I recently came across an article discussing a Passivhaus build with an impressive drawing and description of the wall system the architect used to get an outstanding R-77 exterior wall.  WOW!! That’s an impressive feat! As I was reading through the comments a person brought up what windows to use in their future build because of a limited budget. Double 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!

The articles R-77 wall system beats the other wall system R-values 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.

Just so you know, U-value and R-value are mathematical reciprocals of each other so I convert the U-value of a window to R-value, it’s a simpler way to model wall systems. Double pane windows have an R value of 2-3 and triple pane windows have an R-value of 4-5. For the graph I used R-3&5 for the corresponding window units. If I used the lower R-value 2&4 for the windows, 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.

In the last couple of years several mid level and builder grade window lines now have a triple pane upgrade option.  The triple pane upgrade adds about 10-15% more to the cost of the window package over double pane windows. So, start with your windows and adjust the wall insulation upgrade to fit your budget and climate zone.

Your only as strong as your weakest link! Happy New Year!