The goal of this work is to better understand the influence of window U-factor and solar heat gain coefficient on residential space heating and cooling energy use in the United States. We calibrated our simulation models with residential energy use data and evaluated the affect of window U-factor and solar heat gain coefficient on space heating and cooling energy use. U-factor and solar heat gain coefficient have a comparable impact on heating energy use, whereas U-factor has a minor impact and solar heat gain coefficient has a strong impact on cooling energy use. Homes in Madison, Wisconsin, and Baltimore, Maryland, that have mechanical heating and cooling show ·that total annual energy cost savings are comparable with the different types of low-e glazing that are on the market today for the same frame type. For homes without air conditioning, the low-e windows with a higher solar heat gain coefficient provide greater savings. In cooling-dominated climates, like Phoenix, Arizona, and Miami, Florida, windows with a low solar heat gain coefficient offer the greatest energy savings. More work is required to obtain better agreement between simulation models and actual cooling energy use, especially for dry, sunny climates.