Most often used in reference to a coating for high-performance windows. The "E" stands for "emissivity," which is the degree of efficiency. A thin metallic oxide coating increases the U-value and/or decreases the SHGC of the window by reducing heat flow from a warm(er) surface to a cold(er) glazing surface. The best location for the coating should be calculated but is often based on whether the primary heat flow control direction is from the inside out (heating climates) or the outside in (cooling climates).