In 1976, in response to the energy crisis, DOE began a program at Berkeley Lab to examine the potential of more efficient window technologies. Lab researchers faced a number of daunting challenges including developing the coatings, documenting how they could be used in a window, demonstrating their effectiveness, and finally convincing consumers that these invisible coatings were important.

Berkeley Lab's initial goal was to develop a clear scientific understanding of the heat transfer mechanisms in windows and identify the technical opportunities for reducing those gains and losses. The concept of low-emissivity, or "low-e," coatings—thin metal or metal oxide films—had been around since World War II, but no U.S. manufacturer had yet developed a product. Low-e coatings are heat mirrors; they allow visible light to pass through, preserving occupants views of the outdoors, but reflect back long-wave infrared radiation, from heat sources such as buildings' heating systems.

Berkeley Lab awarded subcontracts to several firms to develop prototype coatings and new, low-cost, thin-film deposition processes. All of the performance testing of the coatings was conducted at Berkeley Lab and lab researchers developed a new computer model (WINDOW, now in its fifth version) to determine the best use of the coatings in the overall window system.

Encouraged by these efforts and initial product introductions by a few innovative firms, several large manufacturers began investing in low-e window development. By 1982, these major manufacturers began to offer products of their own. By the mid-1980s almost every U.S. window manufacturer was offering low-e windows, and in 1987, low-e windows accounted for 17% of window sales. With many different products on the market, a standardized system for rating the energy performance of windows was needed. In 1989 Berkeley Lab developed the National Fenestration Rating Council (NFRC), a non-profit organization that established energy-rating procedures for windows and developed a labeling system to communicate the information to customers. Most states in the U.S. require all window products to be rated by the NFRC and around 100,000 products have been rated so far. Berkeley Lab scientists remain involved with the board and committees of the NFRC.

Berkeley Lab researchers continue to develop improved energy-efficient window designs. In XX year, they demonstrated that replacing air between two windowpanes with an insulating gas, such as argon, significantly decreased heat loss in cold climates. Computer simulations, as well as test data from the lab's Infrared Thermography facility (see photo) and from actual homes, helped convince manufacturers to incorporate this technique into their products and to inform purchasers that this was a reliable, cost-effective product. When leading manufacturers were interested in pushing the technology further, Berkeley Lab researchers developed a new "superwindow"—a double-paned window with two low-e coatings and a new krypton gas fill that actually preformed better than an insulating wall. A couple of years later, "superwindows" hit the market. More recent inventions include highly insulating aerogel windows and gas-filled panels, an insulation technique more effective than foam. Berkeley Lab won an R&D 100 award for this technology, and has granted an exclusive manufacturing license to a private company.

Forty percent of all windows are in commercial buildings, where cooling and lighting costs are of greatest concern. Building on the heat-reflecting properties of low-e coatings, Berkeley Lab has developed spectrally selective glazings for hot climates. These transparent glazings allow in visible light but filter out both long-wave and infrared radiation from the sun, reducing air conditioning demands.

Berkeley Lab has taken the lead in testing products, analyzing benefits and conducting general studies on the impact of energy coming in and out of windows. To encourage more homeowners and consumers to buy energy-efficient windows, Berkeley Lab developed Energy Star labels for windows in 1997. Berkeley Lab researchers have also written a book and partnered with the University of Minnesota and the Alliance to Save Energy to create a website that contains tools to calculate the performance of different windows in each U.S. state. When approached by both small and large manufacturers, Berkeley Lab provides them with information, advice and technical assistance in producing the best windows possible.

The Berkeley Lab windows team's most recent invention is the switchable electrochromic window, which changes color when a small voltage is applied to a chamber filled with gas. This flexible and dynamic technology can be manually or automatically controlled and can be integrated with other building systems, such as lighting and heating/cooling mechanical systems, to optimize interior environmental conditions, occupant comfort, and energy-efficiency.¹

As the key advisor to The New York Times in the construction of their new, all-glass headquarters building, Berkeley Lab has brought to bear many unique skills. Lab researchers have both important relationships with industry players and a track record of objectivity. When The Times was told by others that its pioneering vision for a highly energy-efficient building was impossible, Berkeley Lab proved that all the technology could be made to work reliably and produced economically. The Lab supported manufacturers in improving the performance level of their products and provided contractors with extensive research reports that reduced their risk and, as a result, significantly lowered costs. This project has inspired the creation of other all-glass buildings, notably the Freedom tower and a new Bank of America building. Those in charge of constructing these buildings are also receiving advice and support from the Berkeley Lab windows team.

¹ "Advancement of Electrochromic Windows."