (quasi double wall) when using stucco.

    1. pbout | | #11

      John, great link. I looked at using the drain-n-dry product but the parent company Boral said that it wasn't meant to be used over semi-rigid insulation like Rockwool. In fact, when I looked at this last year they said not to install over rigid insulation at all.

      I also don't understand how Matt is detailing his windows in the video; the drain-n-dry just stops at the edge.

  6. Tyler_LeClear_Vachta | | #12

    I understand the desire to have the drainage gap immediately behind the stucco veneer. The detail in the stucco buckets article takes some cues from an EIFS assembly where the drainage gap is immediately in front of the WRB. Keeping the gap small maintains the insulating value with rigid foam, but with a permeable product like Rockwool if insulation gets wet those R-values could drop further.

    As to Matt's stucco video, I had the same question. The window next to Matt has a large overhang, but the detailing isn't clear on the others. Definitely want to drain or divert there. If you watch Matt's other videos on this house similar question arise about weeps in the stone.

    I agree with Peter - your details seem well thought out. If you can get past concerns about 'bounce' and stucco cracking I think your approach will be durable.

    1. pbout | | #13

      Thanks Tyler, comments appreciated.

  7. pbout | | #14

    I'm still trying to nail down a detail on my window install. The detail I originally posted will work but it's a pain to build.

    One thought was about the rain screen, created by vertical 1x4 furring strips: does this area need to vent 'up' right below the sill, or can I create interior channels using Cor-A-Vent or similar so the air flows horizontally to either side of the window?

    The same could be asked for the head detail; can I skip the bottom vent connection on the rain screen, if somehow all the rain screen channels have air channels between them?

    Then I could follow the JLC article posted above to avoid stucco buckets, and have a sloped drainage flashing below the stucco above each window.

    I may not be describing this as well as I can, sorry

    1. Expert Member
      MALCOLM TAYLOR | | #15


      Yes. We don't provide a ventilation inlet to the cavities below or above windows. You could use Cor-a-Vent, but even using plywood strips you can hold the furring back from the sill and head, and stagger the strips, so the cavities still have air-movement to the continuous ones on the sides of the windows.

      1. pbout | | #16

        Malcom, thanks. I just reviewed Martin's article, All About Rainscreens, and particularly the discussion about what to do at the bottom and top of windows. I think my general plan is now:
        - Keep the wood furring strips back from the head and sill to allow for cross-cavity ventilation.
        - Add a diversion flashing above the head, akin to what's in the JLC article linked above, that will be at sheathing level (buried behind the stucco and maybe just below the Rockwool exterior insulation)
        - Stucco returns on head, jamb, and sill, with stucco sill sloped to prevent any standing water
        - On the sill, put a metal (galvi? aluminum?) flashing just below the window nail flange and under the stucco, to catch anything that gets past and divert it back to the rain screen. Lathe goes over this flashing.

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