What is the difference between U-factor and R-value?

When you add insulation to your attic, you’ll want to know the R-value. It’s a measure of how well a product resists heat loss. The higher the R-value—in floors, walls and roofs—the more energy efficient it is. 

You also want your windows to be good insulators, but they insulate in a different way—so their insulation value is measured differently. The energy efficiency of windows is measured by U-factor, which tells you how well the window keeps the heat in your home from transferring to the outdoors. 

The lower a window’s U-factor, the less heat your home loses through it. A low U-factor indicates that a window is doing a good job of insulating your house.
 
Because Arizona winters are mild, most homeowners don’t worry too much about U-value when they buy windows. We’re more concerned with keeping the hot air outdoors than with keeping heated air indoors.
 
But if you want to qualify for a federal energy-efficiency tax credit for your new windows, you’ll need to buy not only an Energy Star product, but a model with an extremely low U-factor. 

You can claim the tax credit only on windows that have a U-factor of 0.30 or less and a solar heat gain coefficient of 0.30 or less. Solar heat gain coefficient measures how well the window blocks heat from the sun from getting indoors.
 
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