Although most homeowners should by now be well aware of the need for adequate insulation in walls and attics, many do not realize that after inadequate insulation the greatest single source of wasted energy (heat loss in cold weather and heat gain in hot weather) is the typical window with only a single pane of glass.

The addition of snug-fitting storm [WORD ILLEGIBLE] will, of course, help to cut this loss considerably, but more and more homeowners, especially in older houses, have begun thinking about replacement windows - that is, about having completely new windows - that is, about having completely new windows installed in place of those presently existing.

The energy crisis, coupled with extension advertising by manufacturers and installers, has helped spark a tremendous increase in the sale of these windows - but it has also resulted in a great deal of confusion on the part of many homeowners who are not familiar with the different types available, as well as with what savings or benefits they can actually expect from an improvement of this kind.

Replacement windows come completely assembled, with the movable sash already installed in the frame, and with permanent weatherstripping already fitted. Since most of these windows are custom-made to fit the specific dimensions required, after the installer removes the old window, usually no on-site fitting or assembling is required, and in most cases the new window can be installed in an hour or less.

One of the principal advantages of these replacement windows is that insulating glass is used in practically all of them - either two separate sheets of glass separated by a dead air space and permanently sealed around the edges, or a factory made double pane that is welded together at the edges with a hollow space on the inside filled with an inert gas that insulates even better than an equivalent thickness of air.

Some of the newer windows go one step further - they use triple-pane glass instead of double-pane. In other words, they have three sheets of glass separated by dead air spaces, and are factory sealed around the edges.

Tests have shown that converting from single-pane to double-pane, either by installing new windows with insulating glass, or by adding storm sash to existing single-pane windows, will cut heat loss through the glass by almost 50 percent. Going to triple-pane glass will reduce the heat loss by about one-third more - to about 65 percent.

Replacement windows with insulating glass are made with frames of wood, aluminum of vinyl - and this is where the prospective purchaser faces the greatest number of conflicting claims from manufacturers.

Because energy conservation and reduction of heat loss are two of the principal factors most homeowners are interested in when purchasing replacement windows, the frame must be considered, as well as the glass itself.

Wood is highly favored by many because wood is a poor conductor of heat, so the frame is a better insulator than metal. Not only does this mean less heat loss, it also minimizes condensation problems on the inside of the frame - a common problem with some metal-frame windows. However, wood windows still need painting, unless a vinyl-clad type is chosen. These have a plastic coating on the outside. The inside may be finished as desired on some, while on others the inside is also vinyl-clad.

Aluminum windows formerly created annoying condensation problems because as the metal got chilled in the cold outside air condensation formed on the inside and dripped down onto sills and walls. Many aluminum windows still have this problem, but most of the newer models have a built-in thermal barrier or thermal break which largely prevents, or at least greatly minimizes, this problem. This barrier consists of a plastic [WORD ILLEGIBLE] in the framework that keeps heat from being conducted from the inside to the outside by eliminating metal-to-metal contact between the two sides.

Aluminum windows are available with either an anoldized, metallic color finish, or with a baked-on enamel coating that comes in a variety of colors. Some also come with a permanent plastic finish, which eliminates future finishing problems and is described as virtually condensation free.

Windows built with solid vinyl frames generally cost more than comparable ones of aluminum or wood, but they offer excellent insulating properties because the vinyl itself is a poor conductor, and the frames have dead air pockets inside to provide added insulation against heat loss. They are also virtually maintenance-free because the plastic is integrally colored and never needs painting, and their insulating qualities also eliminate practically all condensation problems.