Two Colby residence halls built in the early 1950s and renovated in 2008 received LEED gold certification from the U.S. Green Building Council. Colby now has more LEED-certified buildings than any other private college in Maine and is the first private college in Maine to achieve the gold level. Pierce and Perkins- Wilson, which were once fraternity houses, are now more energy efficient and include local, recycled, and sustainably harvested materials. The two buildings, which were expanded as well as renovated, are the fourth and fifth projects at Colby to receive LEED (Leadership in Energy and Environmental Design) certification. Major energy improvements came from replacing windows, expanding the number of heating zones, adding motion sensors and timers for reduced electrical use, and installing more efficient lighting and equipment. Energy recovery systems allow for the capture of heat energy from warm-air exhaust, which is then used to partially reheat the incoming fresh air. Pierce and Perkins-Wilson, originally both 8,160 square feet, each received a highly efficient 1,800-square-foot addition. Building features include new study/lounge space, an elevator for better accessibility, recycling centers, and interior bike rooms. Other LEED-certified buildings at Colby are the Schair-Swenson-Watson Alumni Center (certified at the LEED silver level and opened in 2005 as one of the first academic buildings in Maine certified by the program), the Diamond Building, which opened in 2007, and the Cotter Union/Bookstore project, which opened last year and attained LEED silver.