High-performance windows increase energy efficient and reduce fading damage from ultraviolet radiation.
The NFRC label makes it easy to assess a window’s energy performance. Which U-factor and solar heat-gain coefficient (SHGC) are optimal depends on climate. Optional ratings for air leakage (AL) and condensation resistance (CR) are included on this sample rating. AL indicates how many cubic feet of air leak through a square foot of window area, and generally ranges from 0.1 to 0.3. CR, which uses a scale of 1 to 100, indicates how well a window resists forming condensation on the inside of the glass. Higher numbers indicate better resistance.
The Energy Star label uses color shading to show the climate zones where a particular window or skylight will perform well.
Builders can turn to the Energy Star program for help selecting the best windows for their climate. In northern regions, windows must be good at keeping in wintertime heat; in the south, they should be able keep heat out in the summer. Windows installed in the middle of the country must balance summer and winter performance. Note that all values in the table are based on NFRC whole-window ratings.
The inside surface of the low-E windows (left and center panes) are much warmer than that of the clear double-pane (right pane). If this window had three bays of clear double-pane glass, the room’s heating thermostat would have to be 2°F to 3°F warmer to provide the same level of comfort achieved with low-E glass.
Spectrally selective low-E coatings do a good job of preventing winter heat loss and reducing summer heat gain while still allowing most of the visible light to enter the space (as indicated by high center-of-glass VT ratings). The chart compares the performance of standard clear glass with that of three types of low-E. Note the performance options available with advanced low-E. Going from high to low solar gain cuts the SHGC by nearly two-thirds with only a modest loss of visible light.