A report commissioned by the National Trust for Historic Preservation found that it is more ecologically friendly to rehabilitate existing buildings than to build new structures, regardless of the number of green whistles and bells they contain, National Real Estate Investor said. "The thought was that in order to build green you had to build new," but the numbers don't add up that way, said Elizabeth Heider, chair of the board of directors at the U.S. Green Building Council. It can take between 10 and 80 years for a new energy-efficient building to overcome, through more efficient operations, the negative climate change impacts that come from construction. Environmental savings from reuse range from 4 and 46 percent over new construction when comparing buildings with the same energy performance level, according to the report. The study conducted side-by-side comparisons of retrofits and new buildings in seven construction categories in Chicago, Atlanta, Phoenix and Portland, Ore.