>A.Marc Rosenbaum of Energysmiths, a sustainable-design consulting firm in Meriden, N.H., responds: In the early 1990s, low-E glass optimized to let in visible light while excluding solar short-wave infrared energy pretty much took over the market. Rather than build different kinds of windows for different parts of the country, most major American window manufacturers — recognizing that most new construction was taking place in the warm climates of the Southeast and Southwest — adopted a one-size-fits-all approach to glazing. So although glass manufacturers continue to offer a variety of different types of glazing, most window manufacturers don't give you many options.

When you compare the performance numbers of three representative glazing products, you can see that the solar transmittance of so-called Northern low-E 178 glazing is about 55 percent greater than that of the commonly available 172 low-E glazing (also called low-E II), while its conductance (U-value) is a tad higher, as is its VLT (see chart,...

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