Modern windows rely on a wide array of technologies to achieve a level of energy efficiency as high as five times that of traditional windows. All that technology can also be confusing. Terms like "low-E glass" and "argon gas" don't mean much to the average consumer. Any firm making a presentation to you about windows is likely to use dozens of such terms. Fortunately, there is a simpler way to evaluate different windows.

An organization called the National Fenestration Ratings Council (NFRC) has developed a rating system based on the U-factor. Most windows now carry this rating so it's becoming easy to make comparisons. You want the lowest U-factor possible. Here are some typical U-factor ratings:

Window Type U-factor

Old metal casement window 1.3
Good quality single-pane window 1.0
Good single-pane with storm window .6
Double-pane with low-E glass .4
Triple-pane with low-E glass .25

When you talk to different vendors, ask them about their window's U-factor rating. You can also look for the NFRC label. Use the U-factor to compare not only the window's energy efficiency, but its construction quality as well. It simply isn't possible for a manufacturer to achieve a low U-factor rating without using high quality materials and precision manufacturing techniques.