It appears that some of the best efficiency is in the vinyl replacement types. I also understand that there are few wood replacement windows on the market, Pella being perhaps the best known. We put Simonton Impressions vinyl (low E, argon filled, "super spacer" glass spacing) in our 1947 brick story and 1/2 and have been very pleased. They were considerably more efficient than the Pella that this contractor also sells (and WAY more than the steel frame casements they replaced!).