Windows are designed to bring in natural light during the day, yet we cover them with curtains and blinds, and rely instead on electric lighting, which costs a lot more money than the sun charges for the same service.

There are some valid arguments for blocking sunlight. It does accelerate the fading of materials such as paint and cloth, and it increases the temperature of a home's interior on hot days.

On the other hand, modern window glass can let in lots of light without heat, and is much more energy-efficient.

Double-pane windows with low-e (emissivity) coating can reduce heating bills by 34 percent in cold climates, compared with uncoated, single-pane windows, says the Alliance to Save Energy, a Washington-based nonprofit advocacy group.

These low-e coatings are spectrally selective, letting in visible light but blocking radiant-heat losses to cut heating bills.

The Efficient Window Collaborative, a group of insulation and window manufacturers that comply with federal energy requirements, says the invisible gas filler in a double-pane window is critical to energy efficiency. Instead of plain air, high-efficiency models use argon or krypton gas, which conduct little heat and help the window's insulating properties.

Traditionally, the material used to create the separation between the two panes of glass, called a thermal break, was metal. New materials are better-insulating and make the overall window more efficient.

Window frames also are insulated for greater efficiency.

Security issues have altered the function of windows in urban areas because, though windows are the eyes on the world, the world can see in also.

Windows are locked, bolted and barred, especially in crime-ridden neighborhoods. Instead of being the "eyes of the house," windows are considered weak points.

But having to close them robs a home's occupants of such benefits as cross-ventilation, and increases the use and cost of air conditioning and fans.

All that aside, windows remain working components of building skins, and manufacturers have been quick to develop new technologies that improve visibility, security and comfort.

The consumer's main concern is getting the results of these new technologies properly installed.

Leakage around windows is so widespread, for example, that the American Society for Testing and Materials International has developed industry standards for installation.

Using these standards, the American Architectural Manufacturers Association has developed a training and registration program for window and door installers.

Among the things most home inspectors check for buyers are the windows. The main purpose is to see whether those windows operate as they were designed.

Sometimes, windows in new construction have been installed too tightly, so they pinch the screens, which won't open and close easily.

In older houses, inspectors usually find that the top sashes of the windows have been painted shut because people rarely move the sashes up and down.

Windows in old or historic houses tend to operate surprisingly well. A lot of old-house owners like living in them, and they have maintained the windows.

According to the U.S. Department of Energy, reducing heat loss or gain in homes often includes either improving or replacing windows.

Among the low-cost options available for improvement are caulking, weather-stripping, retrofit window films and window treatments. Replacing windows will involve the purchase of new materials, which should adhere to certain energy-efficiency standards.

Different combinations of frame style, frame material, and glazing can yield very different results when weighing energy efficiency and cost, according to the energy department.

For example, a fixed-pane window is the most airtight and the least expensive; a window with a wood frame is likely to have less conductive heat loss than one with an aluminum frame; double-pane, low-e window units are just as efficient as triple-pane untreated windows, but cost and weigh less.

No single window is suitable for every application. Many types of windows and window films are available that serve different purposes. Moreover, you may discover that you need two kinds of windows for your home because of the directions that your windows face and your local climate.

To make wise purchases, the energy department suggests that homeowners first examine their heating and cooling needs and establish a list of priorities for desired features, including day lighting, solar heating, shading, ventilation, and aesthetic value.

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