When the skyscraper wars erupted in Chicago and New York in the 1920s and ’30s, architects began to consider how sunlight and solar heat gain through the towers’ extensive windows would affect occupant comfort. Thus, a revolution in glazing technology was ignited. Project teams began replacing leaky single-glazed windows with then-innovative double-glazed insulated glass units (IGUs) and incorporating iron-rich glass, with its signature green tint, to reduce solar heat gain for indoor climate control.
It wasn’t until the energy crisis in the 1970s, however, that the U.S. government began funding research for higher-performing glass to combat the amount of energy—and money—leaking out of windows. The Lawrence Berkeley National Laboratory (LBNL), in Berkeley, Calif., estimates that inefficient windows—essentially holes in otherwise-solid building masses—cost U.S. consumers $40 billion annually in energy loss.
The demand for efficient technologies has continued to grow since. Glass manufacturers now offer several high-performance products, from reflective coatings to dynamic glazing to address this need. These technologies give architects “a broad palette to design what they want in terms of aesthetics and energy efficiency,” says Tom Culp, president and owner of La Crosse, Wis.–based Birch Point Consulting, which advises clients such as the Glass Association of North America on energy-efficient windows, glass-coating performance, and building-code development.
As much of the Northern Hemisphere enters its cold season, here is a primer on how windows went from energy sink to energy efficient.
1860s: The Idea of Insulated Glazing
IGUs are among the earliest glass technologies designed to reduce heat transfer. Consisting of two lites separated by an air- or gas-filled space, double-glazed IGUs are now the standard in residential and commercial construction.
Robert Struble, communications manager at PPG Industries (formerly Pittsburgh Plate Glass), says that the concept of double glazing originated in the 1860s. An inventor filed a claim with the U.S. Patent and Trademark Office for an insulating glass product consisting of two glass panes separated by rope and bound with tar. “Of course, nothing happened with [that product],” Struble says. “Nobody used it, and it was not necessarily commercially viable.”
1940s and '50s: Double and Triple Glazing
In 1945, PPG developed one of the first commercially viable double-glazed IGUs in the U.S. Though they were originally used for Pullman railroad cars, the units found their way into buildings the following year, reducing U-factors from the 1-plus values of single-glazed units to 0.47.
As the use of IGUs increased, other glass manufacturers, such as Germany-based Schott and United Kingdom–based Pilkington, launched their own products. Some manufacturers ventured further into energy efficiency with triple-glazed IGUs, whose U-factors can be as low as 0.15. One downfall is their cost: It can take two decades or more to recoup the cost of triple-glazed windows through utility savings, reports the U.S. Department of Energy (DOE).
1960s: First Coating Technologies
The 1960s saw the beginnings of major advancements in glass technology. PPG developed the first coated architectural glass in the U.S. in 1963 using the same wet chemical deposition process to make mirrors, and refined its technique the following year to create its reflective Solarban product (which turned 50 this year). In its early years, the coating limited the amount of natural light entering the building. Today, coated glazing can have a visible light transmission coefficient of 0.70 and a solar heat gain coefficient of 0.25.
1980s: Low-E Coatings
The energy crisis in the 1970s sparked the development of low-emissivity (low-E) coatings. Pilkington and German firm Flachglas Group created the first commercially viable low-E coatings using thin layers of gold. But the coatings produced a green hue, leading German glass manufacturer Interpane to develop the first colorless low-E coating using silver layers in 1981.
That year, DOE-funded research conducted by the LBNL and Suntek Research Associates, now Southwall Technologies, which became part of Eastman in 2012, resulted in the commercialization of the nation’s first low-E coating for windows. By 1988, 20 percent of windows sold in the U.S. had a low-E coating, according to the DOE.
Today’s low-E coatings contain one or more silver layers that reflect the sun’s ultraviolet and infrared light to help maintain a comfortable interior temperature while allowing for visible light transmission. In the winter, when a building’s conditioned interior is warmer than the outside, the coating works in reverse, reflecting the heat back inside.
Applied when the glass is on the float line, passive low-E coatings, or hard coatings, are ideal for colder climates because they allow some of the sun’s shortwave infrared light to pass through. But most low-E coatings are applied using magnetron-sputtered vacuum deposition. A majority of these so-called soft coatings block nearly all infrared light, making them particularly well-suited for warmer climates and buildings with large expanses of glass.
“Low-E coatings are … good at reflecting heat that is radiated from the room or the environment,” says Dennis O’Shaughnessy, PPG’s associate director of glass research and development. Low-E coatings can come in single, double, and triple layers.
1990s: Window Shades and Fins
Awnings, louvers, fins, and other shading devices that mechanically attach to building façades also limit the amount of solar heat gain and glare entering through windows. According to the National Institute of Building Sciences, buildings outfitted with shading systems consume 5 to 15 percent less energy due to reduced cooling needs.
While shading devices have been around for centuries, contemporary systems are more functional and decorative. Examples can be found on the Innovation, Science and Technology Building, by Santiago Calatrava, FAIA, on the Florida Polytechnic University campus in Lakeland, Fla., and the JGMA-designed El Centro building on Northeastern Illinois University’s satellite campus in Chicago.
Firms producing innovative sun-shading devices today include: Architectural Grilles & Sunshades, in Mokena, Ill.; MG McGrath, in Maplewood, Minn.; and YKK AP America, in Austell, Ga.
1990s: Electrochromic Glazing
Electrochromic glazing is a dynamic system that has been around for at least 25 years, with most installations completed in the last decade. Examples include the Hamilton Garden at the Kimmel Center, in Philadelphia, by BLT Architects with Sage Electrochromics, and the H. Marcus Radin Conference Center, in Clovis, Calif., by Henderson Architectural Group with View Inc.
The glazing is coated with ceramic layers at thicknesses measuring in microns. When low-voltage electricity is applied, lithium ions move among the ceramic layers, darkening the coating. The tinted glass limits the amount of sunlight that enters the building, reducing solar heat gain and glare.
“The material also blocks out light from the sun in the near infrared spectrum ... so what you end up having is a tunable heat and light valve for your building,” says Helen Sanders, Sage’s vice president of technical business development. “You can let as much light or heat in as you want based on what’s going on outside and what you need in the building.”
Analysts project that such dynamic systems will gain significantly more market share in the coming years.
2000s: Thermochromic Glass
Thermochromic glass is another dynamic glazing system, but unlike its electrochromic cousin, it requires no wires or electrical components. Instead, the passive coating tints automatically when heated by direct sunlight, acting much like a mood ring does when exposed to body heat.
As the thermochromic film darkens, it blocks ultraviolet light and reduces solar heat gain on the building while still affording some view out. The technology can be integrated with other energy-saving solutions, such as low-E coatings.
Thermochromic glass manufacturers include RavenBrick, founded in Denver in 2006, and Jenison, Mich.–based Pleotint, which introduced its commercial thermochromic product, Suntuitive, in 2011.
2000s: Double-Glazed Facades
Though double-skin façades have been popular in Europe since the 1980s and ’90s, they didn’t catch on in the U.S. until recently due to their high construction costs. The cost of energy is higher in Europe, improving the return on investment for the technology there.
The concept involves erecting a single- or double-glazed curtainwall façade outboard of the building envelope, which can be made of any material. An air cavity between the two planes promotes natural ventilation, while also serving as a thermal buffer. The system is often used in conjunction with solar shades and other glazing technologies.
In 2009, the Cambridge Public Library expansion in Cambridge, Mass., by the Boston-based team of William Rawn Associates and Ann Beha Architects, became one of the first U.S. buildings with a European-style double-skin façade.
Today, PNC Bank in Pittsburgh is using the technology on its new building, the Tower at PNC Plaza, designed by the local Gensler office. While energy savings depend on the regional climate and the façade system’s design, Nana Wilberforce, PNC’s energy manager, expects that the double skin will help cut the tower’s energy consumption by half. “Our design goal is to make the greenest high-rise in the world,” he says.
2000s: Window Framing
Window frames have also evolved to meet increasingly stringent energy-efficiency goals. Aluminum’s durability has made it a preferred material for window frames, but its high thermal conductivity is a downside. To reduce heat transfer through the framing, manufacturers have developed thermal breaks or thermal barriers that fit within the frame structure.
According to the DOE, a low-E-coated, double-glazed window with a 1/2-inch air space in an aluminum frame without a thermal break has a U-factor of 0.70. The same aluminum window with a thermal break would have a U-factor of 0.52. If a wood or vinyl frame is used with a thermal break, the U-factor drops to 0.39.
Manufacturers making energy-efficient framing systems include YKK AP America and Schüco, whose U.S. business is based in Newington, Conn. “In the last couple of years, there’s been a real evolution in window framing, curtainwall framing, and storefront framing,” Birch Point’s Culp says. “It’s [often] still an aluminum frame, but there are different technologies embedded into it to separate it into two halves and block heat loss.”
2010s: Thin-Film Photovoltaics
While solar panels have been integrated into building structures for years, one of the newest energy-producing technologies is thin-film photovoltaics (PVs). Unlike other glass coatings that only reduce energy consumption in buildings, thin-film PVs can also harvest solar energy.
Thin-film PVs are made of many different materials deposited in thin layers on a conductive substrate. Cadmium telluride (CdTe) PVs are currently the only thin-film option that costs less than conventional crystalline silicon solar cells, but the toxic materials used in their production have raised concerns.
In August, Tempe, Ariz.–based First Solar, a leading thin-film PV manufacturer, announced that it had developed a CdTe PV that converted 21 percent of the sun’s energy into electricity, besting its own record of 20.4 percent energy conversion set in February. Other thin-film PV manufacturers include Bristol, Pa.–based Dunmore and Portland, Ore.–based Solo Power Systems.
Researchers at the Massachusetts Institute of Technology and other institutions are working to advance thin-film PV technology by developing solar cells that are undetectable to the naked eye and others that can be integrated into colored glass.
This article has been updated.