In the superwindows, argon gas, which is inexpensive, nontoxic, clear and odorless, fills up the cavity to increase thermal performance. Susan Douglas says the latest thing in window design is a cavity vacuum between the panes. A Bright Future. If you're seriously thinking of replacing your windows, the South Carolina Energy Office has some advice. Look for the highest R-value, of course, but also look at window type.
Casement windows are the most efficient as a general rule, with awning and sliding windows next.
Double-hung windows have the highest leakage rates, but still might be the most practical for you. Closely examine the window fit to make sure there are no leak-prone gaps. You can tell if air is leaking where the window frame meets the house by holding a lighted candle or lighter at the sash joint. If the flame flickers, there's a leak. All Rights Reserved.