CHANCES are a lot of your hard-earned heat is going right out the window -- literally. Metal door and window frames are among the biggest heat thieves. A few inexpensive precautions can help prevent them from robbing you of energy this winter.

Hallow metal storm doors and windows and sliding glass patio doors with metal frames make direct contact with the bitter winter cold, bringing cold right into the house. These hallow metal frames, although less expensive than wood-framed doors or windows or metal frames with a "thermal barrier," may prove more expensive in the long run.

When the chilled metal surface meets the warm, humidified air inside the house, condensation occurs, and the door or window frame "sweats" as a result. As the temperature drops, the "sweat" may turn to a layer of frost or ice, and can reach a quarter-inch or more in thickness, inside the house. Ice buildup within the frame is intensified in the presence of home humidifiers. The area around the window or door is subsequently cooled, and heat is lost from inside to the outside of the house.

A patio door insulation kit can help solve that problem. Strips of thick foam-cup insulation are applied directly to the problem area -- the metal itself. Each kit contains 38 feet of the insulating material and a squeeze bottle of adhesive. This should be enough to cover the frosty metal on two large patio doors or several small windows.

Instructions are included with the kit. Time involved in insulating a door is about 15 minutes. The foam strips, made of foamed plastic similar to the kind used in hot-drink cups, are 1 7/8-inch wide, which will protect most standard metal frames. The strips have a simulated teakwood grain on the one side, a neutral tan color on the other side, which may be painted to match trim with a latex paint. Other materials may dissolve the plastic. A water-based casein adhesive is furnished, insuring adhesion even in the presence of moisture. The adhesive strips are removable in the summer with a commercial solvent.

The kits are available from the Dalmor Corporation, 701 E. Irving Park Road, Roselle, Ill. 60172. The price, $12.95, includes cost of handling and shipping of each kit.

Hechinger's and Scott's Lumber and Garden Supplies carry a self-adhering foam tape in several widths that mediates between the cold metal and warm air and blocks out cold drafts. Polyfelt weatherstripping can be purchased in rolls at most hardware stores and applied around window and door frames to prevent ice buildup. Polyethylene-coated tape is waterproof and also helps keep out the chill and ice.

A more permanent, but far more complicated solution is to use strips of wood to sheathe metal patio doors. Measure carefully and fit the wood snugly against the metal for insulation.

The ultimate solution is to replace your uninsulated metal doors and windows with the newer kind which have thermal stops, insulation between outside and inside metal, or wooden-framed windows and doors.

Many people pay most of their insulating attention to windows, often overlooking what may be the biggest heat-loss culprit -- the doors. "Everybody concentrates on windows. You come in and close the door and forget about it. Nobody sits next to a door. You come in and sit next to a window, so you feel the heat loss," said Jack McGrath of Hechinger's.

"You lose most of your heat around doors. That's how I found out I had a problem with heat loss -- standing barefoot by our back door. The molding can be improperly fitted, sides of doors uneven; maybe proper molding wasn't put up. A felt strip can be placed around doorjambs to seal out drafts. If the door wasn't properly fitted and sealed, felt stripping can be applied to the doorjamb itself so when the door closes, it's flush." McGrath said.

Storm windows are installed by many people to prevent heat loss, but if not done properly, they can spirit away much of the heat saved. "Metal storm windows, if not caulked properly, are defeating what you wanted to do," McGrath said. "A good storm is necessary. A lot of people don't want to go to the time and expense of installing storm windows. A lot of people try to shortcut."

In the event that those sneaky drafts evade your most intensive insulation efforts, you might try a simple and decorative trick of the English Victorian period. A long, sausage-shaped object made of fabric and stuffed with sawdust can be put along the joint between the upper and lower window sash or along the sill and on the floor in front of doors. These draft excluders were traditionally made of red material. You can sew them from any fabric remnant. If sawdust is not easily available, a heavier filler, such as beans or shredded foam rubber can be used. Aquarium gravel will work well and will resist water from condensation on window and door frames.

They are also available commercially in many drug and department stores. Called "Draft-Dodgers," they are sewn to resemble animals and go for about $4.95.