Since 1977, when Sweden introduced its stringent energy code, almost all new homes in Sweden have been equipped with triple-glazed windows. Here in the U.S., where energy codes are more lax, triple-glazed windows are still rare.

For a minority of U.S. builders, however — especially cold-climate builders of superinsulated homes — triple-glazed windows are considered essential. Since few U.S. manufacturers offer high-solar-gain triple-glazed windows, most Americans get these windows from Canadian manufacturers.

Look for a low U-factorMeasure of the heat conducted through a given product or material—the number of British thermal units (Btus) of heat that move through a square foot of the material in one hour for every 1 degree Fahrenheit difference in temperature across the material (Btu/ft2°F hr). U-factor is the inverse of R-value. and a high SHGCSolar heat gain coefficient. The fraction of solar gain admitted through a window, expressed as a number between 0 and 1.
In any climate, a window with a low U-factor performs better than one with a high U-factor. The lower the U-factor, the better. (For more information on low-U-factor windows, see “Passivhaus Windows.”)

Most cold-climate builders want windows with a high solar heat gain coefficient(SHGC) The fraction of solar gain admitted through a window, expressed as a number between 0 and 1. (SHGC) and a high visible light transmittance (VT). After all, solar gain helps heat a house during the winter. During the summer, when solar heat gain is less desirable, a properly sized roof overhang will shade south-facing windows during the hottest hours of the day. (For more information on the desirability of windows with a high SHGC, see “High-Solar-Gain Glazing” and “Windows That Perform Better Than Walls.”)

Most low-U-factor windows have a SHGC that is unacceptably low — at least for cold climates. Designers of cold-climate houses have to balance conflicting needs — the need for windows with a very low U-factor and as high a SHGC as possible. Most builders end up choosing triple-glazed windows with a U-factor ranging from 0.19 to 0.26 and a SHGC ranging from 0.39 to 0.47.

The higher the visible light transmittance (VT), the better. Windows with a low VT look gray and depressing. To get an idea of what range you’re looking for, consider the advice of Robert Clarke, a technical specialist at Serious Materials and the former president of Alpen Windows. According to Clarke, any window with a VT below 0.40 “would not be ethical to sell as clear glass.”

When comparing U-factor, SHGC, and VT specifications between manufacturers, be careful. While leading manufacturers provide whole-window values (as required by NFRC labeling rules), less reputable manufacturers often trumpet glazing-only numbers. These glazing-only numbers often seem to show much better thermal performance than whole-window numbers, but they are misleading. To avoid comparing apples to oranges, insist on whole-window specifications that include the window frames.

Suppliers of triple-glazed windows
The five leading Canadian manufacturers of triple-glazed windows with pultruded fiberglass frames are Accurate Dorwin, Duxton, Inline Fiberglass, Fibertec, and Thermotech Fiberglass.

New England builders looking for less-expensive triple-glazed windows have sometimes settled for triple-glazed vinylCommon term for polyvinyl chloride (PVC). In chemistry, vinyl refers to a carbon-and-hydrogen group (H2C=CH–) that attaches to another functional group, such as chlorine (vinyl chloride) or acetate (vinyl acetate). windows from Paradigm Windows in Maine. Like almost all of the triple glazing offered by U.S. manufacturers, however, all of Paradigm's triple glazing has a very low SHGC.

A new entry into the field of high-performance windows is Serious Materials of Sunnyvale, Calif. Although Serious doesn’t offer triple-glazed windows, it sells low-U-factor windows with Heat Mirror glazing. (Heat Mirror glazing has two panes of glass with one or more suspended plastic films between the inner and outer pane.)

Assembling manufacturers’ information on U-factors, SHGC, and VT is time-consuming. To help out window specifiers everywhere, I’ve created a table showing these specifications for casement windows from seven manufacturers. (This table, or one like it, may or may not appear in my upcoming Fine Homebuilding article on triple-glazed windows.)

Note: The second table includes Energy Rating (ER) numbers; ER is a window-rating system developed in Canada. For more information on ER, see “Windows That Perform Better Than Walls” and Stephen Thwaites March 27 posting, below.

Last week’s blog: “Forget Vapor Diffusion — Stop the Air Leaks!”