A note from Sam Rashkin: It’s the Window Stupid…

Written by Sam Rashkin, DOE

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.

For more information on energy efficient windows, visit the Efficient Windows Collaborative.