ST. LOUIS — What is the life cycle of a building? Each year, in the United States, new home construction accounts for 50 million tons of carbon emissions. The majority stem from harvesting, manufacturing and transporting building materials — from lumber and masonry to glass and tile to roofing and mechanical systems. But what happens at the end of the cycle? When a building is no longer inhabitable or needed, what happens to all the energy invested in those materials? “It’s demolished,” Hongxi Yin said. “It goes to the landfill.” Yin, an expert on sustainable construction, is the InCEES professor in advanced building systems and architectural design in the Sam Fox School of Design & Visual Arts at Washington University in St. Louis. Over the last decade, he has led a series of research studios exploring ways to improve building efficiency, supply chains and construction processes.