Energy efficiency is key to new windows
The best way to reduce heat loss is to add layers of glass. As more panes are added, the R-value increases. Two layers, or double-glazing, cuts heat loss almost in half. Triple-glazing cuts heat loss by two-thirds, but it also reduces light transmission by 10 percent.
Another technique to increase energy efficiency is to coat the window. Windows equipped with low-emissivity (low-e) coatings allow visible light through, but block heat. That means heat has a much harder time escaping on cold days and entering on hot ones. Low-e coatings also screen out ultraviolet rays, which fade furniture. Some manufacturers make windows "tuned" to hot or cold climates. The basic difference is that hot-climate windows block more solar radiation to reduce cooling costs while cold-climate windows admit more solar radiation to lower heating costs. How effective the window is at holding or rejecting solar heat is measured as the solar heat gain coefficient. This number is shown on the NFRC label and ranges from 0.2 to 0.5. If you live in a warmer climate, seek a lower number; in the colder states, choose a higher number.
The insulating value of a low-e window can be improved about 20 percent by sealing low-conductivity argon or krypton gas between the panes. Krypton is more expensive but insulates better than argon. Its use allows the window assembly to be thinner. Don't worry, these inert gases occur naturally in the atmosphere. They are harmless if the window breaks in .
Finally, the seals around the frame should be tight or air will leak in. Look for units with a rating no higher than 0.2 cfm/ft (cubic feet of air leakage per minute per foot of window edge).
In most cases, you'll probably want to stick with wood frames. Wood is the traditional framing material. It is easily milled into any complex shape. From a thermal point of view, wood framed windows have a U-factor between 0.3 and 0.5, and high R-values. They're not affected by temperature extremes nor do they promote condensation. Wood's downside is that it requires regular painting. Window makers have responded to this by offering cladding. Cladding means wrapping the exterior portions of the window in aluminum or vinyl. That eliminates the need to paint. Cladding manufacturers can match virtually any color you choose. In energy terms, vinyl cladding is similar to wood while metal cladding slightly lowers the thermal performance of wood frames. In either case, look for seams that are fusion-welded, contain gaskets or use a lip to keep moisture out. You don't want water to get under the layer of cladding, because if it were to freeze, it would cause a rupture.