When the Portland Museum of Art starts construction on its $100 million expansion, it is expected to showcase an innovative building product that’s gaining popularity here but is not yet produced in Maine: mass timber. The 60,000-square-foot building is designed to be “a mass timber superstructure,” says the builder, Matt Tonello of Consigli Construction. It will be “all mass timber above the foundation.” The museum’s director, Mark Bessire, has become an advocate for the product, saying its use in the new wing will create a welcoming environment. …The use of the mass timber is starting to find more favor among architects, engineers and builders in Maine who value its structural stability, light environmental footprint and the wood’s natural beauty. Also contributing to its increasing adoption is the product’s fabrication, which facilitates a more simplified and cost-effective building process, when compared with the use of steel and concrete.