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Thermal break for in-betweenie window box?

Peter Staecker | Posted in Energy Efficiency and Durability on

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:

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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  1. GBA Editor
    Martin Holladay | | #1

    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.

  2. Expert Member
    Michael Maines | | #2

    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.

  3. Peter Staecker | | #3


    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.

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