Efficient windows save energy, add comfort

For many years, windows were basically one sheet of plain glass between you and the outside world. They were simple, but they weren't very energy efficient. If fact, they were often one of your home's biggest energy wasters. If you're building a new home or replacing older windows, you now have a variety of choices. Building codes for new houses now require double-glazed windows, but additional enhancements to those windows can make your home significantly more energy efficient and comfortable.

Double-glazed windows transmit less energy because adding the second layer of glass allows for an insulating air space between the two panes of glass. Older double-glazed windows occasionally develop leaks that let air and water in between these two panes, causing the windows to fog up. Unfortunately, about the only way to solve this problem is to replace the window. Fortunately, good quality modern double-glazed windows are much less likely to have this problem.

Double-glazed windows can be made even more efficient by coating the inner side of one of the glass panes with a low emissivity (low-E) coating and filling the space in between with an inert gas — usually argon — that has a higher insulating factor than plain air.

Low-E coatings act like an invisible mirror that reflects only parts of the energy found in light. They reflect infrared — the warm part of light. So, in hot weather, they reflect the heat of the sun back outside, and in winter they reflect heat back into the house. Like the coating on sunglasses, low-E coatings also help filter out ultraviolet rays, which fade window treatments, upholstery, carpets and artwork.

The two most common framing materials for windows are vinyl and wood, which offer about the same insulating value. Aluminum windows conduct energy easily, but can be made more energy-efficient if an insulating material is incorporated between the inside and outside of the windows.

A highly efficient window will cost more than a basic window, but will save you money over the years in reduced heating and cooling costs — and reduced damage from ultraviolet light. In addition, efficient windows make your house more comfortable by reducing drafts.

Heating and air conditioning usually account for only 30 to 50 percent of your energy costs, and energy-efficient windows reduce heating and air conditioning costs by 15 to 30 percent compared with old-fashioned single-paned windows. How much you actually save depends on the efficiency of your replacement windows and the type and condition of the windows you are replacing. If you have leaky, single-paned windows, you may see significant savings. If you use storm windows or have double-glazed windows, the savings may be minimal.

There are other factors that may influence your decision to buy new windows. New windows can reduce maintenance time and cost. Vinyl windows eliminate painting, and most modern windows can be cleaned from inside. New windows may also improve your home's appearance and resale value. Low emissivity (low-E) coatings reduce ultraviolet light, meaning that your window treatments and furniture will fade less. In addition, efficient windows make your house more comfortable by reducing drafts.

For more information on energy-efficient windows, see the website of the Efficient Windows Collaborative at www.efficientwindows.org.

Look for the Label

Two labels can help you find the most energy-efficient windows — the ENERGY STAR label and the rating label from the National Fenestration Ratings Council (www.nfrc.org).The ENERGY STAR program (www.energystar.gov) also rates windows for energy efficiency — including suggesting which windows are appropriate for different climates.

Here are the energy performance factors rated by NFRC. Not every window will be rated in all areas.

U-Factor
U-factor measures how well the window prevents heat from moving through it. The lower the U-value, the better its insulating value. The Energy Star program recommends a U-factor of 0.40 or less for north Georgia.

Solar Heat Gain Coefficient
Solar Heat Gain Coefficient (SHGC) measures how well the window blocks solar heat. Energy Star recommends a SHGC of 0.55 or less for this area.

Visible Transmittance
Visible Transmittance (VT) measures how much light comes through a window. It's a number between 0 and 1; and the higher the number, the more light is transmitted.

Air Leakage
Air Leakage (AL) is a rating of how much air can leak through the window. The lower the rating, the less air passes through cracks in the window structure.