Replace single-pane windows with double panes.

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Double-Pane Windows
No Pane, No Gain
Replacing single-pane windows with energy-efficient double panes can reduce your heating and cooling needs by close to one-third. But because each window can cost a budget-busting few hundred dollars, you may want to replace the windows one side of your house at a time. Or if that's too much to take on right now and you live in a cold climate, consider installing less expensive storm windows. Give your doors the once-over, too; an air-trapping door sweep costs about $15, is easy to install, and can reduce the air that escapes through the gap between the bottom of your door and the floor.