Windows are the weakest link in a well-insulated home, with a square yard of conventional single-pane glass exposed to direct sun on a hot day generating as much heat as an electric space heater.  On a cold day, the same glass will lose more heat than the same area of insulated wall.  Double-glazed windows, using two sheets of glass with air or gas sealed between them, are up to twice as expensive but also up to twice as efficient.  Use an outer pane that will block unwanted solar radiation and an inner pane that will reduce heat loss from inside.  Consider replacing your single-pane windows with low-e-coated or Energy Star windows.  If that is not feasible, simply using storm windows can reduce your winter heating costs by 25-50 percent.  Energy-efficient windows can lower your heating and cooling costs by up to 35 percent each year, and substantial tax credits are available.

Keeping heat in (or out): Understanding U & R Values

Windows lose and gain heat by conduction, convection, radiation and air leakage. This heat transfer is expressed with U-values, or U-factors. U-values are the mathematical inverse of R-values. So an R-value of 2 equals a U-value of 1/2, or 0.5. Unlike R-values, lower U-value indicates higher insulating value.

Conduction is the movement of heat through a solid material. Touch a hot skillet, and you feel heat conducted from the stove through the pan. Heat flows through a window much the same way. With a less conductive material, you impede heat flow. Multiple-glazed windows trap low-conductance gas such as argon between panes of glass. Thermally resistant edge spacers and window frames reduce conduction, too.