I am attempting to model several options for replacing my house’s windows and I’m getting slightly weird results.

This is the Albuquerque, NM area, which is a predominately a heating climate, but does have a cooling season, and gets lots of sun year-round. My cooling bills are very low since I use evaporative cooling, so I want to focus on reducing heating energy, leading me to want to maximize the free solar energy available in my sunny climate. However, I also want to alleviate the summer discomfort from the west windows overheating.

So I’m modeling putting high-SHGC windows on various facings. But RESFEN is giving me slightly odd results. When I model Pella 350 series triple-pane windows, the heating bills fall and the cooling bills rise the more high-SHGC windows I add, which is as I would expect. But for Marvin double-pane windows, the program is reporting the opposite: going with more high SHGC windows increases the heating bills while reducing the cooling bills!

This is the opposite of what I would imagine, and the opposite of what it reported for the triple pane windows. How can it be that with the double-pane windows, going from low SHGC to high inverts the results seen when doing so with triple pane windows? And how could the low SHGC double-pane Marvins possibly result in lower heating bills compared to the high-SHGC Pellas, which have better U-values and higher SHGC? I’m just confused.

Here are the full results I’m getting:


Current windows (estimated): U-0.83 SHGC-0.65

Pella triple pane high SHGC: U-0.23 SHGC-0.46
Pella triple pane low SHGC: U-0.18 SHGC-0.24

Marvin double high SHGC: U-0.31 SHGC-0.55
Marvin double low SHGC: U-0.3 SHGC-0.22

- Base case: $314 heating, $222 cooling, $536 total

- Pella triples, low SHGC on NEWS: $322 heating, $133 cooling $455 total
- Pella triples, low SHGC on NEW: $316 heating, $136 cooling, $452 total
- Pella triples, low SHGC on NW: $309 heating, $144 cooling, $453 total
- Pella triples, low SHGC on N: $299 heating, $156 cooling, $455 total

- Marvin doubles, low SHGC on NEWS: $246 heating, $200 cooling, $446 total
- Marvin doubles, low SHGC on NEW: $251 heating, $199 cooling, $450 total
- Marvin doubles, low SHGC on NW: $272 heating, $195 cooling, $467 total
- Marvin doubles, low SHGC on N: $306 heating, $187 cooling, $493 total


I’ve also attached screenshots of the two most extreme Marvin cases.