Double glass is a mandatory these days for the window to be highly energy efficient. However, besides clear double glass will not do much good if it is not enhanced with gas fills such as argon or krypton, and high performance sealing spacers. Crucial ingredient of the quality insulated glass unit is Low-emissivity coating on the interior surface of the glass. Its job is to deflect most of the harmful UV-rays from the Sun, that not only damage your furniture, hardwood floors or carpets, but they also (most importantly) make interiors of your home warmer. This on the other hand leads to increased loads of cool air from your HVAC equipment and bigger energy bills in the summer. Keep in mind that during the hot long summer days, windows with LowE act as magnifying glass and make you uncomfortable. It just make sense that especially windows facing south are equipped with LowE glass.