Double glazing best with glass or plastic

We have an older-type row house with sash-type windows. What features should we look for when purchasing storm windows? We've heard that plexiglass is far more energy efficient than glass. Do you recommend interior storm windows instead of, or in addition to, outside storm windows? Where can we get more information? John A. Strylowski Alexandria, Va.

Any single pane of glass -- or plexiglass, for that matter -- will transmit heat and cold through the glass.

Thus, double glazing is the way to go.

No matter what type of material you use -- glass, plexiglass, or plastic -- you should have at least two layers for it to do a really effective job in reducing energy costs.

"Otherwise," says an Owens-Corning spokesman, "you gain no appreciable increase in insulation quality."

If you have double-glazed outside storm windows, you really don't need anything inside the windows.

Besides storm windows, of course, there are energy-saving shades and drapes on the market as well as clear plastic films which are applied to the inside glass. They all are designed to decrease the heating bill in the winter and air-conditioning bill in the summer.

Why don't you get in touch with a window manufacturer, such as Andersen of Bayport, Minn., or Pella Windows & Doors, 100 Main Street, Pella, Iowa 50219. There are others. Check with your local supply house, hardware store, etc.

You might also get in touch with PPG I ndustries Inc., One Gateway Center, Pittsburgh, Pa. 15222.