A Guide to Energy-Efficient Glass

A Look at the Past, Present and Future of High-Performance Glass Solutions
Katy Devlin
October 13, 2014
The new LEED Platinum $250 million headquarters for TD Ameritrade in Omaha, Nebraska, highlights the energy-efficient capabilities of glass facade systems. The building maximizes daylighting and energy savings with its high-performance glass facade, rainscreen system and solar thermal array. The vision glass was triple-glazed insulating glass units with PPG Solarban 70XL low-E coating on the No. 2 and No. 5 surfaces. The rainscreen glass was 1/2-inch laminated glass with a white PVB interlayer. Oldcastle BuildingEnvelope was the glass fabricator; Sota Glazing the custom curtain wall manufacturer; and Architectural Wall Systems was the glazing contractor. Photo by Tom Kessler Photography.

The building industry has worked to improve the energy efficiency of glass since the Civil War, when in 1865, inventor Thomas D. Stentson patented the first insulating glass units using rope as a spacer and tar as adhesive. However, it wasn’t until the early 1980s, with the development of low-emissivity glasses, that the energyefficient glass movement truly gained momentum.

The last three decades have seen a proliferation of energy-efficient glazing solutions, with ever-improving low-E glasses, the introduction of dynamic glass technologies and new IGU solutions. And the glass industry has responded, with a range of products to satisfy architects’ thermal, solar and visible light requirements.

This guide presents a comprehensive look at the energy-efficient glass market, from the release of the first low-E glasses until today, and provides a glimpse at the advancements to come. View an interactive timeline of energy-efficient glass milestones, and a product guide to the most recent energy-efficient glass products.

Editor’s note: This guide focuses on energy-efficient solutions for glass products, and does not include high-performance solutions for framing or insulating glass components. Product information for those segments can be found elsewhere on GlassMagazine.com.

High-performance history

One of the major milestones in the history of energy-efficient glass products was the introduction of coating on glass. “Coating on the glass didn’t happen until 1963. And in 1964, PPG released Solarban, with a light- and heat-reflectant coating that blocks both infrared and visible light,” says Rob Struble, brand and communications strategy for PPG Industries.

The availability of reflective coatings led to the boom of the highly reflective glass-clad buildings of the late 1960s and 1970s.

The industry’s next big leap came on the heels of the energy crisis of the 1970s. Development of low-E began in the United States in 1976 with a partnership between Lawrence Berkeley National Lab, and Southwall, www.southwall.com, funded by the U.S. Department of Energy. That partnership developed the first low-E glass in 1981, and by 1988, low-E windows captured 20 percent of the residential market in the United States, according to the DOE.

Low-E glass features nanocoatings on the surface of the glass that block unwanted parts of the light spectrum (infrared), while allowing visible light to pass through. The commercial introduction of spectrally selective low-E glass proved to be a game changer for energy-efficient glass, sources say.

“When you look back to the ‘80s, when low-E came out, that was a huge breakthrough for glass performance,” says Keith Boswell, technical director of the San Francisco office for Skidmore, Owings & Merrill LLP.

“Since the launch of low-Es, the coatings have just become more and more sophisticated,” Struble adds. “We have seen the trend toward allowing higher visible light, while continuing to lower solar heat gain.”

The late ‘80s and early ‘90s saw the influx of soft- and hard-coat low-Es, and in 2000, post-temperable low-Es came onto the market, allowing fabricators nationwide to more easily process low-E glasses.

“The introduction of post-temperable products made high-performance coatings available in local and regional markets. [Architects] now had multiple companies they could get bids from, with reasonable delivery times,” describes Glenn Miner, director, construction, PPG Flat Glass. “This created exponential growth of low-E in commercial buildings to the point where you’d be shocked if you saw a commercial building going up in North America without low-E glass.”

The coatings improved, with manufacturers introducing low-Es with double silver layers, and, in about 2005, triple silver layers. Companies also worked to improve color and clarity, Miner says. “These coatings on the glass are 1/1,000 the thickness of a human hair. In any triple silver product, you might have 12 to 15 layers of material. Only three of those layers are silver, giving manufacturers opportunities to reduce the haze, improve the color and adjust other features that have to be considered,” he says.

“We must be mindful of the energy code requirements; [however] aesthetics [remain] a key factor to the architectural community, and to the broader implementation of the highest performance coatings,” adds Serge Martin, vice president, marketing, for AGC Glass Company North America.

In recent years, the glass industry has started to ‘max out’ the potential improvements in solar heat gain reduction and visible light transmittance with traditional low-E products, sources say, making the development of alternative energy-efficient glass solutions even more critical.

“Given that we have reached the limits of physics in terms of light to solar heat gain ratio, incremental improvements in solar control are likely to come from tuning the key aesthetic
performance parameters around the triple silver LSG performance to provide products with a range of colors and visible light transmissions, which we see happening currently,” says Helen Sanders, senior vice president of operations at Sage Electrochromics.

Glass companies are also working with architects to optimize glass performance based on geographic location and building orientation. “We find that architects are beginning to take advantage of the solar heat gain. In certain climates, and with certain building designs, architects want to have that SHG in the winter,” Struble says.

“What is the most energy efficient design? It’s the most economic use of heating as well as cooling, which means some climates want the passive low-E that allows some heat gain,” Miner says. Manufacturers are currently working toward another series of low-E products that will provide peak solar control for maximum efficiency in static glass systems, he adds.

During the last 30 years, performance improvements to glass coatings have come hand in hand with improvements to the complete insulating glass unit. While the introduction of low-E glass into an IGU greatly improved solar performance, the addition of argon fill in the mid to late 1980s provided a jump in thermal performance. IGUs with warm-edge spacers entered the market, offering improved thermal performance for certain applications, and triple insulating glass units became a sensible solution in some geographic regions.

Dynamic glass

As traditional low-E glasses reach the limits of SHG performance improvements, the industry is building a growing range of dynamic glass possibilities. “The major developments or evolutions [for energy-efficient glass] will likely come in the field of dynamic glazing—electrochromic, thermochromic, etc.,” Sanders says.

Dynamic glass products have reached a tipping point in recent months, with large scale manufacturing coming online for several manufacturers. However, the technology goes back decades.

In 1989, Sage was established with the goal of developing a smart electrochromic glass solution that offers both a clear state and tintable state. By 2003, the company had completed its first commercial installation. In the last several years electrochromic glass reached large-scale production with new facilities from Sage and View.

Thermochromic dynamic glass manufacturers entered the market several years later. RavenBrick, www.ravenbrick.com, developed its first RavenWindow thermochromic prototype in 2007; and Pleotint launched its Suntuitive thermochromic product to the market in 2011.

“Without question, dynamic glass is the next step in the evolution of energy-efficient glass,” says Geoff Brown, project manager for Pleotint. “When utilized in an insulating glass unit, and paired with the latest low-E coatings and gas-filled cavities, dynamic glass enhances those energyefficiency benefits while also helping buildings and homes preserve views, optimize natural daylight, and further reduce energy costs.”

Looking ahead

Driven by demand from the building community, rising energy costs and toughening energy code requirements, glass manufacturers are focused on R&D to develop better-performing glass solutions. The energy-efficient glass industry of tomorrow will feature more dynamics, triple IGUs, new low-Es (including new fourth-surface products) and warm-edge technologies. And, it will likely include emerging product technologies such as vacuum glazing.

“The latest energy codes (ASHRAE 90.1-2013 and IECC-2012), as well as the ‘green codes’ (ASHRAE 189.1-2014, and California’s Title 24), will lead to double glazing, thermally broken frames and glass with low-E being essentially required in all climate zones,” says Chris Dolan, director of marketing, North America Flat Glass, Guardian Industries. “This is based on use of the prescriptive codes and contingent of adoption of the new codes by the individual states. Triple glazing will be required in the far North, and argon and warm-edge spacers will continue to increase in usage. Interior surface coatings will grow as an option to triple glaze on some projects or to create a super-performing triple glaze IG unit.

The industry is also eyeing the energy-saving potential of retrofits in the existing building stock. “When talking about the future of energyefficient glass, the real question is how to best utilize those technologies, not only in new construction, but also in window and curtain-wall retrofit projects,” says Mike Nicklas, business development manager, J.E. Berkowitz LP/Renovate by Berkowitz. “There are thousands of aging buildings across the country that still have original, single-pane glass in their windows and curtain walls. … There are costeffective ways to update a building’s original windows and curtain walls to significantly improve their energy, thermal and acoustic performance.”

As the industry works to continue to improve energy performance, it is working equally hard to ensure the betterment of aesthetics and optics. “At the end of the day, looks are still everything. Construction is a tremendous combination of performance and art,” Miner says.

Katy Devlin is editor for Glass Magazine. E-mail Katy at kdevlin@glass.org.

  • The Terminology of Energy-efficient Glass

    The measure of the rate of non-solar heat loss or gain through a window system in terms of Btu/hr.-sq. ft.-ºF. The lower the U-factor, the greater the resistance to heat flow, meaning a better insulation.

    The measure of the resistance of a glazing material or fenestration assembly to heat flow. The inverse of the U-factor or R = 1/U.

    The fraction of the solar radiation admitted through a window or skylight both transmitted and absorbed, and released inward.

    A fraction of the visible light spectrum transmitted through glazing. The higher the VT, the more visible light transmittance.

    The ratio between theW SHGC and VT. A gauge of the relative efficiency of glazing in transmitting daylight while blocking heat gains. The higher the number, the more light transmitted without adding excessive amounts of heat.

    SOURCES: Bill Lingnell, technical director, Insulating Glass Manufacturers Alliance (U-value, R-value and SHGC). U.S. Department of Energy (VT and LSG).