Claudette Reichel | 6/25/2008 11:03:33 PM
In a warm climate, reducing solar heat gain (and air leakage) through a window is even more important than a high insulating value. This is especially true for west- and east-facing windows. Still, a window that “does it all” will save energy year-round and can allow the entire HVAC system to be downsized enough to offset the higher cost of high performance windows.
ENERGY STAR labels on windows are based on climate zones and show a map of the region for which a window is best suited. When choosing windows for new or existing homes, look for the ENERGY STAR label as well as the NFRC label to see the unit’s performance ratings. The IRC building code includes an energy code that generally requires ENERGY STAR qualified window units. (Firgure 1, Energy Star Window Label)
NFRC: National Fenestration Rating Council is a national nonprofit organization that publishes a directory of windows that have been tested according to their criteria for whole unit performance. (Firgure 2, NFRC Label on Tested Window Units)
SHGC: Solar heat gain coefficient is the fraction of solar heat that actually penetrates a window and enters the living area of a home. The lower the number, the better for reducing solar heat gain. In the southern climate zone, SHGC should be 0.4 or less, which normally is an insulated window (two layers of glass with an air space between them) with a low-e coating inside one of the glazings.
AL: The rated air leakage measures air infiltration of the window unit (in cubic feet per minute –cfm – per square foot of area). The lower the number, the more airtight the unit.
U-factor: U-values measure the ability of the window to conduct heat. It is the inverse of R-value. The lower the number, the better its insulating ability (most important to reduce winter heat loss).
VT: Visible Light Transmittance is the percentage of visible light that penetrates a window. The higher the number, the more daylight.
Insulated glass: Insulated glass (IG) has two or more layers of glass with a gap between them and are required for new construction. Inert gas fills, such as argon and krypton, have even lower conductivities than air, increasing the insulating values of the unit. Low conductance sash, frame and spacers between the glazing layers reduce heat transfer through those parts. The unit’s U-factor reflects the combined benefit. (Figure 3, Spectrally Selective Low-e Glass)
Low-e: A low-emittance (or "low-E") glass coating is a microscopically thin film applied to the glass. This coating hinders radiant heat flow to keep heat inside in winter and outside in summer. Choose a Low-E coating based on your climate.
Low solar gain, or southern climate, low-e windows do a better job of blocking heat from the sun and provide the greatest energy savings in this region; they generally have an SHGC between 0 .20 and 0.40.
High solar gain, or northern climate, low-e windows are better at reducing heat loss in winter and allow more heat from the sun to enter the home; they are advantageous for south-facing glass in a passive solar home.
Spectrally selective low-e: Specialized coatings are available that can cut heat gain without sacrificing the view and visible light. This allows for a low SHGC with a higher VT, to preserve daylight without excess heat gain.