Why are so many LEED-certified buildings all-glass? It seemed to defy logic. After all, no matter how much you can reduce artificial lighting by using daylight, the insulation value of glass is negligible compared to solid insulated walls, and anyway there are many overcast days and dark winter afternoons. It is even more puzzling when an all-glass building is shaded. First you wrap it in glass, and then you wrap the glass in something else. I always suspected that this was more about announcing “I am a green building” than about actually conserving energy. And now it’s official. A recent NYT article reported on the results of a New York City study of the actual energy performance of large office towers. (The full report is here.) In the rating system used, 75 is considered the minimum score for a building to be considered “high efficiency.” The all-glass 7 World Trade Center (LEED gold) scored 74, and the Condé Nast Building, highly touted as the city’s first green building when it opened in 1999 received an unspectacular 73. Close but no cigar. Solid buildings with windows, even old ones, do much better: the venerable Empire State scored 80, and Chrysler scored 84. As for the old glass towers, it’s downright embarrassing. The recently renovated Lever House reached only 20, and Seagram scored a miserable 3.