You can use the energy performance ratings of windows, doors, and skylights to tell you their potential for gaining and losing heat, as well as transmitting sunlight into your home.
Heat Gain and Loss
Windows, doors, skylights can gain and lose heat in the following ways:
* Direct conduction through the glass or glazing, frame, and/or door
* The radiation of heat into a house (typically from the sun) and out of a house from room-temperature objects, such as people, furniture, and interior walls
* Air leakage through and around them.
These properties can be measured and rated according to the following energy performance characteristics:
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)
A fraction of solar radiation admitted through a window, door, or skylight—either 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. Therefore, what SHGC you need for a window, door, or skylight should be determined by such factors as your climate, orientation, and external shading. For more information about SHGC and windows, see passive solar window design.
The rate of air infiltration around a window, door, or skylight in the presence of a specific pressure difference across it. It’s expressed in units of cubic feet per minute per square foot of frame area (cfm/ft2). A product with a low air leakage rating is tighter than one with a high air leakage rating.
Source: Office of Energy Efficiency and Renewable Energy, U.S. Department of Energy