Many tips for saving home energy that are doled out this time of year cost more money than they will ever be worth in heat savings -- replacing all the windows in your house, for example. But adding a storm door may be money well spent.

A storm door goes on the outside of your regular entry door, saving energy in two major ways. It creates a first barrier to weather, reducing the effect of air leaks from the primary door. It also reduces heat conduction through the existing door by creating an insulating air pocket.

And many storm door windows can be replaced with insect screens in summer, allowing better air flow and potentially limiting the need for air conditioning.

A storm door also protects the primary door, reducing maintenance costs, and may lend an added level of security against break-ins.

"There's a nice payback, as well as other benefits from a storm door besides the energy savings," said Bruce Thomas, product development manager at Larson Manufacturing, a maker of storm doors.

The storm door industry sells an estimated 5.2 million doors a year, Thomas said. At an average price of $140 each, that puts the industry at nearly $730 million.

But storm doors are not for everyone. From strictly a money-saving view, the heat loss is the foremost factor. And it is only a good investment if your primary door is old but still in decent condition, according to federal authorities. Adding a storm door to a newer, insulated, foam-core door is not usually worth the expense because you will not save much more energy.

Storm doors typically cost $100 to $300. Below are other considerations when buying a storm door:

· Break-even period. It's difficult to estimate how much money you'll save. The storm door will pay for itself more quickly if you use a less expensive, but quality, storm door. Paying more for a stylish door is fine but it lengthens your break-even time. "Adding a storm door that costs about $200 or less is generally a good investment if your existing door is old, but still in good condition," says the Department of Housing and Urban Development.

· Installation. Consumers handy with a screwdriver and drill should be able to install a storm door in two to four hours as a weekend project, Thomas said. Professional installation might run $130 to $150, which could double the cost of the door and will eat into your savings.

· Alternative for replacement. Adding a storm door is far cheaper than replacing your primary door, whether you're doing it for energy savings, looks or added security.

· Back doors. "The same theory applies to any door opening in the house," Thomas said. "Most folks, if they have a storm door on one opening, they have them on all."

· Features. Some doors have useful self-storing pockets for the glass in summer. Doors with exposed wood will require more maintenance and painting, so it might be worthwhile to choose a door with a weather-resistant outer layer.

· Sunlight. Never add a full-view glass storm door if the exterior door gets more than a few hours of direct sun each day, unless the storm door has vents. The glass will trap too much heat against the entry door and could damage it, according to the Energy Department.

· Now is a good time. Uncle Sam will help pay for certified storm doors installed this year and next. U.S. consumers are eligible for a one-time tax credit of up to $500 for installing qualifying storm doors and other energy-efficiency home improvements on their primary home. The credit will pay for 10 percent of the door's cost, but not installation.

· Patio openings. Storm doors for patio doors are hard to find and adding one to energy-efficient patio doors is seldom worthwhile, the Energy Department said. Insulated drapes, when closed for the night in the winter -- or on sunny days in the summer -- are a better idea.

· Disabled access. Automatic door closers and hold-open features are pretty standard on storm doors, but consumers with wheelchairs, walkers and other mobility aids might pay special attention to the usability of these features.