When it comes to window efficiency there’s really two factors that matter:
#1 Window Frame Material (wood, vinyl, fiberglass, fibrex, aluminum)
The gold standard of glass for our hot/humid Austin TX climate has been Cardinal Low-E 366 glass. (This is the glass I have been specifying in my Marvin, Andersen, Gerkin, Fleetwood, and Eagle windows.) Most window manufacturers buy their glass from Cardinal but they might call this Low-E 366 by their own trade name. For instance, Andersen calls it their “Smart Sun” glass but it’s really Cardinal Low-E 366 in their windows.
Here’s the big news, Cardinal Glass has just introduced a new glass that gives triple pane performance using only two panes of glass! Their new Low-E i81 glass boasts a U-Factor of .20. Most Double Pane Low E 366 windows I’ve been installing have had U Factors of .24-.30 so these are potentially 50% better! I’m looking forward to getting this glass into my future houses. -Matt Risinger
Here’s a chart off the Cardinal Website showing different glass configurations and their U-Factors.