Retrofitting all the windows in a building from single-pane to double-glazed windows is an expensive project that is not often justifiable in terms of energy-conservation savings. The benefits, though, are evident in the relative U-values of different glazings. Storm windows are easy to install on one- or two-story buildings, at least for the worst summer or winter weather. They have an insulating value almost as good as triple-pane glass because a large air space is trapped between the glazings. In addition to reducing the thermal losses through windows, storm windows also minimize infiltration, which can be quite a significant energy burden in windy areas.
Other options are to replace windows with thermopanes, using the existing casings. As a last resort, a part or all of the window area can be blocked off with insulating panels or even a gypboard wall.
By the numbers: Single paned windows have an R-value of about 1; double-paned R-2; triple-paned R-3. Since heat transfer is directly proportional to the R-value, twice as much heat flows through a single-glazing than a double-glazing window. Basically, anything you can put in front of a window increases its R-value: blinds, curtains, storm windows. Notice that blinds tilted at 45 degrees give a double-pane window roughly the same R-value as storm windows; add both and you get R-4.
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Excerpt from Energy Conservation Projects.