Opinion and questions on details of European flangeless windows
Hi All
I have searched extensively online, and I feel the following would be the best strategy to have a good air and water sealed “middie” windows in my planned thick wall for a new construction. I’m in Climate Zone 5A.
My wall from inside to outside would be as follows.
Drywall
2″ x 6″ studs with R23 Rockwool Comfort Batts
7/16″ Zip sheathing
3″ Rockwool Comfortboard 80 (R 12.6)
1″ Furring strips
Brick veneer + trims with steel or Hardie sidings
Thickness of insulated portion is 9″ 7/16. And then furring strips. So, the window buck thickness would be 9″ 7/16. I plan middie European windows. So, it would be in the outer half of my studs – kinda middle of the buck.
The attached 3 pictures summarize what I am planning to do. The first two pictures of making a window buck and then flashing it are from kimchiandkraut.net. The last picture is a screenshot from a youtube video by NS Builder. It was interesting to see self adherent WRB (SIGA Majvest) being used as flashing.
Questions:
1. What is the best material to make a window buck?
a) Wood
b) Something like Zip sheathing (7/16 vs 1/2 vs 5/8″) which can provide an air + water barrier on the inner surface of buck, thus minimizing tape use
c) use a study material like advantech subfloor
2. Is adding an acoustic or a watertight sealant (e.g Lexel) between the attachment of window buck to the jack stud needed?
3. Is flashing using tapes (Zip tape, SIGA fentrim or Wigluv) better than using a liquid like Prosoco fastflash or Zip liquid flash? In terms of durability and costs…
It was interesting to see SIGA Majvest used for flashing by NS Builders. It is not recommended to turn at corners, so maybe cost saving if window bucks are very thick…
4. If I use tapes, do you think the Zip stretch tape would do well with flashing the outer 4 corners of the projected bucks, plus the 4 corners where outer surface of the bucks meets the zip sheathing on the wall. In videos where I see Zip stretch used, the window opening outer edge is flush with the Zip, so it goes over the sill pan, jamb, and then on the flush Zip.
5. To make the windows air and watertight, which method is better – in terms of durability and costs.
a) Backer rod plus sealant (e.g. Prosoco airdam) along all inside perimeter, plus SIGA fentrim on outside perimeter except the bottom
b) SIGA fentrim both inside and outside (except the bottom)
6. To get some extra insulation, can I place some rockwool Comfort Batt strips on the perimeter (except the bottom) before doing air and water sealing?
Any other thoughts are much appreciated.
Thank you so much in advance.
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Replies
1. I have used 7/16" Zip and 3/4" Advantech. 7/16" requires precise screwing to keep the corners together. I now only spec 3/4" Advantech, though other wood structural panels would work. Solid wood could also work but it expands and contracts, and if there were any load on it, the grain could crack, so I'd stick with sheet stock.
2. It depends on how you plan to detail your primary air control layer, whether you want a perfect or nearly perfect secondary air control layer, and how stringent you want to be about vapor control. I use the exterior of the sheathing as the primary air control and don't worry much about the other two so I would not use the adhesive.
3. I don't have first-hand experience with fluid-applied products but in the high-performance building world, they are generally considered to be superior to membranes. But there is nothing wrong with high-quality membranes either. It depends how much you want to pay and who will be installing it. (Fluid-applied is generally the most expensive option.)
4. It would probably work but I have always used non-stretch membranes and I think they would be easier/simpler when doing a houseful of windows.
5. Pretty equivalent in performance. Not sure about costs but tape is probably less expensive.
6. I don't understand.
Thank you so much.
Will use Advantech 3/4" as subfloor, so will then do window bucks with that only.
Now I am thinking fluid applied flashing will probably be easier (even though more expensive), and more fool proof, compared to me checking the tape origami job on every window. I will discuss prosoco fast flash with the builder. The joints can be done using the gun-grade and the rest can be done using the roller grade.
Point 6. I meant the gap between the window frame and the advantech (top/sides), is it okay to place thinly cut strips of Rockwool Comfortbatt there to enhance thermal insulation? The backer rod will then go inside of it, and then covered by Prosoco airdam OR just SIGA fentrim.
Got it. Yes you can add insulation to that gap but air-sealing the interior and exterior is more important. I don't insulate the bottom because insulation, even mineral wool, can hold moisture which could lead to mold.
Stretch Tape is geared to reduce the amount of tape origami needed at windows at the 3-plane corners, but you don't have 3-plane corners if you have a buck like the photos (one of the planes is very small) so I don't think Stretch Tape gains anything. You're going to have to do tape origami if you use tape, depending on your buck geometry.
Liquid flashing is a good solution. It's messy and has a temperature-dependent drying time, but there's basically no limits to the geometry it can be used on. If you can frost a cake, you can apply liquid flashing.
Realize that tapes are often not the best for adhering to liquid flashing though, so think through your installation details if you're planning on using tape for window sealing, etc. This may set which direction you choose for flashing the buck. I think some tapes and some liquid flashing are compatible, but you should only use those known-good combinations, so that may set exactly which brand of compatible tape and liquid flashing you would have to use (and whatever those specific products cost).
The cost comparison between liquid flashing and tape will completely depend on the products you're comparing, and who will be performing the installation labor.
Thank you. Now I am thinking fluid applied flashing will probably be easier (even though more expensive), and more fool proof, compared to me checking the tape origami job on every window. I will discuss prosoco fast flash with the builder. The joints can be done using the gun-grade and the rest can be done using the roller grade.
Inside, I can use backer rod, and then Prosoco air dam. One user here who did a backyard test and said SIGAFentrim stuck very well to Fast Flash. But, I'll just stick with liquids then.
https://www.greenbuildingadvisor.com/question/backyard-building-science
harryaiims,
I've got a very similar assembly in my mudroom, which I am renovating at a glacial pace. I am doing outie windows because I want deep interior jambs (to collect more crap in the mudroom, of course), but it sounds like the assembly is pretty much the same as what you are thinking. I framed all my ROs 1 1/2" bigger and built 3/4" plywood boxes that span the depth of the wall from interior framing to exterior insulation (5 1/2" wall + 1/2" Zip + 3" Rockwool). Once I popped the plywood boxes in, I still have my 1/4" all sides RO. Around the boxes, I used structural screws to fasten 3" wide 2x bucks (just leftover framing material) all around the box and back to framing. Those bucks are what I'll fasten the window flanges to (or, if they were Euro style, you would fasten through the plywood box and into the bucks). The top buck I ripped at a 14° angle to drain away from the wall.
If you're properly flashing around the window buck I wouldn't bother with anything like Lexel between buck and sheathing; the flashing should take care of that.
I went back and forth on tape/liquid flash but settled on Zip Liquid Flash. As Chris said above, it is easy but messy. I liquid flashed the hell out of the entire opening and bucks and ran it a few inches onto the Zip sheathing. It's a bit spendy, but at the end of the day is a thinner buildup than lots of tape and it seems there's less room for error or an adhesive failure in a corner. I have to say, I had never used Zip LF before this and I was super impressed. That stuff is TENACIOUS and is a great solution for a lot of problem areas. As Chris said though, think through your sequencing. Zip LF will stick to tape, but Huber does not recommend trying to tape over LF. And if you LF over Zip tape, you need to wipe the tape surface down with acetone first to remove the release agent left by the backs (you'll know it's working because the Zip logo will disappear).
TL;DR, +1 vote for Zip Liquid Flash.
Out of curiosity, If you have 2x bucks fastened back to framing, and everything is liquid flashed, what was the purpose of the 3/4" plywood versus just using the 2x bucks? Was that just to stabilize the bucks to prevent relative motion?
Chris_in_NC,
My thought was that having a plywood box in the RO would maybe be a bit more dimensionally stable over time, so maybe I'd be less likely to get any cracks opening up in the liquid flashing - mostly at the sill.
After doing it, next time I'd just buck it out with 2x material and liquid flash it, and I think that'd be plenty good enough. The plywood boxes probably only cost me $80 and an extra hour of my time, but even then I don't think it was worth it.
Excellent, thanks for the followup.
Thank you. Excellent and clear drawing.
I will not need the extra 2x bucks because my windows would be middie. So they will be in the outer half of my 2"x6" studs. So, the window can be nailed through the advantech 3/4" buck into the jack stud.
Based on all the replies,
I am highly leaning towards fluid applied flashing. It will be more fool proof, compared to me checking the tape origami job on every window. I will discuss prosoco fast flash with the builder. The joints can be done using the gun-grade and the rest can be done using the roller grade. I think roller grade can save some time since my window buck will be deep, and inside surface will need upto 6-7 inches of liquid. Zip liquid flash does not come in roller grade.
Inside, I can use backer rod, and then Prosoco air dam. One user here who did a backyard test and said SIGA Fentrim stuck very well to Fast Flash. But, I'll just stick with liquids.
Yes, I did come across Huber's recommendations of using acetone on the Zip tape followed by liquid flash, and not to do tape over liquid flash.
https://www.greenbuildingadvisor.com/question/backyard-building-science