The federal government recently reported that more than half the heat in the average house is lost through small openings that allow cold air to seep in and heat to leak out. Almost 80 percent of this loss occurs through windows.

Because most windows are built so they can be opened and closed, they cannot be effectively sealed against cold-air infiltration, even if weather stripping is installed around the movable sash. This does not imply that weather stripping is ineffective; it's just that no weather stripping that allows the window to be opened or closed can seal out cold air.

One way to cut down on heat loss through windows is to install snug-fitting storm windows. These can be of wood or metal, traditionally, installed on the outside, or the do-it-yourself type of plastic storm window that comes in a kit and is easily installed from the inside. (Some can be installed from the outside.)

Even in the case of homes that already have conventional storm windows on the outside, adding extra protection on the inside can save even more in terms of energy cost. This is true because far too many outside storm windows do not fit snugly and a certain amount of cold air still gets past them. The combination type that has sliding glass panels that can be interchanged with screens in the summer cannot be made really airtight because enought play is required in the tracks to permit the separate panels to slide easily up and down.

Inside storm windows are also the answer to eliminating cold drafts in apartment houses where installation of outside storm windows in just one apartment would be too expensive, or perhaps even forbidden by the landlord.

The simplest to install and least expensive plastic storm window is the type that uses a clear film of flexible plastic that is installed by taping it to the window frame on the inside. A few of these materials are designed so you can roll up the plastic and reuse it each year, but in many cases it is thrown away at the end of the winter and new material is bought the following year.

Although this type of disposable plastic storm window can be almost as effective as a more permanent type, it has three disadvantages for most homeowners: More people do not like the way it looks; once the plastic is installed you cannot open the window without taking it down and then replacing the tape with new material; you have to redo the whole job each year.

To overcome most of these problems and provide an even more efficient type of storm window, a number of companies have introduced kits that enable the homeowner or apartment dweller to put together storm windows that are rigid, more durable -- and semipermanent. These have rigid plastic or metal moldings that are cut to the lengths desired, then joined to form a frame that hold a sheet of rigid clear plastic or glass. They cost considerably more than the temporary ones of flexible plastic -- from $12 to $20 for a 24-inch by 36-inch window -- ut are easy to remove quickly when a window is to be opened.

Some are designed so the rigid plastic molding is fastened to the window frame permanently with a self-adhesive backing. The molding either comes apart in two sections or strips so you can take the plastic sheet out easily, or it is designed to snap open to permit removing the sheet of plastic glazing material. In either case, no tools are needed, and the clear plastic can be quickly replaced by repositioning it and snapping the molding back into place.

Included in this category is the Mr. In-Sider made by Plaskolite, Inc., 1770 Joyce Ave., Columbus, Ohio 43216; the Barrier made by K.S.H. Inc., 10091 Manchester Rd., St. Louis, Mo. 63122; and the Defender-1 made by Defender Energy Corp., Box L., Mahopac, N.Y. 10541. All are currently sold in hardware stores, department stores, home centers and plastic supply houses in many areas.

In addition to the type designed so the plastic channels or moldings remain permanently in place on the window frame, another type is designed so you can fit the moldings around a sheet of rigid plastic or glass (ordinary glass will break easily if one piece is used on a sizable window, so plastic is generally recommended). The assembled frame is then attached to the plastic or glass so you take the whole thing down in the summer, or when you want to open the window. To hold the completed assembly in place, special plastic clips or turn buttons are used. These swing out of the way to release the storm window so you can take it down when you wish (the clips remain in place).

Among the brands available in this category are Therma Frame, made by Elgar Products, Inc., 23945 Mercantile Rd., Beachwood, Ohio 44122; Mr. Window, made by Aluma-Trim, Inc., 918 Stanley Rd., Brooklyn, N.Y. 11208. Therma Frame has rigid moldings or channels of plastic, while Mr. Window has moldings or framing members of white aluminum. Both are assembled with plastic or glass, then fastened to the window frame with clips as described above. Like the other types already mentioned, these are sold through the same type of retail outlets in most communities.

All these companies make special moldings for ensuring a snug fit against the top of the window sill, and most can be painted.