The Clear Benefits of Energy-Efficient Windows

Fenestration--the design of windows and their placement on the walls and roof of a building--has been practiced by homeowners as well as architects for thousands of years. But it took the energy crises of the 1970s for Americans to realize that poor design and placement of windows were costing them a lot in inflated fuel bills.

There weren’t many good solutions to the problem until a new generation of energy-efficient windows was manufactured for residential use. Here is an overview of the types of windows available and placement alternatives:

* Double-pane insulated windows have been in widespread use since the mid-1950s, but many homeowners have failed to take advantage of them. Double-pane (double-glazed) windows have two sheets of glass, separated by an air space, hermetically sealed into a tightly bonded, single pane. The dead airspace between the glass insulates, preventing heat or air conditioning from escaping. This window’s U-value (a measure of heat flow through a material) is 0.52, compared with a 1.1-rating for a single-pane window. This means double-glazed windows lose about half as much heat as single-glazed, and double glazing greatly reduces condensation, or sweating, on the inside.

* Triple-pane windows refine the idea. They boast U-values ranging from 0.34 to 0.39, and offer a heat retention advantage of 25% to 35% over double glazing.

* Low-E--or low-emissivity--windows look like ordinary double-pane windows, but the inside surface of the outer pane has a microscopic metallic layer that reflects radiant heat, while allowing the visible portion of the solar spectrum through. Low E-glass has a U-value of 0.32, losing 42% less heat than a standard double-pane.

* For very warm climates, low-E glazing is available tinted and is known as high performance sun-insulating glass. Lower U-values and ability to filter out ultraviolet rays are the advantages.

* For even lower U-values, look to mirror glazing. These windows work like low-E windows, but the metallic coating is on a clear plastic film suspended between the double panes, giving a U-value of only 0.24.

As far as placement, in all except the warmest climates, large window openings should be placed on the southeast, south and southwest sides, according to the internal requirements of each space. This allows the greatest possible passive solar heat gain. Make east and west windows moderate in size. Keep northern windows small, and, if possible, have low-E glass. This takes full advantage of solar heat gain during the winter while providing indoor comfort and energy efficiency all year round.

In very hot climates, where air conditioning costs are the prime concern, the largest window area should face north, with lesser amounts to the east, west and south to minimize heat gain in summer.

To improve ventilation, some windows should open directly into the prevailing wind, others on the opposite side of the house. Large windows, representing at least 10% of the floor area in each room, work best. Height difference between inlets and outlets increases airflow.