| Windows are ranked in a number of categories | | | | Shows the solar energy transmitted by a window |
| used to compute their energy efficiency. For | | | | and shows how efficiently a window blocks heat |
| anyone buying windows for your household, more | | | | from the sun. The smaller the SHGC, the less |
| importance needs to be placed on different | | | | solar heat the window transmits. |
| categories based on the climate the residence is | | | | Visible Transmittance (VT) |
| located in. For example, if someone is looking for | | | | Shows the amount of light a window permits |
| replacement windows in Phoenix, Arizona, a sunny | | | | through. The greater the Visible Transmittance, |
| and warm climate, the Solar Heat Gain Coefficient | | | | the more light you see. |
| is probably the most critical ranking to consider. | | | | Air Leakage (AL) |
| Other things, including the position of a window to | | | | Measures the rate at which air passes through |
| the sun and any shade that may be present | | | | joints in a window. Air Leakage is measured in |
| should be considered as well. | | | | cubic feet of air passing through one square foot |
| The United States Government's Energy Star | | | | of window area per minute. The lower the Air |
| program is crafted to make it simple for people | | | | Leakage value, the less air leakage. |
| to evaluate energy efficient products and | | | | Condensation Resistance (CR) |
| practices designed to save them money and | | | | Shows how effectively a window resists water |
| safeguard the environment. The Energy Star | | | | build-up. The higher the Condensation Resistance |
| program ranks in excess of sixty products, | | | | factor, the less condensation build-up a window |
| including windows. Energy Star certification makes | | | | allows. |
| it easy for consumers to be certain they are | | | | Alternative Window Ratings |
| acquiring energy efficient replacement windows. | | | | There are additional categories that are not |
| All Energy Star replacement windows are scored | | | | included on the NFRC Energy Star label although |
| in five categories but Energy Star qualification is | | | | they are helpful and show up on a few windows. |
| based solely on the U-Factor and Solar Heat Gain | | | | Light to Solar Gain |
| Coefficient. The U-Factor and Solar Heat Gain | | | | The ratio of Visable Transmittance to Solar Heat |
| Coefficient are also the categories utilized to | | | | Gain Coefficient measures the amount of light |
| decide if a window qualifies for the IRS Energy | | | | that is transmitted without adding heat. A greater |
| Tax Credit. | | | | number means that more light is transmitted |
| U-Factor | | | | without with no added heat. |
| Calculates the rate of heat transfer and indicates | | | | R-Value |
| how efficiently the window insulates. The lower | | | | R-Value was created as a means to evaluate |
| the U-Factor, the more effectively the window | | | | insulation. It indicates conductivity and measures |
| insulates. The window U-Factor incorporates the | | | | the resistance to heat flow. A higher R-Value |
| thermal properties of the frame along with the | | | | suggests better insulating properties. Almost all |
| glazing. | | | | windows today are ranked using U-Factor. |
| Solar Heat Gain Coefficient (SHGC) | | | | |