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.


Quite simply, the SHGC value indicates how much direct sunlight and absorbed heat that can enter your home through your windows or doors. Sometimes referred to as a shading coefficient and usually expressed as a value between 0 and 0.87, a lower SHGC value is preferred. Just like U- and R-values, Low E glass will increase your window or door’s performance against solar heat gain.


Windows and Doors with the Energy Star® label indicate that the product meets Energy Star® guidelines. Energy Star® is a program run by the U.S. Department of Energy and the EPA. The Energy Star® label can be affixed only to windows and doors that meet specific U-value and shading coefficient ratings for one or more regions of the country. These ratings are achieved through independent testing done to NFRC (National Fenestration Ratings Council) standards. The ratings vary due to the the window or door style, the type of glass and the overall product design. Wincore® is proud to be an Energy Star® partner.