Thermal breaks for window bucks in double stud walls
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.
我的系统是否有缺陷？
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.
GBA Detail Library
由气候和房屋部分组织的一千个建筑细节的集合
答复
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!
我不会担心。您的16英寸窗口雄鹿的R值比您可以安装的任何窗口都更好。
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.
总体答案是效果很小，您不必担心。但是，如果您想担心它，因为您喜欢付出额外的努力，我们不应该妨碍这种努力。
对这种情况的热分析要直观地思考，因为热量在组装的不同部分的不同方向上流动。例如，考虑到窗户下方的底部，事实证明，负责大多数热桥的部分是窗框下方的部分，也许是1英寸“进入内部空间，1”以外窗户户外。下一个3-4“在外观中，对于热性能并不重要。仅用泡沫代替外部似乎是最有可能改善的地方。
因此，要弄清楚有多少效果以及如何改进它，您可能需要使用2D热分析软件。如果您想了解LBNL，则有来自LBNL的免费软件：
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.
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":
我想知道是否简单地用泡沫将窗户放在窗户上（也许是常规的泡沫，也许是奇特的高强度泡沫Charlie发布的）是否足够了。您仍然会有一座胶合板的热桥，但这将是一条长16英寸的路径（SO R-16ish），而不是3“ -4”路径，更令人担忧。
您可能需要特殊的高强度泡沫才能通过泡沫固定窗户。由于高强度的东西很昂贵，因此您实际上可能只需要在窗户本身周围的一条条带，而其余的钱可以用普通的泡沫衬里。
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.
我与马尔科姆（Malcolm）在一起，因为您不想妥协该领域的结构刚度。Sheetrock裂缝可能会发展出来，任何时候您与房屋的窗户连接都不强烈，其他麻烦也可能随之而来。找到一部分北面墙壁，然后将其毛皮擦掉一英寸半，以获得R6添加的纤维素或其他东西，如果它可以平衡它。有点像碳信贷 - 它们不会减轻直接损失，但它们可以弥补其他地方。我找到了一些景点，可以在其中可行地添加2.5英寸的EPS泡沫，以获得5个外部泡沫。在内角的门廊下，我没有在皮草条上悬挂很多重量。我只是为了减轻其他奢侈品和弱点而做到这一点的额外好处。如果您失去结构完整性，就太远了。
为了最大程度地减少雄鹿的热桥，您可以超大RO并使用刚性绝缘材料来弥补降压，从而将直接的热桥从雄鹿到窗户减少。根据窗口的不同，这也可以显着改善窗框的性能，即窗口的最弱（热）部分。
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.
Malcolm, I mean a detail similar to this:http://www.foursevenfive.com/wp-content/uploads/old/2012/12/post08_windowdetail-full.jpg。与欧式窗户宽框架s 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.
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.
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.
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.