How to overinsulate window frame with exterior insulation, WRB detailing
AdamPNW
| Posted in Green Building Techniques on
Hi all,
I’ll be installing flange-less triple pane windows in a 2×6 wall with plywood sheathing, continuous wood-fiber exterior insulation, rain-screen, and cedar board/batten. I’m presuming most builders are still opting to go with a mechanical WRB exterior to the continuous insulation, but I’m having trouble visualizing how to integrate header flashing into that assembly.
Im planning to place the window roughly in the middle third. My issue is that if I’m trying to flash to the sheathing, won’t the WRB be in the way? Any help or details would be a great help, thank you!
Adam
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Replies
AdamPNW,
I may not be understanding your situation correctly, but as a rule of thumb, you flash to behind the WRB wherever it is, not necessarily to the sheathing. So if you had decided to locate your WRB behind the foam you would flash back to there, if it is in front that's where the head-flashing goes.
You can see that in the current blog. The location of the head-flashing is not related to where the window is: https://www.greenbuildingadvisor.com/article/high-performance-window-header
Thanks Malcolm, sorry for the late reply. That makes sense regarding the head flashing.
Ive got a follow up question for you:
Can i nudge the wood fiber insulation over the window a bit (over insulating the window frame) at the head and jambs without compromising the WRB or interrupting any drainage path?
The WRB in this assembly will be on the outside of the wood fiber insulation.
Ive read that over insulating the frame is ideal from a thermal bridging standpoint, but im not sure how this complicates the install.
For one thing, I’d plan to install the wood fiber board and WRB first, then install the windows from the inside. The ext insulation at the sill would need to be sloped away im assuming.
Thanks!
Adam
AdamPNW,
I can't see why your plan wouldn't work. But I should admit I've only ever detailed or installed flanged outie windows, so I may be missing something other would catch.
As I'm thinking through the details more, it might be simpler to use rips of Comfortboard to overinsulate the frame…but installed exterior to the WRB, and held in place with return trim. The WRB could then be detailed conventionally, wrapping over the wood-fiber board and into the RO without zig-zagging around the overinsulation.
Thus, the mineral wool would also serve as the drainage plane since it allows for a capillary break, and will not rot. This could be done at the head and jams, but perhaps even at the sill, since the WRB is now at the plane of the sill plate, and can slope away, allowing water to drain from the mineral wool.
I know id have to figure out how to detail the trim but that shouldn't be a huge problem. Thanks for your input as always Malcolm. I could also repost this as a stand-alone question for the Q and A.
Here’s a sketch of the jam/sill detail I’m imagining.
AdamPNW,
My only questions would be around attachment. What do you fasten the return trim and sill flashing to?
At the sill, the pan should be sloped and/or have a back-dam.
Right, forgot to slope the sill. I like your technique of sloping just the portion of the sill under the window.
As far as attachment, still working out those details. But perhaps the rainscreen furring can provide a nailbase for the trim. I might need a mockup!
Adam
Your windows manufacture instructions are always a good place to starts and rarely consulted.
Walta
Good point, ill reach out go them. Thanks Walta