If you live in the Northern part of the U.S. and are considering purchasing new windows that have an ENERGY STAR certified U-factor of 0.27 or better, then those windows would meet the ‘prescriptive’ ENERGY STAR criteria for the Northern Zone as defined by this map.
You can also look up your home by county in this database:
Here is an overview of the whole set of criteria:
We have also calculated ‘equivalent energy performance’ criteria which should match the annual energy performance of the ‘prescriptive’ criteria in the Northern Zone. Assuming that consumers would typically use a moderate solar gain product, equivalent energy performance options can be calculated. By raising the U-factor by 0.01 and raising the SHGC by 0.05 you essentially get the same annual energy performance as the lower U-factor product in the Northern Zone.