You can check your window for the NFRC rating label. The NFRC (National Fenestration Rating Council) is an independent organization that rates the U-Factor for replacement windows. U-Factor is the amount of heat transferred through a material. The lower the U-Factor, the slower the rate of heat flow and the better the insulating quality. You can also check the Solar Heat Gain Coefficient (SHGC). This measures how well a window blocks heat from the sun. The lower the SHGC, the better a window is at blocking unwanted heat gain.