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Window sealing details for flangeless windows, metal siding, and asphalt paper.

DarrylThom | Posted in Green Building Techniques on

I just posted this on the FHB website as well, but I’m thinking I may get valuable insight from GBA as well.  Apologies if you are seeing this twice….

I am cutting new windows into existing wall construction and I seem unable to find similar examples of the sealing details online.  The building was originally a commercial warehouse style build and we are renovating it into a residential home.  The current construction from inside to outside is drywall, poly vapour barrier, fibreglass insulated 2×6 12 ft stud walls, 1×4 furring, asphalt felt, and then metal siding.  The windows I am installing are PVC framed flangeless windows.  I would like to install the windows in the center of the wall assembly to take advantage of the head protection offered by moving the window further inside the opening.  The two complicating factors for me are integration with the asphalt felt WRB and connection to the metal siding.

If I was to be doing a new build, I would likely not have chosen asphalt paper as the WRB, but that is what I have, so I need to find a way to work with this.  What appears to be the most relevant info I can find on the FHB site is this article,  I am reluctant to use the asphalt paper as a dedicated drainage plane and have some concerns with the sealing materials bonding to the material.

I have attached a diagram of what I theorize may work, but I would like the advice of some people who are more experienced and knowledgeable than I (probably most folks).  The key points to note are that I have what I consider a insurance sealing connection from the asphalt paper to the sill, I then cover that with an additional connection from the J Trim on the metal siding back to the sill.  The idea being that I use the siding for drainage but have the lower connection “just in case”.  I plan to use Siga tapes, based on their performance in the the GBA backyard tests.  I would use the Siga Dockskin to prime the asphalt paper as well.

1)  Will this work?
2)  If I use a backdam, is the third layer of Siga Wigluv that goes under the window frame necessary?
3) The article referenced above suggests sealing the bottom of the window frame for air sealing may work for inset flangeless windows.  Thoughts?
4)  Examples of cutting into existing WRB layers that I have seen illustrate using the the cut WRB to wrap back over the sill and then do the sealing.  As I do not feel asphalt paper is the best performing WRB and have concerns about bonding to it, I have thought to cut it after a short lap on to the sill so my sealing materials can bond to the wood of the sill directly.  Thoughts?
4)  I plan to use Hardie trim boards for the exterior returns.  What is the best attachment method to not compromise the sealing work under it?
5)  Am I complicating this more than I need to?  These tapes will be a significant cost to purchase, is there a simpler more cost effective approach I have just looked right past?

At this point, I have the building and the windows.  I have not purchased any sealing materials.  If someone wants to revise this whole plan and use a completely different methodology, I am open to listening.

Thank you in advance for taking the time to think about this and offer your help.


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  1. Expert Member
    Akos | | #1

    Since you have metal siding, I would go with a metal sill and trim all around. Any metal shop can bend you up the pieces and even color match your siding. The flangless vinyl will have a nice groove around the perimiter for the metal to slide into making the seal much better. The rest of the details you have shown should work.

    I'm a big fan of outie windows, deep sills are very handy and look better on the inside. Going this route would bring the window out to your WRB plane would simplify your window install significantly. You can even go with flanged window if you can get some vertical strapping in behind the felt on the sides.

    Inset window does protect your window more but now your weak point is your window sill as it will see a lot more water. Generally windows/walls fail at the sill, so keeping water away from that is always the best.

    1. DarrylThom | | #3

      Thank you for your input Akos. You are making me reconsider a couple decisions. I had been leaning to using Hardie Trim for asthetics. We would prefer a wood style window treatment but also want to uae firesafe materials as we live in a forested mountain area. I felt Hardie Trim would be a middle ground on that. You may be right in the utility of a metal trim integrating with the window though. I felt that using the sealing of the tapes may mean I could worry less about the function of the exterior trim.
      It ia interesting that you say most leakage concerns occur in the sill area. I have limited knowledge there and seem to recall reading that controlling water intrusion at the head was of primary importance. I have likely been making the decision on where to place the window in the frame based on mistaken premises to begin with. Again, I may need to reconsider my approach here.

      What were your thoughts on the approach of connecting the drainage plane to the exterior by bridging the sill to the j trim with tape, and them covering that with the exterior trim?


      1. Expert Member
        Akos | | #7

        Water leaks at the head is an install issue. They are easily prevented by basic details.

        There should be a head flashing, this should extend out past your trim, flashing should be sloped with end dams on sides, flashing should be lapped by the WRB ( head flashing needs to go under your felt).

        Leaks at the sill are the nature of windows, if your windows don't leak there now, it will eventually. This is why sill details are very important (sloped sill covered in flex flashing, back dam, sloped sill trim (metal or wood) that drains out past your siding).

        Mounting your windows in plane with your WRB will mean much simpler details and no origami flashing. Less chance of getting the details wrong and having water damage.

        Generally the peel and stick connects to the WRB, this makes window replacement down the road much easier. You can run it out to your J mold just make sure it is fully covered by your trim as most will not hold up to long term UV exposure.

        P.S. Your back dam in your drawing is in the wrong spot. It should be behind the window or at least under the inside edge of the window.. Also make sure you are not blocking the weep holes on your window with any of your tape/flashing/trim details.

  2. Expert Member
    1. DarrylThom | | #4

      Thank you Malcolm. I have read those articles and felt that my scenario with the asphalt felt and the metal siding didn't fit perfectly with the information presented. As I recall, the thick wall article shows a wall where the WRB is in line with the window and focuses more on the return details, which also weren't metal siding. The other article also has a modern WRB as compared to the asphalt felt I have. I do not feel the same confidence in tieing my drainage plane to the asphalt felt, which Is why I have tried to move that to the exterior. Not sure if that is a good idea or not...

      1. Expert Member
        MALCOLM TAYLOR | | #5


        Like Akos, I'm more comfortable with outie windows that those situated further in, but that may simply be because that's where most of my experience lies.

        Window heads are important p0ints to get right, but they are also easier. The great majority of the damage to framing I've seen when removing old windows has been at the sills.

  3. DarrylThom | | #6

    Thanks Malcolm. I think I'll do a reset on my planning and reconsider setting the windows as outies. I feel like I have just been operating on the assumption that inset windows offered greater benefit in the physical protection of the head area. Based on the feedback thus far, I may have been on the wrong track. Not too late to change plans though.

    I appreciate the input from you and Akos. Happy holidays, as well!

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