As green building increases in the United States, more than 20,000 homes have now earned certification from one of the nation’s most rigorous programs: LEED (Leadership in Energy and Environmental Design.) “There are green homes, and then there are LEED homes. This milestone is evidence that the residential market is increasingly recognizing this fact,” said Nate Kredich, vice president of residential market development for the private U.S. Green Building Council, which launched the voluntary program in 2008. At least half the homes fell in the affordable housing category. Another 79,000 U.S. homes have registered for certification, which requires energy efficiency, water conservation, air quality and sustainable building materials. The LEED process can cost several thousand dollars in fees and inspections, so it’s mostly done by companies or affordable housing groups working on multiple units. It offers four ratings; platinum is the highest, followed by gold, silver and “certified.” Green homes are expected to grow from 17% of the U.S. residential market in 2011 to 29% to 38% by 2016, according to a 2012 McGraw Hill Construction study.