A rating factor that measures how much heat is lost through a given window. The lower the U-value, the less heat is lost and the more insulating properties the window offers. The U-value is directly opposite the numbers of an R-value. A low U-factor equates to a high R-value, and both mean the window offers good insulating properties. (A high U-factor would mean a low R-value, and both indicate the window allows a lot of heat to escape the building.)

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