Thin Triple Glazing Can Meet Increasingly Stringent Codes and Efficiency Programs and Will Inevitably Expand to the Commercial Sector

Beginning with the 1970s energy crisis, approaches to achieving energy conservation by minimizing heat loss through building envelopes focused on what amounts to brute-force methods in pursuit of ever-lower U-factor. These efforts included attempts at severely reducing glazed areas, thicker wall insulation and double-glazed windows, evolving in the late 1980s to triple-glazed.

More recently, such efforts have shown a bit more technical finesse. A prime example is the advent of thin triple glazing. Today’s cutting-edge thin triple design sandwiches a third layer of very thin glass (on the order of 0.7- to 1.2-millimeters thick) between the two outer lites of glass (up to 3.2-mm thick), adds a second low-E coating (applied to glass surfaces two and five, counting from the outermost face as surface one), and replaces argon gas filling with the more highly insulating krypton. These designs are replacing an earlier interim approach in which the center “lite” was a thin film.

ALTHOUGH TRIPLE GLAZING NOW REPRESENTS ONLY 2 TO 3 PERCENT OF THE WINDOW MARKET, PRIMARILY RESIDENTIAL IN COLD NORTHERN REGIONS, THERE IS PRESSURE TO GROW THIS SHARE TO MEET INCREASINGLY STRINGENT U-FACTOR REQUIREMENTS IN CODES AND EFFICIENCY PROGRAMS.

A LOOK AT TODAY

Currently, most builders end up choosing triple-glazed windows with a U-factor, ranging from 0.19 to 0.26 (1.08 to 1.48 in SI [metric] units)—a great improvement over the U-factor of about 0.50 (2.84 in SI units), common for early double glazing without a low-E coating and insulating inert gas.

Although triple glazing now represents only 2 to 3 percent of the window market, primarily residential in cold northern regions, there is pressure to grow this share to meet increasingly stringent U-factor requirements in codes and efficiency programs, such as ENERGY STAR.

Specifically, energy performance criteria as set forth in the new proposed U.S. ENERGY STAR 7.0 specification would offer two options for the U.S. Northern Zone: a prescriptive value of no more than 0.22 U-factor (a significant reduction from the current 0.27 value) or a range of 0.23 to 0.26 under the equivalent energy-performance options (compared to current values of 0.27 to 0.30). In Canada, a maximum U-factor of 0.21 (1.19 in SI units) is called for and/or an Energy Rating of at least 0.34. The International Energy Conservation Code of 2021 imposes similarly strict criteria, as do various state codes (notably California).

Also, because solar gain helps heat a house during the winter, many cold-climate homeowners and builders seek windows with a high Solar Heat Gain Coefficient (SHGC), in addition to low U-factor. The proposed maximum permitted SHGC for ENERGY STAR 7.0 in the Northern Zone is reduced to no more than 0.17 prescriptive or either 0.35 or 0.40 under the equivalent performance option. By comparison, current first-generation triple-glazed units have a SHGC, ranging from 0.39 to 0.47, while some thin triples can offer lower SHGC values with many low-E coatings available today.

Thin triple glazing with krypton gas infill, rather than the currently common argon, can meet the most stringent of these requirements. Such a unit would in fact be necessary to obtain ENERGY STAR “Most Efficient” designation for use in the Northern Climate Zone. Current leading-edge thin triple designs turn in U-factors of 0.15 or less (0.85 in SI units)—and all at a total insulating glass thickness and overall weight similar to that of double-glazed units.

BECAUSE SOLAR GAIN HELPS HEAT A HOUSE DURING THE WINTER, MANY COLD-CLIMATE HOMEOWNERS AND BUILDERS SEEK WINDOWS WITH A HIGH SOLAR HEAT GAIN COEFFICIENT, IN ADDITION TO LOW U-FACTOR.

The engineering infrastructure for widespread thin triple application is being prepared to keep up with product development. For example, a key code reference is ASTM E1300 (2016), Standard Practice for Determining Load Resistance of Glass in Buildings, an update of which is in development and will consider the inclusion for thin triple insulating glass in future updates.

A LOOK AT THE FUTURE

Given ENERGY STAR mandates and code development trends, there is great potential for growth in thin triple demand. Although earlier triple-glazed units have typically been too heavy, too thick and too expensive (resulting in too long a payback period), especially for replacement applications, thin triple technology has the potential to cure these shortcomings.

Regarding cost, as the volume of thin triple products increases, the price premium is expected to come down. Also, if better windows allow HVAC to be downsized, the savings opportunity can help offset the higher initial cost.

About the Author

Bill Lingnell
Bill Lingnell serves as the technical consultant to the Fenestration and Glazing Industry Alliance. He has consulted about many facets of glass and wall systems used on architectural construction projects.

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