Don't underestimate the power of windows in a high performance home
In a November 2014 update, Sam Rashkin, chief architect of the Building Technologies office of the U.S. Department of Energy, discussed the value of high-performance windows in a tight building envelope.
A note from Sam Rashkin:
It's the Window Stupid ...
I have personally delivered over 25 zero energy ready home (ZERH) training classes across the country. Consistently, one of the biggest "ah-hah" moments in the four-hour course is the huge impact windows have on overall wall assembly performance.
Even with just a 15% window-to-floor-area ratio, windows represent a giant thermal hole that disproportionately upsets all the good work you do on the insulated wall assemblies. Who knew?
Below is a table I developed that compares the overall R-Value of the entire wall assembly with various cavity insulation levels.
Assuming approximately an R-3 window (e.g., U-value = 0.30) representing 15% of the wall area, we can invest substantial cost to increase the wall cavity insulation from R-18 to R-39 with only a marginal increase in the overall wall assembly R-value (e.g., R-11 vs. R-15).
In other words, we've more than doubled the wall insulation at substantial cost and only realized about a 33% improvement in overall wall assembly R-value due to the impact of much lower R-value windows.
With these same assumptions, we can increase the R-18 insulated wall over 300% to R-60 and only get a 50% improvement in overall wall assembly R-value (e.g., R-11 vs. R-17).
Now look at the power of high-R windows. We get nearly the same overall wall assembly R-value with an R-10 window (e.g., U-value = 0.10) and R-18 insulated wall as an R-3 window and R-60 insulated wall (e.g., R-16 vs. R-17). Yes, windows are a really big deal! There is a desperate need for reasonably priced, high-R windows.
Watch Sam Rashkin discuss the new Zero Energy Ready Home program.
Read more about energy-efficient windows and doors.
Companies: U.S. Department of Energy