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Installing flangeless windows in wall with 6″ exterior EPS insulation

BertM | Posted in Green Building Techniques on

I am building a new house in southern Ontario, Canada.  The exterior walls will be constructed using standard 2×4 framing, with OSB sheathing (which will be the air barrier), and then 6″ of rigid EPS (GPS, actually) insulation.  The plan is for house wrap over the insulation as WRB, then 3/4″ strapping, with long screws into the studs to hold the insulation in place.  Vinyl siding will be mounted to the strapping.

The windows are triple-glazed European tilt&turn, with no flanges.  I want to install them at or near the outside of the wall surface to maximize the useful interior window sill width.

I have read the very useful article here:

But I’m not sure of the construction details of the plywood window bucks.  The triple glazed windows will be quite heavy.  We have several 60″ x 60″ windows on the south side of the house.  The plywood cantilevered buck seems a rather flimsy thing to hold up all that glass 6″ out from the framed wall.

How should the corners of the buck be constructed to take the load?  

The suggestion of a 2×4 under the outer edge of the buck sill makes sense, but does it need to be supported somehow, or is it only transferring load from the center of the sill to the corners of the buck?  

Thanks for any advice.

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  1. Expert Member
    Akos | | #1

    5'x5' is not a big window. It is heavy but not excessive. A plywood box nailed to the inside of your rough opening is extremly strong, it will not sag. You can reinforce the corners with some Simpson corner hardware.

    If you want extra beefy, you can also make bucks out of LVLs. I've used these for the bottom piece under larger tilt and slide doors.

    Make sure to get the metal mounting clips for the tilt and turns, makes the install much easier.

    1. BertM | | #4

      Thanks for the response. I like the idea of adding angle brackets to the corners.

      I will ask my window supplier to make sure the metal mounting clips are supplied with the windows.

  2. George_7224612 | | #2

    Your desire to maximize the useful interior width of the sill conflicts with the way the euro windows work. Since they open in, you'll have to move whatever is stashed on the sill any time you want to open a window. Also, with the window mounted on the exterior of the wall, the sash will only be able to swing in 90 degrees, so it'll stick way out into the room. And, I believe that the building science guys say that for best performance the window should be mounted in the center of the wall thickness.

    Given the difficulty of hanging those very heavy windows out on the exterior, you might consider changing your wall construction to 2 X 6 with 4" of exterior insulation and moving the windows to center wall. You'll lose some window sill but simplify your life.

    1. Expert Member
      MALCOLM TAYLOR | | #3

      Window weight or placement aside, I'd much rather build a house with the 2"x6" framing and less exterior insulation as you suggest.

    2. BertM | | #5

      Thanks for the response, George.

      You're right, swinging the window inward will clear off the sill very thoroughly. However, I think the tilt opening is the way we will use these windows almost all the time. The turn option seems to be there mostly as a convenient way to clean the window. However, it had never occurred to me that the window could swing 180 degrees and lie flat against the wall. Is that a common use? The window would have to be mounted far enough in that the hinges are inside the plane of the drywall.

      I have read quite a few arguments for innie, outie, and mid-placed windows. If the experts can disagree, then I feel free to choose what I prefer. :) My wife loves plants in the house, so having big window sills will be a significant positive feature of our new house.

      1. George_7224612 | | #6

        I've had to look at these concerns in my own project, Bert. I agree that we will likely use the tilt opening most of the time. Not sure how well it will work if the window is crammed down the end of a tunnel. On the other hand, for the window to swing flat against the wall would require that it be close to the surface of the drywall. It's unlikely that there would be enough clearance of furniture, etc to allow a full swing of a 5' window. The compromise I've decided on is to mount the window mid-depth in the wall. That should allow air flow when the window is tilted and offer the best protection for it and best performance. We like plants in the house, too. I'll encourage my wife to use plant stands on wheels so they can be easily moved when it's time to clean the windows.

        1. Expert Member
          Akos | | #7

          For my home, most of my windows are 3 section with only the middle operable. This lets you open the window all way and still have deep sills. Works very well. It also means you can have plants on at least 2/3 the sill. I was hesitant in the begging when ordering as I though it might look weird, but it looks fine installed.

          Generally, tall and narrow tilt and turns work much better than wide ones. The only spot I have wide is for patio access and there it is a slide.

          90% of the time we only use the tilt as well, but do occasionally open the fully especially a nice summer day.

          1. BertM | | #9

            Akos, it's nice to hear from someone who has actually lived with the tilt&turn windows, and to get confirmation that the tilt mode is the primary use.

            When you do open the window fully in turn mode, do you worry about a gust of wind catching the open window and slamming it into the frame or wall?

          2. Expert Member
            Akos | | #10

            My windows do open fully, so there is very little chance of wind catching it. There a couple in the house that are similar to your setup (1 fixed, 1 operable) and there it does occasionally catch the wind. This only ever happens on very windy days though. They do get slammed shut a bit roughly, but not enough that I ever though about adding holding straps. Make sure they install the windows level and square, otherwise the window will want to close on its own from gravity.

            The tunnel effect for single operable windows is there, but it is not bad. I have a bathroom window like that and it still works quite well for intended purposes when tilted in. Try to get the jamb extension near the edge of the window (might have to notch the backside of them around the mounting clips).

          3. BertM | | #11

            Akos, thanks for these good tips. Installing the windows plumb and level was already important, but now even more so.

            Good idea to keep the jamb extensions as thin as possible to maximize the space around the window in tilt mode. How deep are your inside sills? Mine will be about 6".

        2. BertM | | #8

          I had not thought of the "tunnel" restricting air flow in the tilt position. I'll have to think about that one some more....

          Our 5x5ft windows are actually in 2 panels, with only one half operable. The operable window will therefore only have a "tunnel wall" on one side, leaving the other side to ventilate freely. The top will still be more restricted, though.

  3. frankcrawford | | #12

    I have white PVC frame tilt n turn Klearwall windows in my passive house in Calgary for the last 4 years and they are great. They are installed near the center of my walls, about 6in recess on the outside and 6in on the inside. Remember that the frames are thick, wide and the worst performing part of the window. Most triple pane tilt n turn windows can only have a 4ft wide opening. So a single opening window 4ft wide by 5ft tall would allow low light in and be better performing then a 5ft wide with a thick frame in the middle. I use the tilt function most of the time and I think its effectives would be reduced if installed as an outtie window.

    1. BertM | | #13

      Thanks for sharing your experience, Frank. You must have incredibly thick walls: 6" + 6" + the thickness of the window frame ~= 15" or 16"? I guess Alberta winters are tougher than Ontario winters. Putting my windows to the outside of my 10" walls will give me about the same 6" interior recess you have.
      Good insight about the amount of space taken up by the window frames. Where were you two months ago when I was ordering windows.... :)

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