Low-e replacement windows: which coating works best?
November 12, 2011
Low-e glass has become the standard for energy-efficient windows. However, there are several types of low-e glass. All windows labeled "low-e" may not produce the same results for different climates. Energy Star requirements vary for each of the four climate zones in the U.S.
U.S. Climate Zone Map from energystar.gov
You must consider low-e glass products with the best U-value and Solar Heat Gain Co-efficient (SHGC) for the location of your home.
U-values, SHGC and other specs for low-e glass products can be found on the glass manufacturers' websites. In the U.S. the three primary manufacturers are PPG, Cardinal and Guardian.
PPG's premier low-e glass is Solarban 70XL®. This low-e glass has a visible light transmittance of .64, meaning the glass is clearer than most other low-e glass, which is visibly dark. With a low U-value of .24 this glass can be well-suited to winter in northern climates, but because of its low .27 SHGC, it is great in warm-weather climates where solar heat is less desirable. (A lower SHGC means less solar heat is transmitted through the window). Sungate 500®, which has a U-value of .31 and a SHGC of .70, is better-suited to northern areas where you want more solar heat gain in the cold, long winter.
Cardinal offers its 366® glass product, which is comparable to PPG's Solarban 70XL®. With a U-value of .23 and a SHGC of .25, it satisfies both the northern U-value requirements as well as the southern SHGC requirements, but this glass is slightly dark. Their Clear Low-e 180® is better for the northern zone because of its lower U-value, .31, and its higher SHGC of .63.
Guardian has similar glass to the other two manufacturers. ClimaGuard 75/68® is for areas where homes require more heat – it has a U-value of .32, and its .68 SHGC allows more sunlight in. It is also well-suited where passive solar heating is used. Guardian's glass for southern zones is ClimaGuard 55/27®. Its low U-value of .29 keeps heat out in the summer, and the SHGC of .27 further prevents the sun from heating things up.
As you can see, low-e glass does not assure optimum performance unless you get the correct low-e glass for the area in which you live. The three glass manufacturers all have similar products, but care should be taken to make sure the low-e glass you choose is the best one for your climate.