U-Values represent the amount of heat that escapes through a wall, window, roof or other surface. The lower the U-Value, the more energy efficient a material is. R-Values are the direct opposite. These measure an object’s resistance to heat flow. The higher a material’s R-Value, the lower its U-Value, and the less energy it will lose. An R-Value depends on the number of layers of glass in a window, what type of gas is between those layers, and whether one or more of those layers of glazing have been treated with a Low E coating.