The wavy glass in your home's old wood-clad windows stole your heart, but air leaks and high fuel prices are bleeding it now. A quarter of home heating costs, and up to 40 percent of cooling costs, literally fly out those windows. Good news is, you can improve the efficiency of your existing windows for a lot less money than putting in new ones. On your side may be old-growth-timber frames, found in most homes built before 1950. The wood is more durable and stable than today's plantation-grown stock. "You can't buy a new window that will last as long," says Dick Fritzer of in West Hartford, Connecticut, whose repairs have given many century-old windows another 100 years. You or your pro can significantly reduce air leakage in a double-hung by repairing sash damage, insulating weight pockets, and adding new weatherstripping. Add a storm window and you can match the efficiency of new double-pane units.