Window sealing details for flangeless windows, metal siding, and asphalt paper.
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, https://www.finehomebuilding.com/2020/05/05/flangeless-windows-done-right. 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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