Commercial and residential buildings account for nearly 40 percent of energy used in the U.S. With energy costs soaring, the green building materials industry has grown to $156 billion a year and spawned an array of start-ups and technologies. One technology is smart windows, which use specialized glass and films that darken to help cool buildings and cut air-conditioning costs as much as 25 percent. Smart windows work by responding to electric current, light, or heat. They haven't been particularly popular with consumers, but the rise in energy prices and new building codes designed to reduce energy usage have given the sector a boost. Here are four promising businesses vying for the market.

Sage Electrochromics
Based in Faribault, Minnesota, Sage has been making smart windows for businesses and schools since 2003. The glass darkens when a user flips a switch to send a low-voltage current through it. In 2010, Sage partnered with the French company Saint-Gobain, one of the world's largest producers of construction materials, to open a U.S. manufacturing facility. The companies' research and development operations will merge, and Saint-Gobain will distribute Sage's glass in Europe.

Number of Building Installations: 100+

Pleotint, based in West Olive, Michigan, makes a temperature-sensitive film that darkens as it absorbs solar heat. The company began producing the film in 2010; it is sold to fabricators, who press the film between two glass panels. The product is targeted for new construction and retrofitting windows in houses and offices. Like Sage, the company is also courting international markets. For instance, its products are distributed to real estate and construction companies in China for upscale residential markets and commercial buildings.

Number of Building Installations: 10

Founded in 2007 and based in Milpitas, California, Soladigm treats glass with a material that conducts electric current. Like Sage's glass, Soladigm's darkens when electric current is applied. Among the company's investors is General Electric, which chose Soladigm as one of 12 clean-tech business partners in a competition. Soladigm is building a manufacturing facility in Olive Branch, Mississippi, that is expected to open in 2012. The company's primary target is commercial buildings.

Number of Building Installations: 0

Switch Materials
This Vancouver, British Columbia, company, launched in 2007, sells film to glass and window manufacturers. Switch's product, unlike its competitors', responds to light and electricity. The tint darkens in sunlight but can be reversed by sending an electric current through it. The company has received grant money from the Canadian government for pilot projects with architects and glass manufacturers, in which they are using the film in glass for several commercial buildings in Vancouver. Switch Materials expects the film to be commercially available by 2012.

Number of Building Installations: 0

The Line: Sage Electrochromics leads the market and will grow only stronger now that it has joined forces with Saint-Gobain. Pleotint's temperature-based film is less complicated and easier to produce, so it could do well if the housing market rebounds, especially in southern climes. But builders and dwellers who want windows with glare control may find electrochromic windows, which respond to electric current, more useful, because they darken on demand. Given its backing from GE and a new facility that could produce its technology in large volumes, Soladigm may be well poised to capitalize on the market for smart buildings.