/ Green Design

Skylights, Patio Doors, Entry Doors and More

Besides offering a view while washing dishes, windows provide natural light and reduce the need for electricity. The same applies to skylights, patio doors, and entry doors with glass panels.

Maximizing the use of natural light to save energy and to make your home more comfortable is also called daylighting. Daylighting takes into account window placement and coverings as well as the windows themselves.

On the other hand, windows can be a major source of heat loss in winter and heat gain in summer, forcing you to overwork your furnace and your air conditioner. Old or poorly installed windows cause drafts or allow condensation to develop.

If you're not replacing windows as part of your remodel, you should buy energy-efficient storm windows for winter and also add weatherstripping and caulk around windows to air seal them. In the summer months, outdoor vegetation and awnings can protect your home from heat gain. So can shades, blinds and window films.

Window Stickers to Watch

To find energy-efficient windows and skylights, look for products with stickers from the National Fenestration Rating Council (NFRC). The NFRC measures the following key properties: U-factor, solar heat gain coefficient (SHGC), air leakage (AL) and visible light transmittance (VT).

  • U-factor indicates the rate of heat loss of the entire window. (It's the opposite of R-value, which indicates the insulating value of the window). The lower the U-factor, the more energy efficient it is. Along with SHGC, this is the most important number to watch.

  • SHGC measures how much of the solar radiation that hits the window will enter the home. It is expressed as a number between 0 (0 percent) and 1 (100 percent). The lower the SHGC, the more radiation blocked by the window.

  • AL measures the cubic feet of air infiltrating a square foot of the window area. The lower the number, the less air can get in your house. Casement, awning and fixed windows tend to be tighter than sliding, single-hung and double-hung windows.

  • VT indicates how much visible light passes through the window. It is expressed as a number between 0 and 1. The higher it is, the more light is transmitted-this is good for daylighting.

Windows, doors and skylights can qualify for the Energy Star program based on U-factor and SHGC. The requirements vary by climate. In the northern part of the country (Alaska, plus Washington to Maine, from the Canadian border to the bottom of Nebraska), windows must have a U-factor less than or equal to 0.35 and skylights must have a U-factor less than or equal to 0.65. They can have any SHGC. In the southern zone (Hawaii, Florida, and the southern tips of Texas, Louisiana, Mississippi, Alabama and Georgia), windows have must have a U-factor of 0.65 or less and an SHGC of 0.4 or less. The SHGC is the same for skylights, but the U-factor can go up to 0.75.

Energy-Efficient Features

Factors that affect a window's energy efficiency include the glazing (the panes themselves), coatings, gas fills, spacers and the material used for the frame.

When it comes to windows, glazing doesn't refer to a shiny coating. Rather, the glazing is the glass or plastic panes themselves. A double-glazed window is the same thing as a double-pane window. Windows can be single-, double-, triple- or quadruple-glazed: the more panes or glazing layers, the more energy efficient they will be. (Usually two does the trick.) Tinted glass also helps to reduce solar heat gain.

Low-E Coatings: Glazing can be coated. Low-E (low-emittance) coatings or films are thin layers of metal or metallic oxide applied to the window pane. The coatings are nearly invisible, lower the U-factor and help to reduce heat loss. Low-E coatings differ by SHGC, and can go on multiple panes. The right combination depends on your climate.

Gass Fills: Windows with multiple panes can have more than just air between the glass. Argon and krypton gas fills provide better insulation, lower the U-factor and are not toxic.

Spacers keep the layers of glass the right distance apart from each other. In the past, they were made from aluminum, whose conductivity can lead to heat loss and condensation. Now, manufacturers offer "warm edge" spacers made from less conductive materials such as stainless steel, silicone foam, fiberglass or vinyl.

Fiberglass and insulated vinyl are the most efficient choices, but standard vinyl, wood, vinyl-clad wood and wood composite frames all are close behind. Aluminum frames, especially those without a thermal break, are the least efficient option, and better suited to warmer climates.


For an interactive energy-efficient window selection tool, visit the Efficient Windows Collaborative at

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