The articles fail to mention, however, that windows are not only a source of heat loss but also a source of heat gain. In the north, the best insulating value doesn’t necessarily result in the lowest heating bill. The featured windows employ one or more highly insulating, low-solar-gain low-emissivity coatings. While this is a great choice for cooling climates, it blocks the free heat from the sun in heating climates.
The article on vacuum glazing says that these windows could be net-energy gainers even facing north and that this could save U.S. homeowners $15 billion per year. With careful edge and frame construction this is quite possible. Today’s best heating climate windows are 15-plus years old and not newsworthy. But interestingly, while they are usually not net-positive facing north, they would almost certainly outperform the described vacuum glazing facing south. And on a whole-house basis in a heating climate, they would produce equivalent or, more likely, greater savings.
While we’ve all been conditioned to think more is better, when it comes to windows for houses in heating climates, the “more” is usually the solar gain, not the insulating value.
– Stephen Thwaites, P.Eng.
Thermotech Fiberglass Fenestration Ottawa, Ontario
Mr. Thwaites raises a good point that windows with low-emissivity glazing—and consequently low solar-heat-gain coefficients—may not be the best choice for south-facing windows in heating climates.
EBN has long argued that a “one size fits all” approach in specifying windows doesn’t make sense (see, for example,
Vol. 5, No. 2). We should have made this clearer in the article on vacuum-insulated glazing, but we did address “tuning” windows by building orientation in the review of Alpen windows. Indeed, that is one of the hallmarks of Alpen, which usually provides different glazings for different orientations when making windows for a building project.
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