Source: Courant.com. In a study covering existing and new houses sold between May of last year and April 30 of this year, the Earth Advantage Institute, a nonprofit group based in Portland, OR, found that newly constructed homes with third-party certifications for sustainability and energy efficiency sold for 8 percent more on average than non-certified homes in the six-county Portland metropolitan area. Existing houses with certifications sold for 30 percent more.