If you’re trying to lower your energy bills, you have probably plugged many of your home’s air leaks and have added insulation to your attic floor. Now you may be wondering, “What should we do about our old windows?”

Unfortunately, there is no clear answer to this question. Sometimes it makes sense to leave old windows exactly the way they are. Sometimes it makes sense to repair the windows’ weatherstripping and add storm windows. And sometimes it makes sense to replace old windows with new energy-efficient windows.

Before providing a framework to help you decide what to do, it’s important to address a few basic questions about window replacement.

Here’s the good news: if you install new energy-efficient windows, your energy bills will go down.

Here’s the bad news: your bills won’t go down as much as the window salesman promised. In fact, your new replacement windows will save you so little money on your energy bills that the payback period for this investment may be more than 100 years — far longer than the new windows are likely to last.

Of all the data-crunchers who have looked closely at energy savings attributable to window replacement, none are more credible than Michael Blasnik, a Boston-based energy consultant with access to utility bill data for millions of U.S. homes. “I’ve looked at a lot of window replacement data,” Blasnik explained at the Building Energy 12 conference in Boston. “I’ve heard window salespeople say that you can save 50% on your heating bills if you replace all your windows. In fact, the amount of energy saved by replacing all of the windows in a home is generally on the order of 1% to 4% of the heating energy usage.”

Exaggerated marketing claims by companies selling replacement windows have exasperated…