Insulating Glass Unit Performance

In terms of IGU performance, the primary performance considerations that have led to increased reliance on single-source production are structural capabilities—especially for larger units—as well as techniques for controlling solar performance. For the former, the glazing contractor must deliver unitized glazing products in aluminum or steel frames that meet key performance standards including tests for static structural load, air and water infiltration and, for hurricane-resistant glazing systems, impact resistance and cyclic loading.

Yet even before the framed glazing is tested, the fabricated glass itself must demonstrate the proper levels of performance as either heat-strengthened or fully tempered flat glass material, as defined in the standard specification ASTM C1048, or as a laminated architectural flat glass, defined in ASTM C1172. These two specifications are important touchstones for glass used in general building construction.

▶ ASTM C1048 includes coated and uncoated glazings as well as spandrel glass with one ceramic-coated surface and both transparent and patterned glass, whether they are heat-strengthened (HS) or fully tempered (FT). It requires that “all fabrication, such as cutting to overall dimensions, edgework, drilled holes, notching, grinding, sandblasting, and etching, shall be performed before strengthening or tempering and shall be as specified,” according to ASTM International.

▶ ASTM C1172 is a specification standard for the quality requirements of cut panels for laminated glass, which “consists of two or more lites of glass bonded with an interlayer material for use in building glazing.” A number of interlayer materials can be used with varied numbers and thicknesses of glass plies for such applications as safety and security needs, detention facilities, resistance to hurricanes and cyclic wind loading, blast- and bullet-resistant settings as well as places requiring reduced sound transmission.

Another important test is ASTM C1376, Standard Specification for Pyrolytic and Vacuum Deposition Coatings on Flat Glass, which covers all the “optical and aesthetic quality requirements for coatings applied to glass for use in building glazing,” according to ASTM International. Both pyrolytic and magnetron sputtering vacuum deposition (MSVD) coatings are used for applying low-E coatings to glass to improve building energy efficiency by reflecting or absorbing infrared light, which is heat energy. Pyrolytic coatings, also known as hard-coat, are applied during manufacturing through chemical vapor deposition (CVD). Vacuum (sputtering) deposition methods, also called soft-coat, are applied off-line by a fabricator, and are integral to single-source fabrication methods.

Both CVD and sputtering are effective techniques to help building enclosures control unwanted heat gain, as measured by SHGC, to improve energy performance and occupant comfort, as well as to reduce unwanted condensation. They are also used to control aesthetic and architectural variables, whether for vision or spandrel glazing or overhead glass assemblies. However, experts in glazing note that sputtering is a highly flexible technique that allows for a wide variety of product options for insulating glass assemblies.

A single-source fabricator tests and verifies—under a single entity—that an IGU meets the minimum criteria in these glass standards. In addition to specifying glazing to meet these critical specs, architects also must consider the aesthetic qualities of fenestration and glazing systems that meet the most rigorous performance needs. Two key drivers of architectural expression are palette and production quality, which are both controlled in the manufacturing and fabrication processes. Palette refers to the color and reflectivity of glass, as well as the application of graphics and design of framing, joints, and other related elements.

Photo © Brian Savage, courtesy of Viracon

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