Do Europeans Really Make the Best Windows?
These cold-weather windows are relatively new to our shores and perform well, but some lesser-known North American products may be just as good.
Synopsis: Homes in cold climates need windows that will keep the cold air out and admit as much light as possible into the home’s interior. Only a handful of window manufacturers in the United States and Canada sell high-performance windows suitable for cold-climate homes, so manufacturers from Germany and Austria have eagerly filled the market niche by introducing a host of high-performance units to American builders. Contributing editor Martin Holladay explains the performance ratings of both European and North American windows, while noting that comparisons between the two are difficult given the different protocols used to test them. He examines the anatomy of a typical European window, then describes the characteristics common to North American windows.
A good window seals out cold, windy weather and admits as much light as possible into a house’s interior. While those functions seem rather obvious, some claim a new class of window can perform these duties better than any window made in the United States.
Only a handful of window manufacturers in the United States and Canada sell high-performance windows suitable for cold-climate homes. Sensing an opportunity, manufacturers from Germany and Austria have eagerly filled the market niche by introducing a host of high-performance units to American builders.
Defining high performance
The two most important measures of a window’s performance are its U-factor and its solar heat-gain coefficient (SHGC), numbers that can be found on a new window’s National Fenestration Rating Council (NFRC) label.
A window’s U-factor is the inverse of its R-value; the lower the U-factor, the better the window is at resisting heat flow. A low U-factor is desirable in all climates. While the best double-glazed windows have a U-factor of about 0.27, triple-glazed windows have U-factors as low as 0.17.
SHGC is a measure of how much solar heat is admitted through a window. In general, windows with a high SHGC help to heat a house (a desirable feature during the winter) while windows with a low SHGC help to prevent a house from overheating (a desirable feature during the summer). Cold-climate houses need windows with a high SHGC (at least 0.39), especially on the south elevation. On the other hand, hot-climate houses need windows with a low SHGC (0.30 or lower), especially on the west elevation. Cold-climate builders should look for windows with a low U-factor and a high SHGC. The higher a window’s visible-light transmittance (VT), the better. VT indicates the amount of visible light that enters a window.
Many high-performance European windows meet stringent standards established by the Passivhaus Institut in Darmstadt, Germany. (For more information on the Passive House standard, see “The Passive House: Green Without Gizmos,” FHB #210 and online at FineHomebuilding.com.) To be certified, a Passive House window needs a European U-factor no greater than 0.80 W/ m²•K°.
In central Europe, such low-U-factor windows maintain an interior pane temperature of 17°C (62.6°F) or more on the coldest day of the year. The Passive House U-factor requirement can be achieved only by windows with insulated frames and triple glazing with two low-e coatings and warm-edge spacers.
U.S. distributors now sell Passive House windows from several European manufacturers, including Internorm and Silber in Austria, and Henzmann, Optiwin, Pazen ENERsign, and Unilux UltraTherm in Germany.
The European difference
For the most part, the glass in European Passive House windows is quite similar to the glass used in the best Canadian windows: argon- or krypton-filled triple glazing with two low-e coatings and warm-edge spacers. That’s why many energy experts report that the thermal performance of the best European windows is about the same as that of fiberglass-framed, triple-glazed Canadian windows. Katrin Klingenberg, founder of the Passive House Institute US, gives a bottom-line analysis: “Our experience has been that the overall performance of the fiberglass-framed Canadian and U.S. windows is almost as good as the German Passive House windows if you look at the overall systems design [using Passive House Planning Package software].”
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