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Community and Q&A

Is ThermoPly acceptable for window buck material?

TIM LANGE | Posted in Energy Efficiency and Durability on

This DER is a 100 year old house in Zone 7. Leaning strongly to using 3″ or so of spray foam on exterior of old siding. Vertical furr strips made from 3/4″ plywood, 3 1/2″ wide would be screwed to the frame using blocks of XPS. This will allow SPF to flow under the furrs filling the gap and providing more support.

For the window bucks, I’m considering a picture-frame of 3/4″ plywood around the RO screwed to the frame using same XPS blocks. The picture-frame and vertical furrs would be in the same plane and provides attachment for cladding and window trim.

Instead of lining the window buck with OSB or plywood, I’m wondering if ThermoPly could be used strategically. For the bottom of the ‘buck, would use 1/2″ plywood or OSB with a layer of ThermoPly over the top. We haven’t decided on windows and may use a window without nail-fin (Euro Style tilt n turn).

The sides and top of the window buck would have just ThermoPly lining the RO and fastened to the plywood picture-frame. The SPF would be directed behind the picture-frame to fill in the gap all around the ‘buck.

Since the ThermoPly is a WRB itself, flashing the RO could be done with a liquid applied flashing at the joints of the ThermoPly. Also use over the picture-frame to extend the WRB from the ThermoPly to the SPF.

See the attached .pdf for a SketchUp representation.

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  1. GBA Editor
    Martin Holladay | | #1

    My first reaction is that your suggested approach resembles the "Dudley box" approach. You may be interested in reading about Dudley boxes. Here is the link: "Window Installation Tips for a Deep Energy Retrofit."

    I'm not sure why you prefer ThermoPly to plywood or softwood lumber. I'm guessing that you either (a) want to save a little money, or (2) think that there will be a significant difference in thermal performance between plywood and ThermoPly. (It think the difference in thermal performance, if any, would be trivial.)

    Remember that ThermoPly can't hold screws or nails -- a signification disadvantage around a window, where you may need to fasten exterior trim.

  2. TIM LANGE | | #2

    Thanks Martin,
    I have read thru the Dudley Box a couple of times and am borrowing the picture-frame of lumber framing the RO for attachment of cladding. I also looked at ThermalBuck product but haven't crunched the numbers to see what it would cost in comparison to regular bucks and flashing. I don't like having to use long fasteners when securing a J-Channel for instance and it won't hold a nail if needed for window trim.

    I'm trying to take advantage of the SPF properties of flowing and bonding to other materials. Don't need plywood lining the RO for strength; can use ThermoPly as a back-stop to the SPF and once the SPF has been applied it will provide structural support to the wood picture-frame as well as the ThermoPly.

    The WRB properties of ThermoPly could be used as a way to reduce the amount of Wet Flash I'll need to use. If ThermoPly material lines the RO, then the Wet Flash only needs to lap over it 1" to make the WRB continuous from the SPF over the picture-frame to the ThermoPly lining the RO.

    I'm leaning toward Wet Flash or similar due to using the external SPF as the WRB and it will have an irregular surface that tapes may not seal to as well as a liquid applied material. I've calculated a material cost for a 38x62 RO to be $20 when using a 3.5" width picture frame and lapping the inside of the RO by 1" (onto ThermoPly) and the outside of the picture-frame to the SPF by 2"

    For attaching window trim, could still nail to the width of the 3/4" picture frame, carefully.

    Next, I'll compare the cost of using conventional flashing tapes to the Wet Flash. May be cheaper to use tapes for the flat surfaces and Wet Flash for the boundary to the SPF.



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