Choosing the right window system depends on how much of the window you need to replace to solve your existing problems. There are at least three different systems: sash replacements, sash replacements in a unitary frame and “prime windows,” or full window units with structural frames and sills like those used in new construction. If your old window frames are rotted and you see signs of water damage in the wall framing, you’ll probably want to go with prime windows (be sure to find the source of the leak and fix that first). If, on the other hand, the frames are sound and square, you may be able to get away with simply replacing the sash.
Replacement windows come in vinyl, aluminum, wood, clad wood or fiberglass. Each looks and performs differently, and cost differences among materials vary widely. As a general rule, the most expensive choice is