Tests have shown that windows having only a single pane of glass and no storm window account for a high percentage of heat lost during cold weather. Adding storm sashes will cut heat loss anywhere from 30 to 60 per cent, depending on outside temperature and the window area involved.

In spite of this, many people have not added storm windows because of the high cost of having the usual triple-track combination units installed, or because they live in a rented house or an apartment, where installing storm sash outside would be impractical.

Now, however, several companies manufacture inexpensive kits that enable anyone to install their own storm windows, using clear plastic instead of glass, and fitting them onto either inside or outside existing windows. In many cases these are just as efficient at reducing heat loss as professionally-installed aluminum-and-glass models installed outside, and in some cases they are even more efficient (plastic is a poorer conductor than glass). In addition, these kits simplify problems (and reduce the cost) of having storm windows fitted to extra-large or odd-sized windows as well as basement, porch, storm vestibule, mobile home and camper windows. Adding plastic storm windows inside, even on a house where storm windows are intalled outside, will reduce heat loss by 10 to 15 per cent, while reducing interior condensation problems during very cold weather.

Although they can be installed over any window, it should be noted that there is one drawback - the main window cannot be opened when the unit is in place. Unlike triple track or sliding storm windows, which permit you to simply slide them open after the prime window is opened, the plastic sheet would have to be removed first.

The simplest and least expensive form of do-it-yourself storm window consists of a clear piece of flexible vinyl film that is stapled or tacked around the outside of the window frame. The vinyl is sold by the yard, or in sheets of various sizes, and often comes in kits that include heavy cardboard or thin wood strips that form a "frame" around the plastic. These have to be replaced each year.

A great improvement over this simple use of plastic is a new kit introduced recently by the Mortell COmpany, Kankakee, Ill. 60901. The kit contains heavy weight clear polyethylene film, with extruded vinyl strips that form a permanently installed frame. As the drawing shows, a matching rubber spline fits snugly into the channel in the vinyl frame strip so that when forced into place above the plastic film, it grips the plastic firmly.

To install this storm window, first nail the plastic moulding with the rustproof nails provided. The clear plastic is spread over this by tacking it temporarily along the top, then pressed into the moulding grooves by forcing in the rubber spline on top. The excess plastic is then trimmed with a knife or razor blade.

At the end of winter the spline is pulled out and saved, but the plastic film is discarded and replaced next year. The same moulding and spline can be used to create screens during the summer - simply replace the clear film with a sheet of fiberglass screening. In the fall switch to the plastic to re-create a storm window.

Another type of plastic do-it-yourself storm window that includes a permanently mounted plastic frame or moulding is the In-Sider, made by Plaskolite, Inc., 1770 Joyce Avenue, Columbus, Ohio 43216. This kit contains a rigid, clear plastic styrene sheet instead of a plastic film and is re-usable year after year.

The vinyl moulding has a pressure-sensitive adhesive back so it may be installed against the window frame by merely pressing it into place. This moulding is hinged so it opens to accept the clear plastic sheet, and closed to hold it firmly in place. The mouldings (which can be painted) remain permanently in place. Each year the sheet is removed by simply opening the framing strip.

Because styrene is not suited to outdoor exposure, this storm window kit is intended only for inside installation. It costs a bit more than the first type, which uses a flexible film. A typical kit for windows up to 38 x 65 inches retails for approximately $14.85, while the Mortell kit for windows up to 38 x 58 inches is priced at $8.50.

In addition to these storm window kits, the Reynolds Metals Co., Richmond, Va. 23261, also markets aluminum mouldings for making your own permanent storm sash and screens. An illustrated brochure is available with detailed instructions for assembling and making these units. For screens you can use either plastic or aluminum screen wire, and for storm windows you can use either regular glass, or sheets of rigid clear plastic (acrylic or styrene), sold in many lumberyards and hardware stores.