EcoSmart Stud 
The Wall That Started It All: R-77 Exterior Wall in a Passivhaus
I recently came across an article showcasing a Passivhaus project that featured a stunning R-77 exterior wall. WOW!!! That’s an impressive feat! But as I was reading through the comments, one reader posed a simple but significant question: “What windows to use in their future build because of a limited budget. Double pane windows or triple pane windows?”
I have written about this before in the article Weakest insulation link, the Windows. That article has costs of wall systems and corresponding R-values and how they compare.
I had just finished putting together a spreadsheet for builders to use to compare wall systems R-values and costs. So, I decided to check out how the R-77 (comp B) wall system with double pane windows would compare to a less insulated wall system (comp A) with triple pane windows (see graph). While I expected it would be close, I didn’t think it would be that close!
Modeling the Numbers: Double Pane vs Triple Pane Windows
TheR-77 wall system beat the lower R-wall by 100% in all wall segments. The overall effective wall R-value without windows is higher also. But as soon as 10% is inserted into the equation for windows things change dramatically! Now the effective R-values are very, very close for the two wall systems.
Why Window R-Value Matters in High Performance Wall Systems
The R-value and U-value of a window are reciprocal values. So for modeling purposes, I convert U-values to R-values. It gives a more direct comparison between wall and window performance. Here’s the reality:
- Double pane windows generally offer an R-value of 2 to 3
- Triple pane windows range from R-4 to R-5
In my graphing model, I used R-3 and R-5 for double pane windows and triple pane windows, respectively. But when I ran the model using R-2 and R-4, the lower R-value wall system (comp A) effective insulation with windows is now is actually higher than the R-77 (comp B) wall!
While this is an extreme example this does hold true for most high-performance wall system comparisons. Adding triple pane windows to the exterior wall assembly will beat out most any other wall system with double pane windows.
The Cost of Triple Pane Windows: Is It Worth It?
In recent years, many mid-range and builder-grade window lines have introduced triple pane window options. The triple pane upgrade adds about 10-15% more to the cost of the window package over double pane windows.
For builders focused on installing the best energy efficient windows, this is a small price to pay for a big step up in performance. And because triple pane windows significantly reduce thermal bridging, they complement any investment in high performance wall systems.
So, start with your windows and adjust the wall insulation upgrade to fit your budget and climate zone.
You’re Only as Strong as Your Weakest Link
The takeaway? You could have the most impressive high performance wall systems on the market, but if you pair them with underperforming windows, your efforts might fall flat.
As building standards rise and energy codes tighten, every detail matters. The next time you’re comparing double pane windows with triple pane windows, remember: your home’s performance depends on more than just its walls.
Happy New Year—and here’s to stronger windows and smarter builds!