* Solar Heat Gain Coefficient (SHGC). The fraction of solar radiation, or short-wave infrared rays, admitted through a window or skylight. The lower a window's SHGC, the less solar heat it transmits.

* Low-E. Microscopically thin, virtually invisible metal or metallic oxide layers deposited on the glass in a window or skylight. Low-E coatings reduce the heat transfer between the layers of glass.

* Low Solar Heat Gain Low-E Coatings. These are low-E coatings that let the visible daylight in and reflect the harmful ultraviolet and infrared out. Because these coatings have low emissivity, they provide an added layer of insulation as well.

* Visible Light Transmittance (VT). The percentage of visible light that comes through the window.

* Argon Gas. An inert, non-toxic gas used to insulate windows in order to reduce heat transfer.

To discern which windows are appropriate for your facility and geography, look for ENERGY STAR[R] windows and review the product's NFRC (National Fenestration Rating Council) label. This will indicate the window's performance capabilities by revealing U-Factor, SHGC, VT, and Air Leakage ratings. Where your building is located (i.e. the climate zone) will influence the ratings you seek. For example, a facility in northern Minnesota should have windows with a U-Factor of 0.35 or less, whereas a building in the Florida Keys requires a U-Factor of 0.65 or less.

Double-pane (or double-glazed) windows feature an air space, which serves as insulation, between the layers of glass and offers excellent energy performance. When paired with other desired attributes, these windows offer very realizable energy savings. "Energy-efficient windows with low-E coating can reduce heating bills by 34 percent in cold climates compared to uncoated, single-pane windows. In hot climates, spectrally selective low-E windows can cut cooling costs by 38 percent," explains the Alliance to Save Energy.

MODERNIZING and MAINTAINING

Replacing inefficient windows can be extremely costly, time-consuming, and a real inconvenience to building occupants. If your budget or your tenants are not in favor of new windows, consider film. "Window films are designed to--depending on which product you pick--stop anywhere from 30 percent up to 83 percent of the total solar energy that strikes the window," says Darrell Smith, executive director, Intl. Window Film Association (IWFA), Martinsville, VA. The use of window film results in an immediate decrease in the cooling load; as an additional benefit, many utility companies provide rebates and incentives to help pay for the retrofit.

Keeping your energy-efficiency goals in mind, look at the film's shading coefficient, defined by IWFA as the ratio of solar heat gain passing through a glazing system to the solar heat gain that occurs under the same conditions if the window were made of clear, unshaded window glass. The lower the number, the better. "An average window film would have a shading coefficient in the neighborhood of 0.45 to 0.55," says Smith. In many applications, window film can help you meet the U-Factor and Solar Heat Gain Coefficient ratings mandated by current building codes--and sometimes can even exceed the building code requirements for new windows.