Thermal break for in-betweenie window box?
Hi, Peter in zone 5 (Boston), considering a PGH design with double stud walls (2×3’s inboard, 2×4’s outboard; 2.0 SPF at the outside wall and cellulose for the remainder) with about 10″ of insulation space (shooting for R-40). (See case 10 of Bldg America report BA-0903: https://buildingscience.com/documents/bareports/ba-0903-building-america-special-research-project-high-r-walls/view).
I’m Looking for R-7 or better windows (mounted as in-betweenies) and interested in minimizing any thermal bridging due to the window box and mounting method shown in the figures of the referenced note. Ideas or construction methods that you can share would be appreciated.
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Replies
Peter,
If you are willing to switch to outies instead of in-betweenies, you could use the ThermalBuck approach.
More information here: An Alternative to Wood Window Bucks.
Peter, if you oversize your rough opening as shown on your GBA links, you can line the buck with Zip-R sheathing, as a substrate for exterior extension jambs and sill extension. Or just use rigid insulation without the OSB, but it's nice to have something to attach the trim to. The details will depend on your choice of window and trim. The Passive House term for this is "over-insulation," where the exterior insulation actually covers most of the beefy European-style window frames. The actual amount of money you will save is small, but there is a measurable benefit.
ADDITIONAL INFORMATION:
Here are two GBA pdfs showing in-betweenies: the window frame appears to be thermally isolated from the box, but the box itself is a thermal shunt from warm to cold.
https://www.greenbuildingadvisor.com/sites/default/files/Flanged%20window%20as%20an%20inbetweenie%20-%20head.pdf
https://www.greenbuildingadvisor.com/sites/default/files/Flanged%20window%20as%20an%20inbetweenie%20-%20sill.pdf
picking up this thread, maybe my situation isn't unique.... in my basement reno I’ve removed the original (mostly rotted) bucks and frames, and for now, i’m reusing the 1980s vinyl sliders that were in decent shape. I built 5/4 bucks pitched out, with a back damn that is liquid flashed to insert the sliders into. so far so good. i’m about to attach interior window boxes to the heated side of the back dam. These boxes span the thickness of the new interior insulated walls. but been thinning about the thermal bridge - maybe I should break the wood-to-wood connection with 1” of XPS? I've attached a drawing and some photos of where I am. thanks for your thoughts. I suppose another approach would be to make the interior boxes from Thermax. Anyone tried that?