GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter X Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

I want to do outie windows in a wall with exterior Roxul ComfortBoard, but what about the window weight?

user-5293988 | Posted in Green Building Techniques on

I believe that outie windows are less work than innies. It is much simpler to build a box on the inside that doesn’t need to shed water than one outside that does. If I have 4″ of Roxul Comfortboard outside of my sheathing and build a plywood box to extend through the roxul, will this box hold the weight of a large picture window that weighs a couple hundred pounds? Any recommendations on ways to hold the weight of a big window as an outie without creating a thermal bridge?

GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.


  1. charlie_sullivan | | #1

    I've been eyeing the Compacfoam product imported by 475 as a good option to use for something like that. It's a super high density polystyrene foam that is mechanically more like plastic lumber than foamboard, but is almost as good insulation as real foam insulation. I don't know how to spec what size you need to use to support the weight.

  2. GBA Editor
    Martin Holladay | | #2

    I suggest that you read my article, Installing Windows In a Foam-Sheathed Wall. The article discusses two ways to support heavy outie windows:

    1. Install a plywood window buck that fits inside the window rough opening and cantilevers outward, with the exterior edge of the window buck flush with the exterior face of the rigid foam (or in some cases flush with the exterior face of the furring strips). This approach is typically used for outie windows or in-betweenie windows when the rigid foam layer is between 2 inches and 6 inches thick. According to Lstiburek, this approach works well for windows that are less than 3 feet wide. “Wider windows get a 2-by underneath the plywood at the sill [on the exterior side of the wall sheathing], to give the center of the window some additional support,” says Lstiburek.

    2. Install a picture frame of 2x lumber, with the lumber ripped from 2x6s or 2x8s. The width of the ripped lumber should equal the thickness of the rigid foam [or mineral wool]. This type of picture frame — the strongest from a structural perspective — is attached directly to the wall sheathing [and the rough opening lumber behind the wall sheathing] with long screws, with no intervening foam. This approach is the one to use if your foam is more than 6 inches thick, or if you are installing outie windows on a house with mineral wool insulation on the exterior side of the wall sheathing.

  3. 39Chev | | #3

    Regarding 2 above...

    I plan on 2 inches of EPS on the exterior of my wall without furring strips (hanging vinyl siding directly through the foam). Would it make sense to run my first 1" thick layer of EPS right up to the window opening and then make a frame out of 1" lumber and nail through the foam to the window framing in the wall? That way there would be a thermal break and something to nail the window flange to on the outside and the second layer of 1" foam could but-up to the framing lumber.

  4. GBA Editor
    Martin Holladay | | #4

    I suppose the answer to your question would depend in part on the weight of your windows. This thread discusses large, heavy windows that need to be well supported. If you are talking about that type of window, I probably wouldn't recommend sandwiching EPS between your picture-frame furring and the wall sheathing.

    If the window is narrow and not particularly heavy, I'm sure that your method would work.

  5. user-5293988 | | #5

    Charlie - hmmm the Compacfoam, it looks like it could have potential. I will contact the folks at 475 to see if it will work for me.

    Martin - Thanks for the info. If the compacfoam won't work, I guess I'll have to give a little on my thermal envelope and opt for 2x material to get the needed strength.

  6. DIYJester | | #6

    I installed large 4x6 tempered glass picture windows in a 3/4" plywood buck. Both my wife and I stood in this buck with almost no deflection in the wood. I used quite a few large screws to ensure the buck was held well within the window frame. This was with 4" of xps and a 2x6 wall.

  7. sfriedberg | | #7

    Martin, could you expand briefly on the need to use picture frames if you are installing outie windows on a house with (exterior) mineral wool insulation.

    Is this because the WRB is assumed to be behind the insulation and the picture frame makes it easier to extend the WRB? I'm guessing "no".

    Is this because rigid foam would provide an adequately stiff backing (at a reasonable insulation depth) for the window nailing flanges, but mineral wool would not? I'm guessing "maybe", in which case would a flanged plywood box serve the same purpose as the picture frame?

    Some other motivation?

  8. GBA Editor
    Martin Holladay | | #8

    Q. "Is this because rigid foam would provide an adequately stiff backing (at a reasonable insulation depth) for the window nailing flanges, but mineral wool would not?"

    A. Yes.

    Q. "Would a flanged plywood box serve the same purpose as the picture frame?"

    A. Yes -- assuming that you are talking about something like a Dudley box (see photo below). For more information on Dudley boxes, see Window Installation Tips for a Deep Energy Retrofit.


  9. user-5293988 | | #9

    Mike M -

    That is good to know about the strength of a 3/4" plywood box. A question I have: was the xps installed (and supporting the plywood) or was the plywood flying free 4" out from the wall when you "tested" it?

  10. christopherw | | #10

    I'd love to hear about it if anyone learns more about Compacfoam or something similar for this scenario.

  11. KeithH | | #11

    One question I think you should ask with a big picture window and a large outie offset: am I setting this window up, intentionally or accidentally, to be used as a window seat? If so, I'd probably think carefully about whether a little thermal bridging in the forum of stronger window framing is better than the safety risk of someone overloading your window box and sending them to the ground on top of a large piece of glass.

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |