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Windows—Is this a good energy savings investment?

Your home's windows are probably the weakest part of your home's thermal barrier. If your home has single-pane windows, as almost half of U.S. homes do, consider replacing them. New double-pane windows with high performance glass (low-e or spectrally selective) are available.

When shopping for new windows, look for the National Fenestration Rating Council (NFRC) label; it means the window's performance is certified. Also, be sure to look for the Energy Star label.

In colder climates, select windows that are gas filled with low emissivity (low-e) coatings on the glass to reduce heat loss.

In warmer climates, select windows with spectrally selective coatings to reduce heat gain.

If you are building a new home, you can offset some of the cost of installing more efficient windows because doing so allows you to buy smaller, less expensive heating and cooling equipment.

If you decide not to replace your windows, there are steps you can take to improve the performance of the windows you have.

Cold Climate Window Tips

  • Install exterior or interior storm windows. The cost of new storm windows is usually about one-quarter the cost of new primary windows. Storm windows can reduce your heat loss through the windows by 25-50 percent. They should have weather stripping at all moveable joints; be made of strong, durable materials and have interlocking or overlapping joints.
  • Install tight-fitting, insulating window shades on windows that feel drafty after weatherizing.
  • Close your curtains and shades at night. Open them during the day.
  • Keep windows on the south side of your house clean to maximize solar gain.

Warm Climate Window Tips

  • Install white window shades, drapes, or blinds to reflect heat away from the house.
  • Close curtains on south- and west-facing windows during the day.
  • Install awnings on south- and west-facing windows.
  • Apply sun control or other reflective films on south-facing windows to reduce solar gain.
  • Sunscreens are often the least expensive window-shading option that preserves a view through the window. The must be installed on the exterior side of your windows to be effective. Sunscreens reduce the solar heating of insulated glass units.

Energy Star
Energy Star is a program developed by the U.S. Dept. of Energy (DOE) and the Environmental Protection Agency (EPA) to help consumers identify energy-efficient products and services. Visit www.energystar.gov.

Low emissivity
Low emissivity (Low E) coatings on glazing or glass control heat transfer through windows with insulated glazing. Low-E coating is a microscopically thin, virtually invisible, metal or metallic oxide layer deposited directly on the surface of one or more of the panes of glass. The Low-E coating reduces the infrared radiation from a warm pane of glass to a cooler pane, thereby lowering the U-factor of the window. Different types of Low-E coatings have been designed to allow for high solar gain, moderate solar gain, or low solar gain. A Low-E coating can also reduce a window's visible transmittance unless you use one that's spectrally selective.

Spectrally selective
A special type of low-emissivity coating is spectrally selective. Spectrally selective coatings filter out 40 to 70 percent of the heat normally transmitted through insulated window glass or glazing, while allowing the full amount of light to be transmitted.

Spectrally selective coatings are optically designed to reflect particular wavelengths but remain transparent to others. Such coatings are commonly used to reflect the infrared (heat) portion of the solar spectrum while admitting a higher portion of visible light. They help create a window with a low U-factor and solar heat gain coefficient but a high visible transmittance.

U-factor
The rate at which a window, door, or skylight conducts non-solar heat flow. It's usually expressed in units of Btu/hr-ft2-ºF. For windows, skylights, and glass doors, a U-factor may refer to just the glass or glazing alone. But National Fenestration Rating Council U-factor ratings represent the entire window performance, including frame and spacer material. The lower the U-factor, the more energy-efficient the window, door, or skylight.

Solar heat gain coefficient (SHGC)
SHGC refers to the fraction of solar radiation admitted through a window, door, or skylight—transmitted directly and/or absorbed, and subsequently released as heat inside a home. The lower the SHGC, the less solar heat it transmits and the greater its shading ability. A product with a high SHGC rating is more effective at collecting solar heat gain during the winter. A product with a low SHGC rating is more effective at reducing cooling loads during the summer by blocking heat gained from the sun.

National Fenestration Rating Council
NFRC is a non-profit organization that administers the only uniform, independent rating and labeling system for the energy performance of windows, doors, skylights, and attachment products. They provide fair, accurate, and reliable energy performance ratings so that:

  • Architects, builders, code officials, contractors, homeowners, and others can compare different products and make informed product choices.
  • Building officials, state government employees, and others involved in code development and enforcement can determine if products meet local codes.
  • Government- and utility-run energy efficiency programs can establish performance requirements and standards.
  • Manufacturers have a fair and level playing field to compare products and an accurate method of showing the energy benefits of new designs or technology.

Fenestration is defined by Merriam-Webster as "the arrangement, proportioning and design of windows and doors in a building."

Sources:

Wisconsin Energy Conservation Corporation

National Fenestration Rating Council

John Krigger, Saturn Resource Management. Author of numerous energy efficiency books including Surviving the Seasons and Residential Energy: Cost Savings and Comfort for Existing Buildings

U.S. Dept. of Energy, Energy Efficiency and Renewable Energy

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