Energy Performance Ratings for Windows, Doors and Skylights

Energy Performance Ratings for windows, doors and skylights are designed to tell you their potential for gaining and losing heat, as well as how much sunlight they allow into your home. If you’re thinking about replacing these items, and want to install energy-efficient products, you should pay attention to these ratings.
Knowing how well your windows, doors and skylights perform is important. In a typical home, up to 30% of heating and cooling is lost through its windows and doors. To put that in perspective – the amount of energy lost through doors and windows in the U.S. every year is roughly equivalent to all the energy we get from the oil carried by the Alaska pipeline!

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.
Energy Performance Ratings are determined by measuring the following energy performance characteristics:

■U-factor
The rate at which a window, door, or skylight conducts non-solar heat flow. 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)
This is the 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 also more effective at reducing how hard your A/C has to work during the summer, because it blocks 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.

■Air leakage
This is 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.
Installing energy-efficient windows, doors and skylights can significantly lower your heating and cooling bills. Simply by choosing products with efficient Energy Performance Ratings, you’ll keep more of your hard-earned energy dollars in your pocket. And, you’ll gain greater comfort because your home will be better able to retain heat and cold. Next time, we’ll talk about daylighting, sunlight transmittance, and what it means for your energy bills.

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