Triple Glazing: Is it worth it? A tale of U-Values and G-Values. Essentially the energy efficiency of windows comes down to two measureable factors. First is U-Value. This refers to the heat that is lost through any substance that makes up the shell of your home, it’s a standard applied to walls and roofs, as well as windows. Secondly we have G-Value, also known as solar gain. This refers to the amount of heat that is absorbed through the windows when the sun hits them.