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Q. As a HERS rater working in north-central New England, I've noticed that the U-values of the windows that most builders are installing are quite good — below .35 — but they have low SHGC (solar heat-gain coefficient) values. While it seems that windows with higher SHGC values would be a good idea in cold climates, I've found that most manufacturers offer only low-E windows with these relatively low SHGC values. Why?

>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, below).

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Because it allows more of the sun's infrared energy to pass through, 178 glazing is definitely preferable in cold climates — except for cases involving large expanses of west-facing glazing or significantly overglazed southern facades.

An even more effective glazing for the south side of a passive solar building might be Pilkington Energy Advantage glazing, which is better at allowing the sun's heat and light in, though slightly worse in insulating value.

In fact, there is a great variety of glazing available that can be used to optimize the energy performance of homes. Different glazings can be used for different climates and facade orientations, allowing solar heat in where appropriate and excluding it elsewhere. But as you've pointed out, the greatest barrier is availability from the big manufacturers.

For that reason, recently I've been recommending pultruded fiberglass windows from Canadian manufacturers like Accurate Dorwin (888/982-4640, www.accuratedorwin.com) and Thermotech Windows (888/930-9445, www.thermotechwindows.com), as these manufacturers offer a wider choice of glazings and can accommodate triple glazing.