Actually, “insulated” glass is a misnomer, but it is often used to describe energy-efficient windows. What the term means is that there are two or more panes of glass separated by insulation at the edges and air or gas in the center. The result is greater thermal efficiency.
Air infiltration is the amount of air leaking into a building through any kind of opening. The amount of cold or hot, humid air entering the house determines how hard the heating or cooling system has to work, and windows and doors are a prime source of air infiltration. Look for a low rate of air infiltration when you choose your windows and doors.
Modern window technology seals an inert gas (usually argon) between the panes of glass. The gas is a far better insulator than plain air, so it further increases the window’s thermal value.
Low-E stands for low emittance. Simply put, the glass is coated with a virtually clear material that acts to cut the transmission of ultraviolet rays from the outside to inside. Low-E glass improves the thermal efficiency of the window, cuts glare and prevents damage that ultra-violet rays can cause to interior furnishings. If you are in a climate that has long, hot summers, Low-E windows can make a Big difference in your air-conditioning bill, too.
Essentially R-value and U-value are two sides of the same coin. The term R-value usually refers to such things as wall and ceiling insulation value, but it does not translate well to windows. So the window industry uses U-values to measure the rate of non-solar heat loss or gain instead. A low U-value indicates that the window does a good job of keeping out the heat and cold.
A. The SHGC indicates how much solar radiation enters a building through a window and is expressed as a number between 0 an 1. The lower the SHGC, the less solar heat it transmits. SHGC can refer to the entire window, or to only the glass in it.