"Stop throwing money out your old windows," warn some manufacturers' commercials. While new windows can reduce your energy bill, don't expect to recoup your investment anytime soon.

Replacement windows can save you between 10 and 25 percent per year on heating and cooling if you have single-paned windows. But they cost between $7,000 and $20,000 for an average house. Custom sizes can add about 15 percent. So new windows probably won't save enough energy for you to pocket any net savings for 20 years or more. But they can make your home more comfortable, quiet, and attractive.

Finding an energy-efficient replacement window has become easier, as insulating features such as heat-reflecting, low-E coatings and argon gas between glass panes have become standard on many lines. You'll also find many new options and extras, including fiberglass window frames.

We tested 19 replacement windows for air and water leakage, durability, and convenience, with the aid of an outside lab. Months of testing found significant differences between replacement window brands and types, plus a spot-reducing coating that wasn't a clear winner (see The great unwashed?).

Wood and fiberglass are top scorers. Typically more expensive than vinyl, the clad-wood and fiberglass windows we tested from major brands (Ratings and information are available to ConsumerReports.org subscribers only) excelled at keeping out cold air and rain when new. We subjected each window to a week's worth of extreme temperature swings that force window components to flex, expand, and contract. Then we tested again for air and water leakage. Models with excellent or very good durability showed no change or little change in performance, respectively. Only two models as air- or water-tight as before.

Vinyl is inexpensive and convenient. It makes up about half of the replacement window market mostly because it's relatively inexpensive and maintenance-free. But vinyl replacement windows tend to leak air a bit more in cold climates. Vinyl also doesn't have the same visual appeal as wood, and it can't be painted or stained. So it might not be appropriate for older homes.

More companies offer installation. Andersen, Marvin, and Pella recommend installers specifically trained on their products. Andersen's Renewal brand and Marvin's Infinity line are sold only to authorized installers. Both of those replacement windows performed very well in our tests, but we didn't include them in our Ratings (available to ConsumerReports.org subscribers only) because we couldn't buy them anonymously, as is our policy.

You might want to go with a manufacturer-recommended installer anyway. Readers who did were more satisfied overall than those who used contractors employed by or recommended by Home Depot or Lowe's, according to our recent Home Improvements Survey. For more tips on hiring an installer, see Finding an installer.