Low-E Glass Gains Ground

Ten years ago,

EBN reported on the growing popularity of low-emissivity (low-e) glazing, which allows visible light to enter buildings while reducing unwanted heat gain and heat loss. Introduced in the early 1980s, with the energy crisis still fresh in America’s mind, low-e glazing gained market share quickly, and by July 1996 the glazing industry had produced its billionth square foot (see

EBN

Vol. 5, No. 4). Low-e glazing has been building momentum ever since, with sales now approaching one billion square feet (93,000,000 m²)

each year, according to Stephen Selkowitz, who heads the Building Technologies Department at Lawrence Berkeley National Laboratory (LBNL).

Low-e windows now represent about 60% of residential window sales (and a smaller percentage of commercial sales), according to Selkowitz, who views low-e glazing as the most significant change in glazing technology in the past 25 years. “After a long gestation period, low-e glazing is now a mature, standard, low-cost, ubiquitous energy-efficiency technology that demonstrates convincingly that it is possible to provide views and daylight in buildings while still reducing energy use,” he told

EBN.

While low-e glazing is generally a good investment for building owners, the technology has proven an even better investment for the federal government. Beginning in 1976, the U.S. Department of Energy spent about $4 million, largely through LBNL, on low-e glazing research and development that led to the first commercial products in the early 1980s. Over their lifetimes, however, the low-e windows installed through the end of 2005 will generate $37

billion in net savings for American building owners, according to a National Research Council estimate.

Despite the success of low-e glazing, however, windows in the U.S. still account for about 4.2 quads (4.4x10¹⁸ joules) of energy use—costing building owners about $40 billion—each year, Selkowitz said, and even the best windows commonly sold today “are still far from what is needed if the country is to move to zero-energy or carbon-neutral building designs.”

Selkowitz sees hope for significant improvements in a few technologies “that could potentially convert windows from net energy losses to net energy benefits.” He sees promise in more widespread use of triple glazing combined with low-e coatings and gas fills. These products, in limited use today, save energy while improving acoustics and thermal comfort, reducing condensation, and introducing the possibility of eliminating heating and cooling duct outlets at the building perimeter. He is also excited about the potential of switchable glazings, which can be dynamically adjusted as needed to allow varying levels of daylight and solar heat to pass through them (see

EBN

Vol. 15, No. 6).

Selkowitz believes these new technologies will be ready when the market demands them. “With industrial partners, LBNL has recently built and tested prototypes of a new window with a U-value of 0.16 (0.9 W/m²•hr) and incorporating switchable glass. While not yet commercially available, this prototype begins to suggest what the next decade will offer in terms of high-performance window options,” he said. “As we struggle to find the next big energy-saving opportunities in a carbon-constrained world, a new generation of high-performance window technologies and systems is now poised to be the ‘low-e success story’ of the 21st century.”

— Jessica Boehland