Windows on Sustainability

Advanced glazings help buildings achieve net-zero envelopes
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Sponsored by Guardian Industries Corp., View and BISEM

Pyrolytic

Pyrolytic low-E glass is glass with low-E coating applied at high temperatures and fired into the glass surface during the “on-line” float glass manufacturing process. The high solar heat gain of pyrolytic low-E glass is good for cold climates, though it may not meet energy codes in other climes, and can be less crisp in appearance compared to other coatings.

Sputter Coatings

By contrast, sputter low-E glass is glass with low-E coating applied through an “off-line” coating process. The off-line process occurs after the float glass is produced, using a Magnetron Sputter Vacuum Deposition (MSVD) coater. Glass is put into a vacuum chamber, where ionized gas bombards the surface of a metal cathode (silver) with ions. Atoms of the desired metal are vaporized and then deposited in a thin film on the surface of the glass. The MSVD works at the molecular level to produce superior performance and offers significant advantages over “hard” (pyrolytic) and traditional “soft” coatings. By using different gasses, such as argon or nitrogen and oxygen, and by layering metallic and dielectric layers in different sequences, a wide variety of coatings can be produced to meet many design and performance requirements. In general, the MSVD process offers more coating options and improved solar, thermal and light-to-solar gain options than the pyrolytic process. Sputter coatings, offered in a wide variety of color and performance options, including post-temperable versions that can be produced efficiently in stock sizes and then fabricated nearer to the job site, can meet and exceed energy code requirements, dramatically lowering heat gain or loss while providing high visible light transmission and optimal transparency.

The commercial market for coated glass is predominantly sputter coatings, and continues to grow as the pyrolytic coatings have seen limited increases in demand.

Glazing Options Performance Comparison

Sputter Low-E Spectrally Selective Coatings

These coatings reflect both long-wave IR and solar near-infrared rays. In other words, they transmit a higher ratio of daylight compared to the amount of solar heat transmission. By blocking solar heat and making maximum use of daylight, spectrally selective glass can now provide a range of visible light transmission on clear float glass between 40 and 70 percent, while also offering lower reflectivity than was possible in the past, as well as a low U-factor and solar heat gain coefficient. Spectrally selective coatings, defined by the U.S. Department of Energy as glass with a light to solar gain of 1.25 or better, can significantly improve building heating, cooling and electric lighting, to the point of downsizing HVAC equipment, which reduces initial capital investment and ongoing energy costs. Spectrally selective coatings can be applied on clear or low-iron glass as well as various types of tinted glass to produce “customized” glazing systems capable of either increasing or decreasing solar gains according to the aesthetic and climatic effects desired. The Department of Energy further maintains that computer simulations have shown that advanced window glazing with spectrally selective coatings can reduce the electric space cooling requirements of new buildings in hot climates by more than 40 percent. Spectrally selective low-E coatings are available with one, two or three layers of silver; each layer improves the coatings selectivity.

Sputter Low-E Hybrid Coatings

These multifunction coatings have medium reflectivity but with higher light transmission which provides improved transparency. In comparison to older, low light-transmitting reflective coatings, manufacturers have added low-E performance and brightened the exterior appearance while still transmitting considerable light and achieving a low SHGC. Originally developed for residential markets, low-E coatings had low color, reflection and high transmission. Manufacturers modified those films to increase the exterior reflection, which can be an asset in the commercial marketplace, while lowering the transmission, which can increase the heat loads. In short, hybrids were modified to have lower transmission, higher reflection and very good solar properties including low U-values and low shading coefficients.

The sputter coating process occurs after the float glass is produced, using a Magnetron Sputter Vacuum Deposition (MSVD) coater.

The sputter coating process occurs after the float glass is produced, using a Magnetron Sputter Vacuum Deposition (MSVD) coater.

Source: Guardian Industries Corp.

 

 

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Originally published in Architectural Record
Originally published in September 2012

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