U-VALUES & R-VALUES
Also known as the U-factor, the U-value measures how much heat loss or gain can be transferred through a window or door. The lower the U-value, the greater the product’s resistance to heat flow and the better its insulating value. U-value measurements generally fall between 0.20 and 1.20. For windows and doors, a low U-Value means less heat is able to penetrate your home during warmer months AND less heat is able to escape during colder months. The inverse (one divided by) the U-value is the R-value. The R-value measures the ability of a window or door to resist heat transfer. The higher the R-value, the greater resistance the window or door has to heat gain or loss. During warmer months, a high R-value means more unwanted heat from the sun is kept out, and in colder months, a high R-value means heated air stays inside where it belongs. Low E glass can have a positive effect on both U- and R-values.