Too late for outie windows?
I framed and wrapped my house planning on innie windows. Walls will be standard construction with 6” exterior foam. Actually…I installed all the windows that way as well. With that behind me, I am feeling like the structural and performance benefits I based this choice on are overblown, and I’d like to change to outie windows. My rough openings are sized for the windows – not the windows plus extensions. I am trying to determine a way to build rough opening extensions that attach outside the sheathing.
My thought is to build boxes of 2x lumber matching the rough openings in size. I would screw 5/8 plywood flanges to the back of these boxes with quality wood screws. These flanges would sit against the sheathing, and I would screw through the flanges and into the king studs, headers, and sill cripples. I would then predrill and send structural screws through the 2x boxes. I am feeling like with care and plenty of good screws, I can build something rugged enough.
Another issue is flashing. Windows are flangeless – currently installed with flexwrap and straightflash. Housewrap is installed, so my drainage plane is in the wrong spot for outie windows. My thought is to wrap these extension boxes in grace ice and water that extends from housewrap to the interior of the house. At the corners, I would use flexwrap or zip stretch tape, installed shingle style. At the heads, I would cut a new flap in the housewrap above the plywood flange, and install ice and water that begins up under that flap.
Is this a reasonable plan? Any potential problems I am not considering or not addressing properly? Any better ideas?