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Proper order of sheathing, flangeless windows, WRB, and rainscreen?

Jimmy Nguyen | Posted in Green Building Techniques on

I am set to sheath my house after getting all of the double studs finished. I know there have been hundreds of articles on how to flash windows and it seems JLC and Fine Homebuilding magazines feature a “new and improved” method every month, but I was hoping to get some feedback on my particular situation.

Here’s a quick run down of what I have going. I am using 7/16″ plywood for sheathing, 30# felt for WRB, 1×3 purlins run horizontally for rain screen, and LP smartside lap siding turned vertically to do board and batten (as an aside: doing this may void the warranty). My windows are Intus windows with no flanges.

In the October 2015 edition of JLC, there is an article on Installing High-Performance Windows. The author flashes the rough opening and then window directly to the ZIP sheathing and as a precaution, will later add a layer of felt paper. I think I can adapt this method for my situation with the only difference being I have plywood and not ZIP sheathing, which will change the order of the felt paper being applied.

Making things a little bit tricky is I plan to frame out the window openings with the same 1x3s I am using for my rain screen so that my window can be in the same plane as my siding, thus avoiding the need for trimming with sheet metal afterwards.

Do you all think this ordering of steps is good?

1. Plywood sheathing
2. 30# felt
3. 1×3 frame around window opening
4. Flash rough opening with SIGA tape and wrap it over 1×3 frame
5. Install window >
6. Flash flangeless window frame and wrap tape onto 1×3 frame
7. Cover any exposed part of 1×3 frame with additional flashing tape.

An alternative method would see me doing everything I just said except applying the 30# felt after all the windows had been installed and flashed.

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Replies

  1. Jimmy Nguyen | | #1

    Additionally, part of my process is inspired by the Dudley Boxes: https://www.greenbuildingadvisor.com/blogs/dept/guest-blogs/window-installation-tips-deep-energy-retrofit

    They flashed their plywood framed box and flange to foam.

  2. Expert Member
    Malcolm Taylor | | #2

    Jimmy,
    Can you or anyone else explain to me the advantages of flangeless windows? It just seems to be an unnecessary complication. Why are they manufactured that way? is it simply so that they can be installed at varying depths in the window openings?

  3. Jimmy Nguyen | | #3

    Dang Malcom, I was hoping you had the answer for me like you always do. I don't understand flangeless windows either. I had asked my supplier to see if the manufacturer in Germany (I think) could add the flanges, but I never followed up.

    I think that's the only advantage of flangeless windows. According to the author in the JLC article, installing the window in the middle 1/3 of the rough opening is ideal or windows. I prefer it on the outside face so I can have a deep interior stool and not have to wrap the window in extra sheet metal or wood.

    Most of my windows will be protected by generous overhangs. The only vulnerable windows are the ones located on the gable sides.

  4. GBA Editor
    Martin Holladay | | #4

    Malcom,
    Q. "Can you or anyone else explain to me the advantages of flangeless windows?"

    A. I guess I'm old. When I started my career, window flanges hadn't been invented yet. You could order your windows with or without exterior casing -- the least expensive casing option was brickmold casing. Anyone else remember those days?

    The reason that European windows don't have flanges is that they are often inserted into masonry walls.

  5. GBA Editor
    Martin Holladay | | #5

    Jimmy,
    No one has yet invented a tape that sticks to asphalt felt. If you are using asphalt felt as your WRB, you need to plan for lap joints, not taped joints.

    If you want your windows to stick out beyond the plane of your WRB, you are going to need to leave a flap of asphalt felt so that you can tuck something under the asphalt felt -- either flashing or tape -- and then the apshalt felt will lap over the flashing.

    Personally, I don't like to stick windows out beyond the plane of the WRB, because you end up with a horizontal shelf at the window head that has to be flashed. That's a potential leak point. But if you plan ahead, you can do it.

  6. Jimmy Nguyen | | #6

    Thanks Martin. So you are saying I should install the flangeless windows flash everything directly to the plywood and then cover with 30# felt?

    I just looked at the JLC article written by Steve Baczek and he used Proclima tape to transition from window to 30# felt. Shouldn't SIGA tape adhere to the felt just as good?

    I also remember possibly using some Rainguard flashing tape the last time I installed windows with flanges onto felt paper. Either way, I'll do a test of the SIGA tapes adhesion to felt paper.

    Here is the article I was referring to:
    http://www.jlconline.com/how-to/exteriors/installing-high-performance-windows_o

  7. GBA Editor
    Martin Holladay | | #7

    Jimmy,
    The way I read the Steve Baczek article, it sounds like he is depending on a lap, not tape adhesion, at the transition between sticky tape and asphalt felt. Here is what he wrote: "Below the opening, we stapled a 10-inch-wide strip of felt paper that would integrate with the felt-paper WRB that would be installed over the walls. Properly lapped, the felt paper will help to shed moisture under our exterior finish."

  8. Jimmy Nguyen | | #8

    Martin,

    I read it that way too. I believe we have the same thing in mind, but I may have not expressed my thoughts clearly enought. I think I have a good grasp of what needs to be done. I will take your advice and not put the window proud of the siding.

    One other advantage of flangeless windows that I realized is that I can install the windows from the inside and not from rickety scaffolding or ladders. This is a great benefit especially with heavier triple pane windows.

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