Solar Heat Gain Coefficient
|A window rating measuring its ability to reduce unwanted solar heat gain in the home.|
During the summer months unmanaged solar energy creates a thermal heating load. Many homeowners cool their home using fans or air-conditioning. When replacing the existing non-insulated windows with thermal insulated replacement window units the homeowner can reduce the energy needs and costs of cooling a home. Windows with a low solar heat gain coefficient help shade the window unit and keep the home's temperature at a low level.
 Solar Heat
Windows account for most of the fenestration in a house and therefore most solar heat transfers into the house through the home's windows. Window manufacturers have developed tools for managing the amount of solar gain that enters a house before it actually enters the house. The rating that measures the amount of solar gain that enters a house is referred to as the Solar Heat Gain Coefficient (SHGC) rating.
The National Fenestration Rating Council (NFRC) has standardized the measurement that rates the amount of solar heat gain that transmits through a window unit. This standard, the NFRC 200, is a "“Procedure for Determining Fenestration Product Solar Heat Gain Coefficients at Normal Incidents.” This standard indicates a window's ability to admit solar heat gain. Lower SHGF ratings indicate that the window is better able to block the heat from the sun.
 Solar Heat Gain Coefficient
When rating the SHGF the NFRC relates to the maximum amount of solar heat gain that transfers through a window as a "1" and the minimum amount of solar heat gain that transmits through a window as a "0." A window that is rated as a SHGC of 0.70 has 70% of the available solar heat gain transmitting through the window. Less solar heat gain is transferred with replacement windows which display a lower solar heat gain rating.
 Entire Window Assessment
The type of window, as well as the glass, affects the SHGC rating. The NFRC expresses the SHGF rating as an assessment of the entire window performance, not just the glass. The SHGF ratings also include an evaluation of the window's ability to absorb the heat from the sun and conduct it into the house through the entire window. The NFRC rates the SHGF for the whole replacement window, including the effects of the frame.
 Center of Glass SHGC
Glazing is an important component in the SHGC rating. Different types of glazing, including tinted and reflective glass, help to shade a window and reduce solar heat gain. In addition, spectrally selective glass including tinted or coated Low-E glass, blocks or re-radiates the sun's rays and successfully reduces solar heat gain, providing an even higher level of insulation. The NFRC 200 ratings provide homeowners with a uniform measurement which compares the solar heat gain performance of these different methods of blocking. The whole window SHGC is lower than center of glass-only SHGC.
In addition to the heating protection offered by windows with a low SHGC rating, homeowners benefit by reducing the sun's rays which enter the house which protects their furnishings. Solar rays can fade carpets, furniture and upholstery but when these rays are limited, the effects on a home's furnishings are limited as well.
 Climate Considerations
Solar heat gain can provide a home with free heat in the winter but a high level of solar heat gain can lead to overheating during the summer. Homeowners must consider how to best balance summer and winter solar heat gain as well as how to determine the best level of SHGC for their specific climate. The appropriate level of SHGC for a particular home will depend on the climate, shading conditions, orientation and other factors. Homeowners can reference the Energy Star program to access simplified guidance on recommended SHGC ratings for a particular climate. The Energy Star Window Selection Tool compares average energy costs for different locations based on various types of windows.
 Energy Star Windows
The Energy Star Windows Program presents information about a window's ability to limit heat transfer and control the amount of solar heat gain that transmits into a house through the window units. The program is administered in conjunction with the United States Department of Energy and displays the Energy Star seal which indicates that a window unit is an Energy Star window. Energy Star replacement windows may be eligible for a tax credit or rebate, depending on the taxpayer's residence. The Energy Star Window Selection Tool indicates which replacement windows are eligible for a tax credit in which locations.
 National Fenestration Ratings Council
The National Fenestration Ratings Council (NFRC) provides uniform ratings of a window product. The NFRC’s Certification Label displays a window product's SHGC rating along with ratings for U-factor and visible transmittance. Energy Star windows take the U-Factor and SHGC into consideration when identifying an Energy Star window but do not refer to the window's Visible Transmittance. Homeowners can use the information on the NFRC label to compare window units and ensure that the window meets local code requirements and other energy specifications.
The U-Factor measures a window's ability to prevent heat from escaping from a home. U-Factor ratings may fall between 0.20 and 1.20 with lower U-Factors providing better insulating capabilities. Most thermal resistant windows are rated at 0.30 or lower. A low U-Factor is particularly important in cold-dominated climates.
 Solar Heat Gain Coefficient (SHGC)
The solar heat gain coefficient (SHGC) measures the success of a window product in blocking the sun's heat rays. SHGC is expressed as a number between 0 and 1 with lower SHGCs blocking increased levels of unwanted heat gain. A low SHGC is particularly important in a heat-dominated climate.
 Visible Transmittance (VT)
Visible Transmittance (VT) measures the amount of light that is transmitted through a window product. It is expressed as a number between 0 and 1 with higher VTs providing higher potentials for daylighting.
 SHGC Certified Products
Homeowners can assess a replacement window's NFRC certification by accessing the NFRC Certified Products Directory. Window manufacturers join the program to request that the NFRC will rate and list their window product in the NFRC Directory. Thousands of certified products are listed in the NFRC Directory under the heading of "Specialty Products." Energy Star window ratings are based on NFRC ratings but the NFRC also rates window components which are not a part of the Energy Star program.