In an effort to keep its Energy Star label relevant, the Department of Energy is tightening Energy Star criteria for windows, doors, and skylights. Beginning in January 2010, these products will need to meet more stringent standards for the rate of heat loss (U-factor) and solar heat gain (solar heat-gain coefficient, or SHGC) to qualify for an Energy Star rating. For example, the maximum U-factor for qualifying windows in the northern climate zone has been lowered from 0.35 to 0.30 (see chart, below). In the southern zone, the extensive current SHGC trade-off menu for windows with different U-factors has been simplified to a one-size-fits-all maximum value of 0.27.

The Department of Energy has simplified the qualification standards for Energy Star windows across its four climate zones. Choices in the northern zone would allow building designers to balance slightly larger U-factors against increased solar heat gain, though it is not clear whether windows with the higher SHGC values will be widely available....

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