The opposite measurement of the U-Factor, the R-value of a window also measures the insulating properties, specifically measuring how much heat is retained. The numbers of an R-value are directly opposite of the U-Factor. 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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