Large windows of 20 square feet – say 4 feet wide and 5 feet tall – will have an annual heat-loss equivalent to the consumption of about 30 gallons of heating oil. With a storm window this heat loss decreases to 15 gallons per year. If a plastic third window is added, the heat loss decreases to 10 gallons. When such triple windows are installed, the best window should be on the warm side.

If a tight plastic window is put over the outside of a leak-prone inside window, condensation will form on the plastic and a subsequent wet windowsill will cause paint to peel. The resistance to heat flow – the insulation effect – has almost nothing to do with the thickness of the glass or plastic. All the resistance is caused by the thin film of air that clings to the surfaces. The spacing between the layers of glass or plastic should be at least one-half inch. Drapes or curtains tend to thicken that air film and increase the insulation effect of the overall system.