Replacing old drafty windows with modern windows will saves energy, but the cost will not likely justify the energy savings. Break even will only occur after 20 to 30 years. Beware of claims such as 40% savings on your energy bills. Realistically, you may save 10% to 20%. If saving money is your only goal, consider weather stripping and repairing the windows you have.
U-Factor The National Fenestration Rating Council (NFRC) has developed a standardized rating system call the “U-factor” which provides a single number with which to compare windows. The U-factor is a number between 0 to 1: the lower the number, the better. 0.35 is good. In cold climates, the U-factor is the most important factor for selecting a window.
Solar Heat Gain Coefficient (SHGC) In climates where air conditioning is more important than heating, the SHGC is the most important factor for choosing a window. The SHGC represents how much heat from the sun penetrates the window. The SHGC is a number between 0 to 1. For air conditioning climates, a number less than 0.4 is good. For heating climates, a larger number, such as 0.6, is better.
Some manufacturers put argon gas, a better insulator than air, between the panes, resulting in a more efficient window. Most experts agree that the argon does not last forever.
Coatings or films can dramatically improve the efficiency of a window. In a heating climate, low-E glass allows short wave solar radiation into the home for a heat gain, and prevents heat loss by reflecting the longer wave heat from inside your house back into the room. In hot climates, the window can be coated or tinted to reduce heat gain from the sun. Ask a home inspector, or another impartial professional, whether you need to upgrade your windows. A window salesperson will likely give you only one answer: yes.