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Windows give us a view out into the world, but that's not all they do. They also play a big role in how energy efficient and comfortable our homes are. During the warm months, older single-pane windows can allow unwanted heat into the home, and in the winter, they can account for as much as 25% of the home's heat loss.

Today's high performance double-pane windows reduce heating and cooling costs and keep homes more comfortable. If replacing single-pane windows with high performance windows isn't in your budget, consider lower-cost options such as applying solar control window film or installing storm windows.

Solar control window film

Solar control window film can be applied to existing windows to reduce solar heat gain through the glass while still transmitting light and providing visibility. Look for products with a solar heat gain coefficient (SHGC) of 0.40 or lower, or shading coefficient (SC) of 0.44 or lower. The lower the SHGC or SC, the less heat the film will allow in.

Window film should only be used on single-pane windows. Consider using solar control film on all east- and west-facing single-pane windows to reduce heat gain into the home. Window films can be applied by a commercial installer; do-it-yourself products are also available at most home improvement stores.

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Storm windows

Storm windows are temporary windows installed over the standard window to improve energy efficiency and comfort. Some storm windows are designed to be installed on the window's interior, while others are mounted on the exterior. Measure existing windows and order storm windows from a window supplier. Storm window panes are typically mounted in the winter to provide additional insulation and wind protection, and then removed in the spring.

High performance windows

Once upon a time, windows were rarely more complicated than a single pane of glass mounted in a wood or metal frame. These days, windows are available in a dizzying array of options. Today's high performance windows have many features that make them stand out over basic single-pane windows, including:

  • Multiple panes of glass, with an air- or gas-filled space between them, to provide better insulation
  • Improved frame materials to reduce heat transfer and insulate better
  • Special low-e coatings on the glass to keep heat inside in the winter and outside in the summer
  • Warm edge spacers between the panes of glass to reduce heat flow and prevent condensation

In most instances, dual-pane windows are required by California's building energy efficiency code, known as Title 24. The energy efficiency code requirements apply to new home construction as well as additions and major remodeling projects.

If you are considering replacing existing single-pane windows with double-pane windows, it may not be cost effective based on energy savings alone, especially if you live in a mild climate. But there are plenty of other reasons you may be in the market for new windows. Your old windows may have rotted frames or sashes, or you may want to cut down on noise from outside or reduce drafts inside. Or perhaps you simply want to bring more daylight into certain rooms.

When shopping for new windows, choose double-pane products with a U-factor of 0.40 or less. U-factor is a measure of heat transferred by the entire window (frame, sash and glass) either into or out of the building. Windows with a lower U-factor do a better job of insulating, and therefore provide more comfort and energy savings when it is cold outside.

In addition to a low U-factor, the windows should have a solar heat gain coefficient (SHGC) that suits your climate and the window's orientation. SHGC is a measure of the solar radiation entering the room through the entire window. An SHGC of 0.40 or less will reduce air-conditioning costs and provide more comfort in warmer climates. A higher SHGC will allow more sun to heat the room, which is desirable in colder climates and in homes designed for passive solar heating (see our Major Remodeling and Additions Overview for information on passive solar design).

Wood, fiberglass and vinyl frames all insulate much better than aluminum frames. Some wood-frame windows are made with sustainably harvested wood certified by the Forest Stewardship Council (FSC).

Visit www.efficientwindows.org for help in choosing the best criteria for windows in your climate and for your specific application. Check with your local utility company for rebate programs for high performance windows.

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More window tips

  • Check the label. Factory-made windows have a National Fenestration Rating Council (NFRC) label showing the product's U-factor, SHGC and other performance characteristics (visit www.NFRC.org for more information). This information is also usually available on the manufacturer's website. Some suppliers offer a limited number of low-e options, so it may be challenging to get the exact window performance characteristics you desire. Telling suppliers and window representatives what you want—even if it is not available today—may help expand product availability in the future.
  • Replacing existing windows. If you plan to replace existing windows, you will need to choose between retrofit and new construction windows. Retrofit windows are installed relatively easily by removing the existing window glass and slipping in a new window frame assembly within the existing window frame. Be aware that if the existing windows have moisture problems, retrofit windows will not necessarily fix the problems.

    New construction windows require that the entire window assembly (glass and frame) be removed prior to installation. Flashing, building paper and sealants must all be reapplied. Additionally, stucco or siding may need to be cut away during installation and repaired after completion. Replacing the entire window frame and substrate requires more effort and money than installing retrofit windows, but it's a better option if water damage has occurred.

  • Shade your windows. If sun streaming through windows is overheating your home and making your air conditioner run overtime, shading the windows will likely be a less expensive option than buying new high performance windows. Awnings, trellises and shade trees are good ways to control solar heat gain through windows. If you are building an addition or doing a major remodel, consider designing an appropriately sized roof overhang for south-facing windows to block sun in the summer but allow in the sun's warmth in the winter. Interior shading products (curtains, blinds and the like) are less effective at keeping heat out than exterior shading, but they are better than nothing.


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