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Flashing finless (European) aluminum-clad windows

Bill_L | Posted in General Questions on

I have finless, wood-frame, aluminum-clad windows to flash, and have attached a detail of what I’m planning so far. I’m looking to address water that makes it behind the aluminum cladding and comes down along the outside of the wooden frame, at the bottom of the window. I want to move it outside of the wall assembly rather than leaving it to run down behind the clapboards.

My problems with the attached detail are surface tension pulling water inward along the bottom edge of the frame, the visibility of the open gap below the window frame, and the distance between the cladding and the sill.

I’m also not crazy about the flashing spanning over the top of the sill (I have dropped the sill 1/4″ below the flashing to prevent capillary draw underneath the flashing). I don’t want to close the designed gap below the window frame, as I want to both prevent capillary draw at this joint and ensure than any water that manages to get inside the wall assembly has a way out. A way of cosmetically covering the gap so that it doesn’t trap water would be ideal.

I look forward to your suggestions, and will also greatly appreciate any similar details that you might be able to post or provide links for.


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  1. GBA Editor
    Martin Holladay | | #1

    I wouldn't worry about capillarity, and I think there is nothing wrong with a gap between the bottom of the window frame and the flashing. You are the only human being on the planet who will be bothered by the gap; no one else will even see it.

    Your detail doesn't show the air barrier, which is an essential part of window installation. The best place to air-seal a window is on the interior side, by using caulk or a high-quality European tape to seal the gap between the window frame and the rough opening (for example, at the back of the granite stool).

    A while back, Jesse Thompson wrote a good article for Fine Homebuilding on installing windows. While his details apply to windows with flanges, the principles he espouses and describes are a useful background for anyone who wants to develop or adapt window installation details. Here is the link: Windows in Double-Stud Walls: In-Betweenies.

  2. Bill_L | | #2

    Thank you Martin, that is a great link.
    The windows in that article have a drip edge to overlap the flashing with that I don't have, which is why I've shown the flashing running below the entire frame. I'm also trying to use a historic sill to blend with a historic neighborhood (though no regulatory requirements). Overlapping the entire sill with flashing, which would run similar to Jesse's detail, or trying to cheat that and stop the flashing short as shown in my detail, are both something I'd hoped to avoid. I'm considering bending the flashing to route out below the sill, which would require dados in the back side of the sill so water can drain behind it (more gaps for the spiders and wasps to nest in) (it would also mean piercing the flashing to fasten the sill to the house). I think there's no getting around that in order to bring water outside of the clapboards, I'll need flashing to run either over or under the sill, so I need to pick my poison.

    The 1/2" plywood (top and bottom) and 2x sides that make up the jamb are meticulously sealed to the (interior) air barrier layer. Connecting to them, I'm showing ISO-BLOCO expanding foam tape, which claims to be "absolutely airtight", and I have been advised to use that between rough opening and frame on top and sides, with the bottom getting sealed after installation, from the inside behind the granite stool as you have suggested. I have been concerned about the ISO-BLOCO being able to completely fill a rough opening gap for those windows that are not perfectly centered in the rough opening (using the readily-available 18mm expansion size), and have been considering spray foam instead, but Jesse's article has me less certain of spray foam than I was before reading it. Perhaps I need to get the 30mm expansion tape ordered asap. Thanks for the reminder.

    and, as always, thanks again!

  3. jinmtvt | | #3

    Please explain how using expanding tape as any advantage over low expension pur foam ??

  4. Bill_L | | #4

    Hi Jin,
    The article linked by Martin talks about people being surprised by leaks in things sealed with spray foam. If memory serves me, it had to do with skinning over and folds in the foam. That said, I have some doubts about the expanding tape as well. The regularly stocked thickness expands to 18mm, and if a rough opening happens to be a little big (more likely than a little small, the stuff might not completely fill the void. Nothing that some well-adhered 1/4" plywood couldn't solve, but who needs the trouble. I can imagine it complicating shimming also. A lot less messy though, at least in my hands :)

  5. user-1061844 | | #5

    Looking at you window profile, it looks like an insulated European window frame that is self shedding, the only water that will come out of the frame is right under the alum., it can not leak through the frame like fiberglass frames tend to do. Please do not run a piece of metal under your insulated frames, as this will create a thermal bridge that will negate all the insulation in the frame.
    It would be preferred to clip/screw an alum. sill under this drip edge of the window, it will take care of your gap, especially if you then back that up with ProClima window tapes. You can tape the exterior with TESCON Profil or even use TESCON VANA as the connection appears to be flush on the exterior. Both use waterproof SOLID acrylic adhesive, that is age tested for >60 year performance.
    The installation gap can be filled with mineral wool which is vapor open just like the TESCON tapes which will let any humidity that could get in dry out. Another options is to use sprayfoam (as insulation, not an airseal),but this will restrict the drying. Both are much easier and cheaper to install then expanding foam tape.
    The interior of the frame can then be taped as well to get the best airseal and installed value.
    If you are still worried about the water getting under the window at the sill, we at 475. the ProClima US partner will have their new amazingly stretchy, strong adhering and self sealing EXTOSEAL Encors tape available in Jan 2014. So you can seal below you window, without the thermal bridge. But that might be more than you really need with the windows you are getting.

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