Windows are a significant source of heat loss and heat gain in the home. When building a new home, the placement of windows and the type of window used can make a dramatic affect on the energy efficiency of the house. For existing homes, replacement windows and window treatments may be considered to increase the energy efficiency of the home.
Energy-Efficient Options for New Windows
Window Ratings help simplify the purchase of energy-efficient windows. The National Fenestration Rating Council (NFRC) has developed a window rating system that considers solar heat gain in addition to R-value and air leakage. The numbers, which represent the Fenestration Heating Ratio (FHR) and Fenestration Cooling Ratio (FCR), indicate the percentage of annual household heating or cooling energy the window will save compared to a worst-case window with single glazing and aluminum frame. The higher the number, the greater the savings.
Types of Glass
Until recently, conventional, clear glass was the primary glazing material available for residential use. Now several types of special glass are available that can help control heat loss or gain, including low-emissivity glass, heat-absorbing glass and reflective glass.
Low-emissivity glass, or low-e glass, has a special coating on the surface to reduce radiant heat transfer. While the air space in normal double-paned windows reduces some of the heat loss, a significant amount of heat is transferred from the warm inner pane to the colder outer pane. The coatings used on low-e glass reduce the emissivity, thereby increasing the R-value (resistance to heat flow) of double-paned units. The incoming visible light is reflected only slightly, so low-e glass appears almost clear rather than mirror-like. Window units with low-e coatings cost about 10 percent to 15 percent more than regular units but can reduce energy flow though a window by 30 percent to 50 percent. New window units should be the low-e type with a U-value (conductance of heat) of .35 or less to control conduction losses. They should also have a shading coefficient of .5 or less to control radiant heat gain in the summer. If a large expanse of glass is used on the south side for solar heating, then a shading coefficient approaching 1.00 should be used for these windows with the radiant heat gain controlled with shades or awnings or both.
Heat absorbing glass contains special tints that allow it to absorb as much as 45 percent of the incoming solar energy, thereby reducing heat gain. Part of the absorbed heat, however, will continue to be passed to the structure. An inner layer of regular glazing reduces this transfer. Heat-absorbing glass reflects only a small percentage of visible light and, therefore, does not have the mirror-like appearance of reflective glass.
Reflective glass has been coated with a reflective film. It is useful in controlling solar heat gain during the summer, but it also reduces the passage of light all year long, and, like heat absorbing glass, reduces solar transmittance in winter. These two types of glass, therefore, are not desirable for use in passive solar heating applications.
Questions to consider before installing new windows
- What is the long term increase to the value of my home? If the home value is increased by the amount spent on windows, then all utility savings can be considered as profit on an investment.
- Will the new windows save time and money on maintenance?
- Would window treatments, such as inside or outside storm windows, window film, awnings, or interior shades, represent a viable alternative to new windows?
- What is the reputation of the windows and the installer? Improperly installed new windows can be as energy-inefficient as the original windows.
Energy-Efficient Options for Existing Windows
There are many inexpensive alternatives to installing new windows in your home that will make your windows more energy efficient.
Storm windows, for homes with single-pane windows, can be as effective, or sometimes more effective, in blocking heat transfer than double-paned units. Several kinds of storm windows are available. The least expensive is plastic sheeting that can be installed around either the outside or, preferably, the inside of windows. Glass units with wood, metal or vinyl frames can be attached to the window frame with clips or screws. The energy savings and payback periods from installing storm windows range from several months to a year for plastic sheeting and 5 to 10 years for glass.
Window treatments, such as insulating shades, shutters and drapes, provide some insulation to windows in the winter by reducing heat loss at night and allowing sunlight in during the day. Since most homeowners have some form of window treatments, the only maintenance is opening or closing the window treatments to allow sunlight in or to keep it out.
Shading devices, such as awnings, exterior shutters or screens, can be used to reduce unwanted heat gain in the summer.
Reflective films are another method of controlling the balance of heat gain and heat loss through windows. These films reflect sunlight away from the window and reflect heat back into the room.
Caulking and weatherstripping are inexpensive methods used to stop air leaks around windows and most homeowners can apply caulk and weatherstripping themselves. For additional information on caulking and weatherstripping, please see our caulking and weatherstripping web page.
Other Sources of Information
U.S. Dept. of Energy, Energy Efficiency and Renewable Energy Network
A service provided through the U.S. Dept. of Energy, the Energy Efficiency and Renewable Energy Network is a clearinghouse for energy efficiency and renewable energy information.
U.S. Dept. of Energy, Energy Savers guide to energy Efficiency and Renewable Energy
- Selecting New Energy-Efficient Windows
- Advances in Glazing Materials for Windows
- Reflective Glass and Films for Windows
- Exterior Doors
- Storm Windows
- Skylights for Residences
Efficient Window Collaborative
The Efficient Window Collaborative provides information on the benefits of energy-efficient windows, descriptions of how they work and recommendations for their selection and use.
EPA Energy Star Program
Home Energy Magazine
Home Energy Magazine is dedicated to providing energy-efficiency information. Their web site features full text articles on residential energy efficient construction and remodeling.
University of Missouri Extension
University of Missouri Extension is a public educational network combining the resources of the U.S. Department of Agriculture, extension professionals at U.S. land-grant universities, county extension professionals and local governments. University Extension provides information and fact sheets on energy topics. Look for the titles listed below.
- "Home Energy Management: Weatherstripping Your Windows"
- "Shades and Shutters for Energy Efficiency"
Lawrence Berkeley National Laboratory
The Lawrence Berkeley National Laboratory's Windows and Daylighting website provides both consumer and technical information on the purchase of windows and reports on research conducted by LBL into the latest technical advances in the design of windows.