Windows are just one part of a system that we call the envelope of your home. The envelope consists of walls, windows, doors, roof, and some type of flooring in the way of dirt, concrete, rock, or other foundation material.
Like every architect, I know the value of a well-placed window that provides scale, dimension, light, and style to a building. All human beings crave light and want to be able to see out from their homes. But replacing the double pane windows you have in your home isn't going to save you the most money, even if they were installed more then 20 years ago. The hardest thing about energy improvements is that the stuff that makes the biggest difference is always the stuff you can't see.
There are two types of heat loss. Air transported heat loss how drafty your house is and surface transported heat loss how well insulated your house is. Windows actually fall into both categories. Homeowners often tell me they want to replace their windows because they are drafty or old. Well, it may not be the window itself that is drafty, but the way it is installed. The biggest difference most homeowners see with a window replacement is in the installation.