Energy Star Climate Zones
The Energy Star rating system divides the United States into four different zones — Northern, North-Central, South-Central, and Southern. To determine your specific zone, see the official Energy Star government website. Once the zone is known, use the criteria listed on the website to find out what is a good Energy Star rating for windows in your area. For example, in Northern climates, a good U-Factor is less than .3, while the SHGC can vary. In the Southern zone, the U-Factor must be less than .6, and the SHGC, less than .27.
In cooler areas, the U-Factor and air leakage are the most important; in warmer areas, the SHGC and VT are most important. Although specific requirements vary with region, as a general rule, the lower the numbers, the better the window. All of the figures represent how much light, heat, energy, or air is able to move through or around the window. The models with the smallest figures are the most tightly constructed, well-coated, and thoroughly insulated, high performance windows.