sash replacement vs full frame replacement?
I live in a 1912 house with lots of old, single pane wood windows. I'd like to replace at least some of them with something more efficient, yet in keeping with the character of the house. I've looked at full frame inserts from Andersen, Pella, Marvin, and Milgard (Ultra), all of which have wood interiors which I think would look fine. However, I'm concerned about the loss of visible glass. Looks like it's about 1-2" on each side.
So I decided to reconsider the Marvin Tilt-Pac (haven't seen other sash replacements?). I don't like the look of the vinyl rails one bit, but they would preserve the visible glass, and presumably would cost less despite not qualifying for the tax rebate (I think?).
My question is, how much less energy efficient are these? With a good install, are they comparable? Any other thoughts on inserts vs sash replacement?