The Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) is working with manufacturers to bring a “super window” to market that is at least twice as insulating as 99 percent of the windows for sale today, potentially saving billions in wasted energy.
According to Berkeley Lab researchers, the “thin triple” super window design doubles the thermal performance of current Energy Star-rated double-glazed windows and is seven times more insulating than a single-glazed window. Berkeley Lab scientists have built and tested prototypes and are now working with Andersen Corp., the largest door and window manufacturer in the U.S., and separately with Alpen High Performance Products, which specializes in energy-efficient doors and windows. Both efforts are looking to build and test enhanced prototypes suitable for large-scale manufacture.
“Our approach is to attack the problem from two sides: to develop both ‘market pull’ and ‘technology push’ forces,” said Berkeley Lab researcher Steve Selkowitz, one of the inventors of the super window concept. “We are working with manufacturers to assist them with their technology challenges while also working with Energy Star, supply-chain companies and utilities, which can offer rebates and incentives for consumer purchase. Our role is to be a catalyst in facilitating technological innovation and an evangelist in promoting DOE’s energy-efficiency mission.”
Current double-glazed windows consist of two layers of glass with a low-E coating and argon gas in the gap between the glass layers to further reduce heat transfer. The Berkeley Lab super window sandwiches a third layer of very thin glass between the two layers of a double-glazed window, adds a second low-e coating, and replaces the argon gas with krypton, which is much more insulating than argon in the very narrow space between the panes.
While there are other triple-glazed windows on the market, this one is the same width and nearly the same weight as existing double-glazed windows. Because of that, the window sash and frame don’t have to be redesigned, which would have posed a significant cost obstacle for manufacturers.
Energy use associated with all aspects of window performance, including heating and cooling, costs U.S. building owners about $50 billion annually. The single largest contributor to building energy use is heat loss in colder climates. In warmer climates, sunlight transmitted through windows adds to air conditioning costs.
Berkeley Lab researchers want to get to the point where windows could outperform insulated walls in winter. With their thin triple super window, they say that goal may now be within reach.
“A single layer of glass has an R-value, which is its resistance to heat conduction, of R1; double glazing is R2, and the low-e double is R4,” said researcher Charlie Curcija. “Our thin triple glazing is R8 to R10, or about twice as good as the best low-E double glazing today.”
The super window was first invented and patented by Berkeley Lab more than 20 years ago. But at the time, there were no viable sources of large sheets of very thin glass, and the cost of the materials made it prohibitively expensive.
“The thin glass is 0.7 mm thick,” Curcija said. “Twenty years ago we couldn’t find any companies that could make thin glass sheets in volume. Now the flat screen TV industry has pushed the glass industry to create precisely the glass we need for windows, and at a price the window market can afford.”