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How do I flash a no-flange window?

Robert Swinburne | Posted in Green Building Techniques on

I am wondering if anyone can advise or point me to good details for flashing a non-flanged European window (specifically Unilux).

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  1. GBA Editor
    Martin Holladay | | #1

    This a big topic -- look for an upcoming article on the subject by Jesse Thompson in Fine Homebuilding -- but here are the basics:

    1. You want to flash the rough opening, not the window. Make sure that any water that gets in the rough opening is drained to the exterior.

    2. Once the window is installed in the (flashed) rough opening, you want to provide an air seal on the interior; in other words, you want to seal the interior of the window frame to the rough opening. The best way to do that is with European window tape, but in a pinch you can use caulk or canned spray foam. Don't seal the exterior of the window sill; you want to allow any water that gets between the window sill and your sill pan (or site-built sill flashing) to be able to drain.

  2. MHoots | | #2

    We are having some of the same issues with a European window (Stabil) that we have been asked to install on one of our projects. I looked up the details that were published in Fine Homebuilding and they were close to our conditions, but not exactly what we were facing.

    Several concerns that I have:

    1. These windows were purchased to be flush with the exterior. All of the details that I have seen online suggest that they should be recessed.
    2. The graphic of the section in Fine Home Building did not show any type of sill pan and the dam affect was only a piece of tape noted to be "air sealing" tape.
    3. We are installing stucco as well, and would love to see a detail of how to integrate this with this window system.
    4. The manufacturer's details are not working and the code states that we need to install per the manufacturer's specifications. There are other issues with the windows as well. Since we are the general contractor, we have to warranty the system, however the manufacturer should be responsible (in my opinion) for providing details showing how their product should be installed in wood framed construction and to meet our codes.
    5. The detail in Fine Homebuilding has a flange as well. Our window does not.

    Here is the article that Martin said would be coming out and that I was referring to:

    I would love to talk to another builder that has encountered this problem and has come with a detail that works, meets our codes and has the manufacturer's blessings.

  3. albertrooks | | #3


    Here is one example of "European Window Tape" from Siga. The idea is to create an air seal at the interior so that pressure differentials don't suck water back into the assembly. A good interior seal will keep water traveling with gravity. A poor seal will allow wind and pressure differentials to influence it's path.

  4. GBA Editor
    Martin Holladay | | #4

    There is more than one way to flash a window. You're right that the code requires you to install all products according to the manufacturers' instructions. If these instructions seem bad to you, you have few options besides refusing to install the product.

    The detail drawn by Jesse Thompson (and published in Fine Homebuilding) shows a site-built sill pan; the pan is labeled "self-adhesive membrane" in the illustration. While many designers and builders include either a sloped sill pan or an interior dam, Jesse does not.

    Ultimately, it's up to you to decide whether the manufacturer's details, or the proposed details provided by the designer, are something you can stand behind and warranty.

  5. jinmtvt | | #5

    I have been bothered with the same problem for quite some time, since i started looking at deutsch windows and their installation process ..

    I haven't had much luck in finding details of their flashing process and isntallation methods,
    as any of you found such info with appropriate details ?

    If i recall, most system seem to use a bottom metal flashing pan.

    What about warpping the window opening completely with peel stick and then taping the window frame to it from the exterior ?
    I don't see why we need to let wind driven water get between anything .water needs to stay out .

  6. MHoots | | #6

    Thanks for the prompt responses and clarification on the detail in Fine HomeBuilding. While it is not labeled, I do see how the dam is integrated into the design.

    Those windows are flanged and the ones that we are installing are not. We applied the tape to the frame of the window provided by the Zip Wall system to create this same affect, water tested it and it does not leak now.

    I see that many are recommending that we use the jamb tape. Why is this better than minimal expanding spray foam? In a retrofit application, the spray foam conforms to the spaces better. I did read that some foams did not pass the blower door tests, however if applied properly, I do not see why they would not work.

  7. MHoots | | #7


    We tried what you had suggested last week and it seems to have worked.

    I also agree- why are we counting on foam tape applied to the jam to keep the water out? The foam should be an air barrier only in my opinion. It needs to be installed without any gaps- of course.

    I can see how the peel and stick tape is OK if you are framing new, however when retrofitting, it will be difficult to apply and can't guarantee that the opening will be perfect.

    We have a disclaimer in our contract about European appliances- now I need to add one about European windows.

  8. GBA Editor
    Martin Holladay | | #8

    You wrote, "What about wrapping the window opening completely with peel stick and then taping the window frame to it from the exterior? I don't see why we need to let wind-driven water get between anything. Water needs to stay out."

    Taping the window frame to the exterior is no guarantee that water won't get to the rough opening. The entire purpose of these details is to handle water that leaks into the rough opening -- not because we want the water to get in, but because we know from experience that it does.

  9. jinmtvt | | #9

    Sifu Martin : how would water get there if the WRB is flashed on top of the RO membrane ??
    If water is under the WRB from higher, nothing would protect the RO no ??

    Also, i am trying to find products/materials to include no wood or any other water sensitive materials
    within the window assembly ( i plan on using ICF and fully membraned RESIST wall systems in the near future.. ) There seems to be many different plastics that one could use for fastening purposes
    instead of wood for window perimeters...

  10. mfredericks | | #10

    I've also wondered about this detail and have not found much clarification online. However Hammer and Hand recently uploaded a video of a flange-less window install in a Passive House project. The house has liquid applied flashings/WRB but I found it helpful to understand the method of air sealing and moisture management with this type of window. I think their method would be compatible with other WRB strategies.

    Video link:

  11. GBA Editor
    Martin Holladay | | #11

    What does "sifu" mean? I don't think it's an English word, and I don't think it's a French word.

    Water can leak into a window rough opening many ways: through cracks between the window frame and the rough opening, as well a through cracks between the sash and the window frame. All windows leak, eventually.

  12. jinmtvt | | #12

    ( sifu = pere/maitre en chinois .. utiliser horse chine pour designer normalement son maitre de kung-fu ..son instructeur ... je l'utilise ici avec respect dans le sens de instructeur/mentor ..en esperant que ca ne vous derange pas !!! ...avec respect bien sur! )

    I understand it can leak from the window itself, but if the rough opening is completely covered with peel stick or another water proof membrane, water then only needs to be redirected toward drainage and no damage is sustained from the leak(s) .

    J'ai donc de la difficulte a comprendre le point sur lequel nous argumentons sifu Martin ...
    Pouvez vous pointer ce sur quoi je derive ?

  13. GBA Editor
    Martin Holladay | | #13

    OK, we agree. All windows can leak. If the rough opening is properly flashed to collect the water that leaks and to direct the water to the exterior, everything should be hunky-dory.

  14. jinmtvt | | #14

    What about using "tape" on both sides of the window frame ?
    ( taped to interior and to exterior RO and assume void was filled with exp foam )

    I don't see how it could pose a problem if the void is filled with foam or other insulation...
    and would make a super air/water seal ... but again i might be missing something as usual ..

    Also i have seen many people here using plywood for window bucks to finish with no thermal breaks on the wood part... wouldn't it be better to use a no flange window with insulation all around
    except for a certain weight holding part at the bottom and secur the window with long fasterners
    through the insulation ?
    then once could use some additional insulation on exterior/interior to "match" the required framing countour

    is there any insulation board with a good water resistance that finish can be fastened to ?
    SIS ?

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