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Contrast plywood window RO extensions with other method?

GBA Editor | Posted in Green Building Techniques on

Hello folks,
I wonder if I could get some discussion on the pros and cons of two window methods with outsulation.
I’ve seen the plywood RO extenders used.
Contrast that with:
(for a 2 in exterior detail) 1 layer of foam right to window RO, with the RO then surrounded with 1x 3+/- as an attachment surface for the window flange. Second inch of foam buts up to the wood around the RO.
Similar situation for thicker foam I suppose.
Thanks!
Nathaniel

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Nathaniel,
    I assume you are familiar with the "innie" versus "outie" debate?
    If not, you may want to read:
    https://www.greenbuildingadvisor.com/blogs/dept/musings/innie-windows-or-outie-windows

  2. Nathaniel | | #2

    Yes, I suppose both methods I described are outies, correct?

  3. GBA Editor
    Martin Holladay | | #3

    Nathaniel,
    I guess so. I don't really have an opinion here. Probably either way will work.

  4. Riversong | | #4

    Nathaniel,

    Windows don't exist in isolation. How are you attaching your cladding (and what type)?

    If you're strapping over the foam for siding attachment, then the window openings get "picture-framed" with strapping to keep everything on the same plane (as would be the case without outsulation).

    The boxing of ROs still requires a nailbase for cladding, but the primary advantage of the boxing is that you've isolated the window openings (and window frames) from all layers of WRB (since the exterior foam serves at least part of that function). So, if water should get between the two layers of exterior foam, it cannot drip down onto a window unit and find it's way to the structural frame or interior surfaces.

  5. Nathaniel | | #5

    "So, if water should get between the two layers of exterior foam, it cannot drip down onto a window unit and find it's way to the structural frame or interior surfaces"

    That make sense, I suppose. It seems that if water found it's way to the top of the plywood box (behind the foam already) it would be forced to migrate around the outside edge of the plywood, both to the interior, and the exterior. Unless the plywood is typically sealed to the header of the RO? Or am I misunderstanding you?

  6. Riversong | | #6

    I use plywood window and door boxes on my 12" deep modified Larsen Truss frames, though in that case they are internal rather than external boxes. But the principle is the same. The boxes are part of the air barrier and should be sealed to the framing (I use Tremco acoustical sealant), and at the box corners.

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