We live north of Buffalo NY in a 56 year old home. We are looking at replacement windows and recently checked out PACE Window (and Door). products http://www.pacewindows.com/
The rep came out and spent about 2+ hours discussing all aspects of their construction and insulating/non conductivity factors. Also, he told me they have a gas fill rate of 95+% which has a significant affect on the thermal efficiency. They are manufactured at thier own facility outside of Rochester NY.
Any input from you knowledgable folks?