I was thinking maybe 3/4" birch plywood (ripped to 11 1/4" wide)at first, but worry about how it would hold up if there's a moisture problem. What about real poplar or birch with a spar varnish...? Thanks, -m

5/8 or 3/4 Advantech is your best bet

Are you talking about green Zip System sheathing? I was hoping I could use something that would be presentable as a finished surface, rather than doing full extended jambs....

Thanks
-m

ZIP sheathing, also by Huber is not Advantech, but standard OSB. That would work as well; Advantech is a little stronger and "tougher". You'll neeoud to him the windows, caulk/foam between the window box and the windowm then tape the interior of the window to this surface, so it is not a finish surface. You can, however drywall this surface.

What would be the problem with using 3/4" poplar or birch? Or birch plywood? I want to end up with wood window "boxes."

Thanks for the discussion.

There is a bit of mystery here, what are you trying to do? How thick is the double wall? What is the WRB? How are you mounting the windows? How are you "flashing the windows? Where is your air barrier? How is it continued to the windows? You can always fabricate interior jam extensions out of hardwood veneer plywood.

A 3/4" plywood box to connect the outer and inner studs is often used around windows to connect the air barrier and WRB to the window but it is a substantial thermal bridge. FWIW I plan on using Thermoply for the connections and mounting the flange less windows to the inner stud, then adding interior jam extensions after sealing the window to the Thermoply.

Hi..

I guess I'm trying to kill 2 birds with one stone, This was suggested by one builder I spoke to. Originally I was going to do 3/4" plywood bucks (this seems like a common way to do it) and then do full extension jambs out of poplar or birch. The problem I can see doing this is getting the extensions attached smoothely etc. (I was thinking of pocket screws on the back side). The walls are to be around 11" thick.
Should I really worry about the little bit of bridging from the buck?

Thanks,
-m

Forgot to say... using a dimpled house wrap like Hydrogap or Valeron's Vortek or Tyvek D (going under cedar lap siding).

-m

I would consider using 1/2" exterior grade plywood sheathing for the box to attach the outer 2x wall to the inner 2x wall. 1/2" plywood should provide plenty of strength for this application and result in less material cost and less thermal bridging than 3/4" material. You still need to provide an interior finish material to the window blockout since the framed opening (including any plywood/OSB) needs to be 1/2" wider and 1/2" taller than the window frame to allow for shimming and leveling the window in the opening. After the window is set you will need to seal this ~1/4" gap around the window with low expansion foam or caulk with foam backer rod. Your interior finish material (solid 3/4" wood or cabinet grade plywood or 1/2" drywall) will then cover this framing gap around the window frame.

"Should I really worry about the little bit of bridging from the buck?" Probably not! Some thermal bridges are inevitable as they are necessary structural elements. If a 3/4" buck was structurally required then around a 3'x5' window that square foot of r 13 is justified. OTOH if there is no structural need isn't using a lower cost solution with a much smaller heat loss smarter, greener?