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Thermal breaks for window bucks in double stud walls

Chris Chenier | Posted in Energy Efficiency and Durability on

We’ll be buliding double stud walls for a new house, with the inner wall being structural (the outer wall will consist of modified Larsen trusses). Total wall thickness will be around 16″ and windows will be “in-betweenies”. This will be my first double-stud wall house and I’m working in the wall details trying to minimze thermal bridging.

All the penetration details for window bucks on double stud walls I’m seeing on GBA and elsewhere are built with plywood (typically 3/4″) joining both walls. This creates a thin thermal bridge that adds-up. (For example, for a large 6’x6′ window, this corresponds to 1.5 square feet thermal bridge, which I consider significant.)

Has anyone used a plywood buck that would extend from the inner wall just far enough to support the window, and then finish the full depth of the buck with foam? Structurally (which may be a concern for larger windows), I see this as equivalent to bucks that extend outwards for walls that would have a few inches of foam on the outside and “outies” windows.

Is there any flaw with my system that I’m overlooking?

We’re owner-builders, doing the work almost exclusively ourselves, so are not worried about extra labor and we’ll be attentive to details. We’ll be built in a small valley and as such will not be subjected to strong winds.

Thanks.

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Replies

  1. Brendan Albano | | #1

    Before you do a lot of work to eliminate this thermal bridge, it's probably worth assessing your assumption that 1.5 square feet of plywood thermal bridge over the whole house is significant. How many kWh/yr are you actually losing through this thermal bridge? I can't imagine it's very many. Probably no more than if the window itself was 3/4" larger in all directions.

    The only thing that jumps to mind as a problem with your idea is that you will have to come up with all the flashing details for your custom window buck, rather than being able to rely on a lot of time-tested details for flashing a window in a double-stud wall.

    Hopefully a double stud expert will jump in though!

  2. Stephen Sheehy | | #2

    I wouldn't worry about it. Your 16" window bucks have a better R value than any window you could install.

  3. Expert Member
    Dana Dorsett | | #3

    I dunno Stephen, pretty-good window is about R5, and if it's only 3" between the interior & exterior side of the window unit the buck is only good for about R3. If it's less than 3" it's less than R3. A code-min window in my area is better than R3.

  4. Charlie Sullivan | | #4

    The overall answer is that the effect is small and you shouldn't worry about it. But if you want to worry about it, because you'd enjoy making the extra effort, we shouldn't stand in the way of that.

    The thermal analysis of a situation like that is a little tricky to think through intuitively, because the heat flows in different directions in different parts of the assembly. Considering, for example, the bottom side, under the window, It turns out that the part that is responsible for most of the thermal bridging is the part right under the window frame, and maybe 1" into the inside space, and 1" beyond the window outdoors. The next 3-4" beyond that in the exterior doesn't really matter for thermal performance. Replacing just the exterior part with foam doesn't seem like the most likely place to make an improvement.

    So to figure out how much effect there is and how to improve it, you might need to use 2D thermal analysis software. There's free software from LBNL if you want to get into that:
    https://windows.lbl.gov/software/therm/therm.html

    A material that might be useful is the super-high-density foam from 475, Compacfoam. It's closer to the strength of wood than to the strength of foam, but closer to the R-value of foam than to that of wood.
    https://foursevenfive.com/compacfoam-primer/

    I look forward to the day that window manufacturers sell a complete kit to drop in a double-stud wall, eliminating fussing with such details on site, and with all the thermal modeling and optimization done ahead of time.

  5. Brendan Albano | | #5

    The biggest issue seems to be that the heat path through the plywood is only 3" or 4" long as Dana and Charlie have pointed out.

    As to the worth it question, how much energy do you lose at a 1.5 sq ft area of R-3 wall? That's like a single, tiny, code min (or slightly worse) window. You should be able to do a little bit of back of the envelope math to get a very rough sense of how much that matters.

    As far as wanting to fix it on principle, regardless of if it's "worth it":

    I wonder if simply lining the window buck with foam (maybe regular foam, maybe that fancy high strength foam Charlie posted) would be adequate. You would still have a plywood thermal bridge, but it would be a 16" long path (so R-16ish) rather than the 3"-4" path that is more worrying.

    You might need the special high strength foam to be able to secure your window through the foam. Since the high strength stuff is expensive, you really probably just need a strip of it around the window itself, and the rest of the buck can be lined with regular foam.

  6. Expert Member
    Malcolm Taylor | | #6

    A large part of what constitutes an experienced builder is knowing what is important. How tight your subfloor seems need to be, whether a connection is strong enough, if your concrete forms need more bracing etc. You can chase these tiny energy gains, but realistically they completely disappear when the occupants take two trips from the car with grocery bags rather than one, leave their PVR on standby, set their thermostat a degree higher, decide to turn a light on for their return from work in the winter, or a hundred other things. It simply isn't worth it.

  7. Joe Suhrada | | #7

    I am with a Malcolm as you don't want to compromise structural rigidity in this area. Sheetrock cracks might develop and any time your window connection to the house isn't strong, other trouble may follow. Find a section of north facing wall and fur it out another inch and a half to gain R6 of added cellulose or something if it will balance it off. Kind of like carbon credits- they don't mitigate the direct loss but they make up for elsewhere. I found a few spots where I could feasibly add another 2.5" of EPS foam to get a total of 5 of exterior foam. Under a porch on an inside corner where I didn't have a lot of weight hanging in the fur strips. It was just an added bonus for me to be able to do that to mitigate some other luxuries and weak spots. A friend of mine built with a single top plate, another foolish maneuver if you ask me. These are things that are just going a bit too far if you lose structural integrity.

  8. Expert Member
    Michael Maines | | #8

    To minimize thermal bridging at the bucks you can oversize the RO and use rigid insulation to over-insulate the buck, reducing the direct thermal bridge from the buck to the window. Depending on the window, this can also significantly improve the performance of the window frame, the weakest (thermally) part of the window.

  9. Expert Member
    Malcolm Taylor | | #9

    Michael,

    Do you mean lining the RO with foam and sitting the window on the foam? The three types windows I use all come with a standard 1 1/4" flange. Even without foam it is sometimes touch and go to get the nails to bite. then there is the finished casing, usually fasten with brads, which probably won't bite. It all sounds like a lot of work for a fairly small gain.

  10. Expert Member
    Michael Maines | | #10

    Malcolm, I mean a detail similar to this: http://www.foursevenfive.com/wp-content/uploads/old/2012/12/Post08_WindowDetail-Full.jpg. With European-style windows that have wide frames and no flanges you don't always need the 2x4 spacers around the window unit, but the concept works either way. If you haven't tried installing windows with clips instead of flanges, you should--no worrying about smashing the exterior window finish, and more precise installation using shims and screws. I agree, it is a small gain--appropriate for Passive House-level efficiency but probably overkill for most projects.

  11. Expert Member
    Malcolm Taylor | | #11

    Michael,

    I'm sure it's just me being a bit slow, but I don't see the advantage. Aren't you simply insulating a space that doesn't need to be there - that you created by over-sizing the RO? And the buck still ends up as a thermal bridge.

    I see the appeal of clipped windows for thick walls in that you can set them anywhere you want, but here in the PNW, where the primary concern is the ability to stop bulk water intrusion, flanged windows offer a much better starting point. They don't end up relying of a caulked joint for their integrity the way clipped windows do.

  12. Expert Member
    Michael Maines | | #12

    The clipped windows I use don't rely on caulk, but on tape, and when inset they are better protected against bulk water than anything installed at the outer wall plane. THERM modeling shows a noticeable difference between the non-insulated bucks and the insulated bucks. One difference from the image I linked to is that there should be a construction gap around the window filled with foam, so there is no thermal bridging from the buck to the window frame.

    Like I said, it's not at a level worth worrying about for a PGH, but there is a performance difference. I managed the process of getting the Ecocor panelized Passive House system through certification as the first certified building component outside of Europe, and this is a detail they required us to look at closely.

  13. Expert Member
    Malcolm Taylor | | #13

    Thanks Michael, I look at every deviation from standard construction practice through the lens of a PGH. I'm sure it makes sense once you get to Passive House or other very strict standards.

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