Ideal window depth to minimize window perimeter thermal bridging through frame
I am replacing the windows and redoing building envelope in my 30 year old house. Currently have 2×6 framed wall, covered with 5/8 tentest type fibreboard and then wood siding. Windows are original 30 year old solid wood. I am replacing windows with the following: Basic flush mount vinyl high efficiency windows, 4 5/8 thick with interior jamb slot for interior trim to be added to. For building envelope, I am removing Tentestboard and siding, and new wall will consist of 2×6 framing, covered with 7/16”OSB sheathing, DuPont Drainwrap, Silverboard Graphite XS cap nail fastened, and then covered with a rainscreen consisting of vertical 7/16 strapping with coravent SV3 top and bottom, and then finally horizontal 1/2 strapping on top of this. The double furring is installed to allow for installation of Maibec shingles and provide the proper drainage plane. My plan is to install the windows at 1/2” beyond the foam exterior, so basically approximately 2” out from wall frame studs. The horizontal 1/2” furring strips will be stepped out from the window to allow for a 1/2” piece of trim to surround the window, and then on top of this will be the finished trim 5/4 thick top and sides, and a deep angled sill on bottom. So, based on the detailed description of this building envelope upgrade, the dilemma I face is determining the ideal window depth, and minimizing any thermal bridging from the perimeter if the window is too far out. Not sure if I need to insert window closer to surface of foam and then make trim thicker to accommodate for this. I’d like to use a product like Thermal Buck, installed flush to the silverboard foam surface to allow for a better nailing surface and not compress the silverboard with the furring strips, however my windows are already on order and rough opening won’t accommodate the lip of the Thermal Buck. I thought about maybe purchasing Thermal Buck and trimming the lip off of it, basically creating 1” thick rectangle that I would picture frame install around opening and make flush with the foam. Any alternatives or suggestions?
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User-7038920, would you mind sharing your name? The software is buggy and isn’t showing what you included in your profile.
Your plan sounds solid. I would ask where your airtight layer is; on similar assemblies I’ve found it easiest to use the structural sheathing, but standard OSB isn’t always airtight, so CDX or ZIP, with all seams sealed, are better options. Foam shrinks when it's cold and can't be relied on as a perfect air barrier, though I'm sure it's fine in many cases.
As for window location, there is a thermal benefit to moving them inboard, or you will get a thermal short-circuit at your interior extension jambs. Imagine that the temperature throughout the wall is an even gradient from exterior to interior; the temperature at the plane of the interior side of the window is the temperature the extension jambs will be. “Innie” windows are also better protected from the weather and have some built-in summer shading, and provide some visual interest—the exterior of most modern homes is very flat, lacking the shadow lines of traditional architecture. On the other hand, the thermal benefit is not huge, so I wouldn't overly-complicate an assembly to account for it.
All things considered, in a situation like yours, I like to put the window at the sheathing plane, which also makes it easy to tie in the airtight layer and the WRB. You will need exterior extension jambs and sill, which is not as hard as you might think—just make up frames and install them in one piece.
Michael, it’s Clark here and thankyou for the informative and detailed response. Any suggestions on the best way to insert an extension jamb frame? More specifically, am I correct in assuming the extension is placed on top of the OSB, picture-framing the window, and then butt the rigid foam up to it? Assuming the extension sticks out 2” from the OSB, then I will butt the 3 1/2 finish trim to the side of the extension frame, stepped out slightly. I’m putting the window head flashing on top of the jamb extension frame and taking it back to the OSB and taping accordingly. I’ve seen examples where it’s double, i.e. both on top of the window jamb and then a second flashing on top of the finished trim - so I may install this second flashing as well.
Thanks again for the feedback,
Yes, that is one common way to do it. There are other approaches that save a few Btu's but your is probably the simplest. There are some details here in the GBA library. You might also search Fine Homebuilding and JLC websites; they have both had articles on different approaches.