Prior to the fourth century A.D., windows were typically holes in walls, usually covered with animal hides, cloth, wood, or shutters. In some places, translucent animal horns, thin slices of marble, or even paper were set in frameworks of wood, iron, or lead.
A fragment of a Roman window glass plate dating to the first to fourth century A.D. indicates that the Romans were the first to use glass to fill windows. And the first record of glass making, by the Roman historian Pliny, suggests that it occurred around 5000 B.C. in Syria.
However, until the 1700s, the majority of glass making was devoted to containers, solid beads, and amulets, due to the limited technology. Techniques developed enough that by the third century, thin rectangular window panes could be produced by shearing through one side of a blown-glass cylinder. This gave rise to tall narrow windows. Glass became common in the windows of ordinary English homes only in the early 17th century.
The French improved the manufacturing of flat glass in the beginning in the late 17th century and were joined by the Germans in the 18th and 19th centuries. Laminated glass was invented in 1903, increasing the safety and security of much larger windows, permitting glazing. The industrial plate glass making processes made possible modern-style floor-to-ceiling windows.
The Crystal Palace, the site of the Great Exhibition of 1851, became the most famous glass structure because of its immense number of plate glass windows, revealing England’s latest glass manufacturing technology. England had repealed two duties on windows and window glass shortly before the Exhibition, allowing the Crystal Palace to cost about 50% less than it would otherwise have.
Technology Transforms Windows
The first major innovation in glass windows came about 100 years later with the introduction of the curtain wall. A wall of aluminum-framed glass, it was first installed in the Lever House in 1952 and was made possible by steel construction.
Window film was commercialized in the late 1950s and 1960s. According to Gary Clark, architectural business manager at Solar Gard, a major manufacturer of window film, the first films were in liquid form which flowed over glass. It blocked some glare, he says, but there was not much solar control. The next step was dying one or two micrometers of polyester and installing with glue sprayed on glass and overlaying the film on it.
The next development in the 1970s was metallization. Aluminum was heated and vapor coated on film to different densities and visible light transmissions. “This was the look we remember on large office buildings with shiny glass windows,” says Clark.
Sputtering evolved in the late 1970s and became common in the 1980s with the use of ceramics, nickel, titanium, and copper to name a few. A solid block of one of the metals is hit by ion bombardment, in which molecular pieces are knocked off and embedded in polyester, which is then applied to the glass.
The most effective metal today, says Clark, is silver or gold spectrally selective films which permit some portion of the solar spectrum to enter a building. While both are more efficient, they are more costly to use than the other methods, with sputtered gold being the most expensive.
Smart Windows Evolve
Smart windows became the next generation of technology to reduce solar heat gain. They can change visible light transmittance and solar heat gain coefficient (the fraction of incident solar radiation admitted through a window). There are three major types: electrochromic, thermochromic, and photochromic. Liquid crystal and suspended particle devices are additional types but they require continuous high voltage AC and their failure mode is dark. The electrochromic type uses low voltage, low energy consumption, and a failure mode of clear. Thermochromic and photochromic windows change properties based on ambient temperature and light respectively.
California was one of the first states to introduce building energy efficiency standards in 1978. These standards included requirements for window film and double window glazing in residential housing and requirements for maximum connected lighting loads in non-residential buildings. These standards were guided by work at the Lawrence Berkeley National Laboratory, where, since the 1970s, research into both window tinting and highly insulating windows has been carried out. They have been updated regularly since 1978.
In general, electrochromic windows are installed only on buildings under construction because they require installing new frames as well as dynamic glass. Window tinting is used primarily on buildings already built, especially old buildings. Tinting may also be applied to second and third panels in dynamic windows to reduce solar heat gain. Thermochromic and photochromic windows can be installed on either new or older buildings because they are installed from the inside over existing frames.
According to the US Department of Energy, today’s energy-efficient windows with low-E coatings are vast improvements over the standard windows that were sold just 25 years ago. But windows are still responsible for over 4 quads of annual energy use, costing building owners around $40 billion per year. In many cases windows can become net providers of energy to buildings, adding solar heat in winter and daylight to offset electric lighting year round. What follows are profiles of three companies which have led in the development of these technologies.
Window Tinting Is Big Business
Solar Gard, a manufacturer of window film headquartered in San Diego, CA, began in 1977 and became a subsidiary of the French company Saint-Gobain in 2011, says Clark. “It’s been a really good fit with Saint-Gobain,” he says. Saint-Gobain has depth and history—it was founded in 1665 when window glass-making was born.
Retrofitting pre-2000 windows is the biggest market for solar control window films, Clark says. In particular, it is used for old buildings on university and college campuses in the northern wintertime cold where facilities managers don’t want to alter the look of buildings and want to avoid the higher cost of new windows and frames.
Solar Gard manufactures a series of window films using various metals. It also manufactures different types of film for different types of glazing. For example, tempered safety glass is heated to 1,250°F and cooled quickly. Annealed glass is heated in the range of 850°F to 900°F and cooled slowly. It shatters in large pieces more easily than tempered glass which shatters in tiny pieces. “Whether single-pane, dual-pane, annealed, or tempered, Solar Gard has an appropriate product for every glazing option,” says Clark.
Panorama CX is sputtered with ceramics, an inert material which is neutral, non-reflective, with no color added to the film. It rejects up to 58% of total solar energy gain. Low visible reflectivity maintains views inside and outside and provides a natural exterior appearance. It is popular in hot and coastal areas and can reduce air conditioning costs.
Panorama Hilite 70 is silver-based and is Solar Gard’s most efficient and better-performing product at 70% light transmission and 55% solar rejection. It avoids the problem of nighttime reflectivity and does not alter the color of light. This film series can be applied to either single- or double-pane glazing and specific cases can make a single pane window as efficient as a double pane window, Clark says.
Ecolux low-E film is designed to repel heat in the summer as well as retaining radiant heat indoors during winter months. It was developed as the demand was increasing from the government and local public utility commissions to improve R (thermal resistance) and U (heat transfer) values in buildings and homes to prevent winter heat loss.
Clark says Ecolux low-E film, with its greater performance and appearance, is made with precious metals and thus is a bigger investment than Silver AG low-E, which is made with more conventional metals and has a more reflective look.
Clark says Silver AG low-E may be more effective in residential buildings. In a building over 50,000 square feet there will be a lot of people and equipment generating heat, so Ecolux low-E with its lower heat rejection numbers may be more appropriate.
Clark also discussed the safety security window film Solar Gard manufactures. Armorcoat is very thick, clear plastic used for protecting windows during hurricanes. Silicon caulking is applied around each frame and the edges of glass to tie the frame and glass to the building, he adds.
While solar film is 2 mil, Armorcoat thicknesses start at 4 mil and increase up to 14 mil. Awareness for the need for safety film began after the Oklahoma City bombing. “After 9/11, Solar Gard manufactured and installers covered many Capitol Hill buildings with Armorcoat,” he says.
Solar Gard also offers stainless steel films which have a defused, reflective, silver-grey tone and are more affordable. Lower visible reflectance varies from 9 to 48%, depending on the option. Views to the outside are maintained while it rejects high solar heat gain, ranging from 75 to 43%, again depending on the option.
Solar Gard manufactures its Graffitigard or sacrificial film which is applied on the outside of the existing glass windows to protect against graffiti. Graffitigard can be removed and replaced in some cases up to five times for the same cost as replacing the window. It is commonly used in shopping malls, gas stations, and transit system windows.
Solar Gard also offers decorative films and can create custom graphics to lay on the film and then apply them on windows or walls for businesses such as holiday designs, and even frosted glass tints for conferences rooms.
Something Different—Installing Internal Windows
Thermolite, located in West Bend, IN, was established in 1977 and markets exclusively to existing buildings, says president Steve Champlin.
In contrast to other companies, Thermolite customizes internal windows which fit into the existing frames rather than replacing them. Champlin argues that this type of installation can cost one quarter of the cost to replace windows and frames. Furthermore, these internal installations protect the integrity of the building design.
Steel or aluminum frames in older buildings are usually cold to the touch, meaning air is passing through them because they have not been sealed as in new buildings. When another window and insulating frame is installed behind the existing window inside the building, the thermal bridging is reduced and any cracks are sealed to reduce air infiltration, Champlin says.
Thermolite provides two basic window designs: a monolithic or single-pane silver glass with low-E coating, and an insulated double-pane gold glass with low-E film and argon inserted between the two layers of glass. It also offers the RetroWAL, a high-performance curtain wall retrofit system, which is also installed on the inside of the existing glass.
Champlin says the company visits the building to be retrofitted before the job starts and measures everything to manufacture the new windows to custom measurements. It buys the plain glass and can use any type on the market.
Champlin explains that a low-E pyrolytic silver coating is applied on the interior side of the glass facing outward that is to be installed in the new frame. If a double-pane window is being installed, argon is injected in the space between the two panes. This new low-E window reflects infrared rays back outside while allowing the visible light to come through. While it may be 24°F outside, the typical space between the old and new windows may be 48°F while inside the room the temperature is 72°F.
Additional solar heat gain can be controlled in double-pane windows by using “between glass aluminum blinds,” which can be controlled by the push of a button on the window frame. Or, window film can be laminated on the opposite side of the pane with the low-E coating or, in the case of a new single pane, on the inside of the existing building glass. Champlin says often the existing windows already have film.
Low-E Glass Has Other Virtues
Some of the alternate advantages of low-E laminated glass include reducing external sound and shielding a lot of radio frequencies, says Champlin. It attenuates wireless routers, cell phones, cordless devices, and protects data in communication devices from outside the building. These advantages are a plus for marketing to banking and military installations.
Champlin also explains that Thermolite’s laminated glass will protect against bomb blasts and hurricanes. The Indigo Hotel in New Orleans sits next door to an identical building which did not have the new laminated windows Thermolite installed in the Indigo Hotel. During Hurricane Isaac about 15% of the windows in the building next door were blown out by the wind. The Indigo Hotel had no broken windows, Champlin says. He explains that the negative suction caused by the hurricane and oscillations in the windows battling the positive forces causes the windows to break. The very air tightness of the Thermolite windows did not allow the negative pressure to push out the glass.
Laminated glass is factory tested with projectiles like 2-by-4 wood beams thrown at it to guarantee it will not break. “Utility bills for electricity and gas have been reduced in government buildings where we’ve just installed monolithic, or single pane, low-E windows,” says Champlin.
By reducing heating and cooling with these windows, “I can look at 25% savings in any building,” says Champlin. “Not only do we reduce heating and cooling for buildings heated by electricity, peak demand is reduced 30% resulting in reduced costs in the following years,” he concludes.
71Above Restaurant Wows With Smart Windows
“Every day, it’s mesmerizing up here and part of our customers’ experience,” says Alex Hasbany, general manager of the 71st story restaurant atop the US Bank Tower in downtown Los Angeles. The windows—that offer a 360-degree view of the city—were manufactured by SageGlass and are a conversation piece and an icebreaker, he says, as they automatically increase or decrease the tinting to control the amount of sunlight coming through the windows depending on the time of day and the amount of glare.
The 71Above restaurant opened in April 2016, following the completion of the US Bank Tower. The owner of the restaurant, Emil Eyvazoff, and the architect, Tag Front, selected Sage Glass two years ago when Eyvazoff first showed Hasbany the space.
Light sensors are installed on the building’s helipad and take readings hourly of the position of the sun, the glare factor, and the time of day which are relayed to the automatic controller. Hasbany says SageGlass set the controls for three stages of tinting: the first clear stage lets 20% of the sunlight in. The second stage lets 6% of the sunlight in and the third stage lets 1% of the light in.
Hasbany says customers couldn’t have lunch or happy hour in the restaurant without SageGlass dynamic windows, due to the heat buildup. The restaurant’s bar is situated so that it faces west and offers “unbelievable views” of the sunset, he says, and adds, “Most customers are blown away.”
Many customers are curious about the windows and some think they have curtains. There are overrides to manually control the tinting and staff may use them to show the tinting off to customers on request, Hasbany says.
SageGlass reports that its dynamic windows block up to 91% of the solar heat, dramatically reducing the need for air conditioning. Hasbany has the evidence. When the restaurant is opened in the morning the glass is clear and the temperature rapidly increases. “Since we’re in a giant fishbowl, it gets really hot quickly,” he says. As soon as the controls kick in the temperature goes down.
Long Gestation for Electrochromic Glass
SageGlass began developing its electrochromic glass tinting technology in 1989 and installed its first installation in 2003 at the Desert Regional Medical Center in Palm Springs. It took a long time to develop the patented technology, says Derek Malmquist, vice president of marketing. SageGlass was acquired in 2010 by Saint-Gobain.
SageGlass includes standard double- and triple-pane configurations in a range of sizes, shapes, and colors that help to increase buildings’ energy efficiency. A special coating on the inside surface of the pane facing outward allows the glass to tint, absorb, and reradiate sunlight outward after receiving a low-wattage stimulus.
SageGlass claims that, due to the energy efficiency of its glazing products, heating, ventilating, and air conditioning systems can use up to 25% less energy, often allowing HVAC system designs to be down-sized. The company can help designers and architects to qualify a building for Leadership in Energy and Environmental Design (LEED) certification credits.
Electronic Controls Are the Brains
Electronic controls connected to low-voltage wiring are the brains of the dynamic glass, says Malmquist. They control the amount of tinting window panels take on, either automatically from a central controller, or at a local wall switch with two buttons that employees can control to modify the tinting. Controllers can also be located in different places around the building and connected together via the building management system.
A cable that extends from individual double- or triple-pane insulating glass units—called a pigtail—controls the tinting, says Malmquist. The pigtail is connected to a cable that runs through the framing system to low voltage wiring on the interior of the building that leads to the programmable automatic controller and to daylight sensors located outside the building. The daylight sensor detects the amount of light available and where the sun is located, based on the time of year. This information is fed back to the controller.
SageGlass can be programmed to tint the dynamic windows to four different levels, based on what customers want, explains David Pender, director of operations, from 60% which is almost clear, to 20%, or 6%, or to 1%. These percentages are equivalent to visible light transmission. For example, Pender says, very early in the morning the lowest windows can be untinted, and, as the sun rises, tinting is increased in those and in the upper panes.
Lighting and electrochromic control systems can also be integrated if the lighting systems offer that capability. A SageGlass mobile app is also available for iOS and Android devices for daylighting controls.
Window shapes and sizes can also be customized. SageGlass dynamic glass can come up in sizes to 5 feet by 10 feet per panel in either new construction or retrofit projects. Shapes include parallelograms, trapezoids, and triangles. Each pane of electrochromic glass can be divided into three zones, known as a SageGlass LightZone, in any combination of clear or tinted colors.
Pender says on a couple of occasions, customers have asked SageGlass to create scenes in skylights. For example, at the Mall of America in Minnesota during the holidays for the past several seasons, the company programmed holiday scenes in the skylight, including a Santa Claus one year and a Christmas tree the next. Even the US Naval Academy asked the company to program “Go Navy” in the skylight of a multi-story building, visible from a bridge, before an Army-Navy game.
Smart Windows Add to Sustainability
The Sheward Partnership worked closely with the developer and contractor to design and qualify a rebuilt building for LEED certification that became Saint-Gobain’s North American headquarters. The developer chose to tear down to bare steel and studs and rebuild a 1960s-era building located in Malvern, PA, that had been unoccupied for 15 years. CertainTeed, the company’s construction arm, is also headquartered there. The building was completed and commissioned in October 2015.
Michael Pavelsky, senior associate and sustainability director at Sheward Partnership says, “We looked at the whole sustainability of the building by designing for energy efficiency.” Early in the design process the group evaluated a number of glazing products and used 3-D computer simulation energy modeling to evaluate each product.
They studied energy performance, indoor comfort, operations, and maintenance. In the end, they chose SageGlass, Pavelsky says. Indoor comfort is enhanced by reducing heat gain and reducing glare coming through the windows. That glare often masks computer screens making it hard for users to view them. There is a big benefit for operations and maintenance, he says because there are no window blinds to repair and replace.
Chloe Bendistis, the sustainability project manager at Sheward Partners says it was necessary to tailor the design to Saint-Gobain’s needs. For example, white boards were installed in office areas and the glare affected the users’ ability to write on and view the content on the boards. She says the problems were solved by adjusting the tinting algorithms in the software that control the electronics which lighten or darken the windows.
Pavelsky says users can control the tinting of the SageGlass dynamic windows in their area through an App on their iPhones or flipping the wall switch to the degree of tint that they want. Otherwise, the central controller modifies the tinting color based on set algorithms adjusted by the amount of sunlight coming in. The exterior light sensor is located on the building’s roof to track the sun and feed the data back to the controller. Pavelsky says there is one sustained design based on four standard colors for the whole curtain wall containing different sized windows.
Bendistis says the occupants love the environment. The interior design was laid out so the majority of the employees would have access to outside light. Even when it is very sunny they still have exterior views.
Delia Milliron, Ph.D., an associate professor at the University of Texas at Austin, is heading up research at the Milliron Group on colloidal nanocrystals, their integration into novel nanomaterials, and applications in next-generation electronic devices and energy technologies.
According to a report published by the McKetta Department of Chemical Engineering at University of Texas at Austin, Milliron and her team of researchers have invented a new flexible smart window material that, when incorporated into windows, sunroofs, or even curved glass surfaces, will have the ability to control both heat and light from the sun. The article about the new material was published in the September 2016 issue of Nature Materials.
The low-temperature process generates a material with a unique nanostructure, according to the report, which doubles the efficiency of the coloration process compared with a coating produced by a conventional high-temperature process. It can switch between clear and tinted glass more quickly using less power.
The next challenge for the Milliron Lab, according to the August 2016 report, is to develop a flexible material using their low-temperature process that meets or exceeds the best performance of electrochromic materials made by the conventional high-temperature processing.