True divided-light windows lose more heat.
Heat loss is highest at the edges of window glass. True divided-light windows lose more energy than windows with internal muntins because there are many more edges where heat can escape. Simulated divided-light windows, with the muntins sandwiched between sheets of glass, retain more of the thermal performance of the larger glass size than true divided-light windows. Because National Fenestration Rating Council ratings are not based on divided-light windows, they may overstate the thermal performance of divided-light windows.
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LEED for Homes: Up to 3 points for high-efficiency windows.
NGBS/ICC 700: Depending on the climate zone up to 12 points for windows that meet U-value and Solar Heat Gain Coefficient maximums.
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