ThermalBuck experience? Window thermal break necessary for condensation control?
With all the emphasis on exterior insulation to reduce thermal bridging, what is everyone doing to thermally break the window frame from the framing? With the high cost of windows, are we missing out on gains by not taking this extra step?
I saw some older posts on Thermal Buck- the product seems fairly easy to integrate into the rough opening, epecially if using a liquid flashing. With a liquid fluid applied like Prosoco or Zip, couldn’t you just flash over the entire foam buck to tie in the sheathing WRB to the sill and jambs? Coating the entire Thermal Buck after it was nailed into the rough opening?
Or maybe with modern high performance windows this is not a concern as they provide enough insulation in the frames?
Appreciate any input from you all! Searched through the archives but couldn’t find much besides wood bucks for exterior insulation assemblies.
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westy123,
I will try to keep this short.
Take an average window size of 32x48 with a whole window R value of R-6, a readily achievable value among high performance windows. The total s.f. area is approx. 10.6 sf.
The rough opening might be a full half inch larger than the window frame, so that plus one 2x frame element makes 2" of width to be concerned with thermally per window. So for a 32x48 window, a total of 332 sq. in. or 2.3 sf of area. Roughly 20% of the window area.
The 1/2 gap can be stuffed or foamed to R-6 pretty easily and the R value of a 2x6 fir stud is approx. R-6.5. So one choice you have is simply increasing the thermal area for the window by 20 percent when calculating for losses. The other choice is spending a great deal of time and effort trying to raise the R value of that 2.3 sf.
You can run your own numbers to see which is a more efficient use of time and money by looking at the energy value gained by bumping a U 0.18 window to a U 0.16 or 0.15 Ignoring the wall choices, another ball of wax.
For cold climates it more useful to get better windows over fussing with the openings.
Heat transfer works like resistors in parallel, not in a series. Christine Williamson says it really well in this video (corresponding transcript is right below it):
https://acelabusa.com/articles/building-science-fight-club-aia-courses/why-glazing-matters-so-much-more-than-insulation
If the frame makes up even ~20% of the overall installed window U-value, it is definitely worth improving. And it probably does since it is such a lower performing part of the window than the glass. One note though, ThermalBuck is meant for flanged windows. It won’t work with the European attachment methods.