Thermotech fiberglass windows are a product I've mentioned before. Read Stephen Thwaites' Column for an excellent series of lectures on why to use fiberglass, why go with triple glazing, low e and other coatings, and the importance of tuning the windows so that you can get better passive solar gain here in the northern tier states and Canada. (Something that is just about impossible with most, if not all, major U.S. made windows) I don't think they offer double hung yet however. I like double hung windows even though they are inherently less efficient than other types because here in Vermont the windows are in some state of "open" six months out of twelve. Window design is not just about the coldest part of the year and double hungs offer some important advantages over casements or awnings in warmer weather.