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“Picture frame” method for outie windows

Loretta Gurr | Posted in Green Building Techniques on

i am installing outie windows in a house with 4” of exterior rigid insulation. the windows are all triple pane in the following sizes: 24” x 24”; 30” x 30”; and 32” x 64”. the method that i would like to use to install the windows is mentioned in the article http://www.greenbuildingadvisor.com/articles/dept/musings/installing-windows-foam-sheathed-wall :

“Install a picture frame of 1×4 lumber, installed on the flat, on the exterior surface of the rigid foam. The 1×4 picture frame is screwed through the rigid foam to the sheathing and framing.”

i haven’t been able to find much else in the way of information on this method. the information i have found is either for a picture frame used in conjunction with a window buck or a picture frame installed directly over the sheathing with the insulation butting up to it.

i am planning on using structural screws from grk (https://www.grkfasteners.com/products/structural/rss ) that are long enough to penetrate the framing by 1.5”.

based on the relatively low weight of the windows and the use of structural screws, can this method be used without a window buck and installed overtop of 4” of foam? also, is there any detailed information available regarding this method that i could present to the building inspector to ensure that he will approve it and to the window supplier to ensure that it will meet their requirements for warranty purposes?

any information greatly appreciated. thanks!

: )

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Replies

  1. User avatar GBA Editor
    Martin Holladay | | #1

    Loretta,
    In my article ("Installing Windows In a Foam-Sheathed Wall"), I gave the following advice: "If you are installing your windows as outies, and the rigid foam layer is only 1 1/2 inch thick or less, or you are installing outie windows to a picture frame, it is probable that you will also be ordering windows with flanges."

    The implication (which admittedly might have been clearer if written explicitly) is that builders who use thicker rigid foam won't be ordering windows with flanges. Instead, they'll be ordering flangeless windows and installing the window with masonry clips.

    Four inches of foam is a lot. If you really want to use the picture frame method with that much foam, you should probably ask an engineer to look at the type and number of fasteners to make the system work.

    Or you could just use window bucks.

  2. Loretta Gurr | | #2

    ok...change of plans!...i will look at window bucks...thanks!

    : )

  3. Ron Rosen | | #3

    Loretta-
    Check out "ThermalBuck" pre-formed window buck material: https://thermalbuck.com/

    These are really stiff-coated high-density EPS foam designed in an "L" shape to be used as a window buck, but much wider than anything you can make yourself. They are available for 4" foam according to the product dimensions page. Just cut them to length with 45 degree ends, caulk the corners and you have a buck.

    From seeing their samples, ThermalBucks are a real game changer for doing foam around windows and doors. They are nearly as stiff as solid wood, but give you the insulation/thermal break you need around an opening. I think they will be much better and faster than trying to fabricate any kind of box on site. Can't wait to try building with them.

    The long GRK cabinet screws (with the thin flat head) are probably a good plan for your windows. I would be cautious about the RSS screws, though. They are structural and you can get too much torque with them. The large heads may also stick up too much and interfere with trim.

    Good luck with your project!

  4. Loretta Gurr | | #4

    thanks for the reply!...i actually looked into thermalbuck a while back but it turned out that the deepest one wasn't quite deep enough for my insulation...i double-checked to see if that had changed but unfortunately it hasn't...it looks like a really good product!
    : )

  5. Mike Stickney | | #5

    Hi Loretta. If I understand your project, you're planning on 4 inches of exterior foam. Curious, why won't the 4" Thermalbuck work for you?

    Mike

  6. Loretta Gurr | | #6

    well, the (current!) plan includes 2x4 strapping overtop of the foam (see attachment)...i have also considered 2.5" foam with 2x4 strapping then filling in between the strapping with 1.5" foam but i would have to give up a little r-value...

    just an aside regarding the strapping...the recommended screw depth for the metal cladding is 1.5"...maybe i'm crazy but it didn't make sense to me to have it attached to 1x4s...

    : )

  7. Kenneth Gartner | | #7

    Hello Loretta,

    I have a similar layout -- typar over 4 inch polyiso over 3/4 t&g sheathing, with a horizontal 1.5 rainscreen base for my 3/4 inch shiplap siding.

    I am using the 'rainscreen' method mentioned at http://www.drjbestpractices.org/installation-instructions/1051, which is similar to the picture frame you describe.

    In my case, we ripped 2x4s in half (ie, 1 3/4 width) and then used pocket screws to create a solid frame, just 1/2 inch wider than actual window size. We are countersinking 7 inch timberframe screws (the kind used for log cabins) through the foam/sheathing into the framing material to a penetration depth of 1.5 inch. Approx 1 screw for each 12 inches along frame. We will use tremco acoustical sealant between the picture frame and the typar/foam. It is plenty strong for our flanged windows. 1.5 inch Truss-head stainless steel screws into the flange, sealed with Sikeflex 1a on three sides (not bottom, for drainage)

    For me, there is still complexity about the drainage planes to sort out but I have many kinds of tape at my disposal to get 'er done before hurricane season is upon us.

    Although one might think that long screws cantilevered across deep foam would be problematic, there was a research article from Joe L.'s building sciences website that convinced me the implementation is sound. It was based on siding weight held per screw, but could be generalized to weight of window as well. I could not use bucks due to architectural limitations, anyway. Since you rainscreenr must already be held on with long fasteners, I would assume the same solution would work for picture frame, logically.

    You are not crazy. You need 2x4 depth for the metal siding, in order to not invalidate warranty. Are your 'purlins' (the horizontal 2x4s used as rain screen) approximately 2 ft on center too? I ended up with hex 1.5 inch screws and found that predrilling 1/8 inch holes gave really uniform lines of screw heads.

    I am surprised you could have such short sill anchors as you show in the diagram. I hope that was six inches of embedment rather than six inches total length. Also hope you uses 2x2in square plate washers instead of the typical small washers so as to avoid splitting the sill under stress.

    Good luck with your house.

  8. Malcolm Taylor | | #8

    Kenneth,
    I was surprised by the length and spacing of the anchor bolts too. I think the IRC requires 7" to be embedded in the concrete. 36" spacing typically only is called out in high seismic zones, as are square washers.

    That said, 6" bolts at 6 ft oc were the norm until recently here. I think I've still got a bag in my shed.

  9. Andrea S | | #9

    Does anyone know if Thermalbucks can be used with flangeless (Klearwall) windows? The instructions on the Thermalbuck site refer only to flanged windows. Thanks
    Andy

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