At some point I really need to rip the walls down on my 2nd floor and insulate them. At the same time, it would be great to replace the single pane windows up there. Of course, I was thinking of using a highly insulated window. I hadn't been doing any actual research on this, but I stumbled upon this link in an advertisement over on EM today. Looks like a window place that can provide some pretty darn high r-value windows. Up to R11 if you don't need it to open, and 7.7 if you go with a casement type! Not too shabby.
Alpen high performance products
My question is, do you guys know of any other suppliers of windows I could look into?