Plywood boxes for window openings in double-stud walls
dowens825
| Posted in GBA Pro Help on
Builder didn’t allow for plywood boxes in the window and door openings of the first 3 walls of our double-stud house project (zone 5 SW MI). Anything we can do short of tearing them out and re-doing them? We’ll be using drywall returns. Does the box serve any purpose other than connecting the two walls? Can we net and dense-pack like everywhere else and just not use the boxes?
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Replies
Dave,
Have you ordered the windows yet?
Was planning to do that today. Obviously they can be made smaller, though I hate to do that. What is the purpose of the box? If needed, could we make the interior opening a little larger and use pocket screws to attach box to the outer window frame?
Where in the opening are the windows going? Innie, outie, center? How are they attaching to the opening? Could you put blocking between and flush with the studs?
My windows were installed in the middle of the opening and needed the plywood box for attachment, but if that isn't your situation, could you use 1/4 ply as the box, just to hold the two walls together and hold in the insulation? If the openings are nice and square, you may be able to manage with the 1/2" smaller rough opening.
Are you stuck with standard size windows, such that adding the box now would mean much smaller windows?
In any event, I'd rip out the door openings and reframe. Obviously, doors need a more robust frame than windows. You don't want undersized doors and they'd maybe be special order and cost more anyway.
Stephen, thanks for your reply. Windows will go on the outside, attached with nailing flanges. We can use strips above and below the window opening to hold the walls together. Builder thought that netting could be used to hold back the dense-pack in the window openings, just like elsewhere. Windows are custom. No door openings are yet framed.
I can tell the builder to tear out the openings and re-do them. The problem right now is that I can't tell him why! I just don't know the function of that box. The outside wall will hold the window, and there's a 5" gap between walls that will be bridged by drywall. Why is the box always specified for double-stud walls?
Dave, as Stephen said, the box is used to install windows in the center of the wall assembly (where they perform the best). Another reason, with outie windows, is to provide nailing for interior extension jambs or drywall returns. Additionally, tying the interior and exterior walls together at the windows will keep everything tight over time.
In my double-stud wall, the windows were more or less in the middle of the wall and were attached to the box with steel brackets, so a sturdy box was needed to hold the windows firmly in place.
In your case, one reason for a plywood box might be to limit any flexing of the window/exterior stud wall unit. If the drywall is attached to both studs and the studs aren't firmly connected to each other, you could get some cracked drywall if the window moves with strong winds and the interior studs don't move as well. That's why maybe a piece of 1/4 ply, glued and screwed to both studs, might be enough. I assume the windows aren't so thick that they will cover the exterior studs completely, so you could simply attach the plywood starting inside the window jamb.
I'm no expert though, so maybe someone else will chime in with a better solution.
Dave,
The plywood boxes make the wall stronger and are the preferred way to go. Whether or not you choose to cobble together an alternative method now that a mistake has been made is up to you.
I can't help wondering, though: does this job have plans and specs? If so, what do the plans indicate? Usually, the framers are required to follow the plans and specs.
Maybe you can use 1/4 plywood and 3/8" sheet rock in the window jambs.
The boxes are, no doubt, part of the air barrier. You might consider using Thermaply to make the boxes, it is about 1/8" thick.
All of your replies are appreciated. The builder understood that he had strayed from the plan, but had hoped there might be a good workaround so that we wouldn't lose momentum. Thanks to your comments, I've gained a much better understanding of why double walls are constructed this way. Window openings are being re-done to accommodate the full plywood boxes today.