The Forest Service’s Forest Products Laboratory is helping to standardize environmental performance reporting and build accountability in the sustainable building design community. This project investigated the environmental performance of the pioneer mass timber building (“Design Building”) built at the University of Massachusetts-Amherst in 2016. The building is a four-story cross-laminated timber-concrete composite 87,500 ft2 structure. Life cycle assessment (LCA) tool was used to conduct a cradle-to-grave assessment on the material effects of structure, envelope, and interior partition assemblies, and operating energy and water use over 60-year period. The Forest Products Laboratory and US WoodWorks working with the Athena Institute assessed the environmental impacts using the Athena Impact Estimator for Buildings (IE4B) and prepared an environmental building declaration (EBD). The IE4B tool provides a cradle-to-grave life cycle inventory profile to estimate a whole building’s environmental footprint. The inventory results comprise of flows to and from nature: energy and raw material flows plus emissions to air, water and land. Seventeen impact indicators were provided in the LCA results including global warming, ozone depletion, acidification, water consumption, and waste, etc. The six most commonly presented LCA indicators were used to produce the building EBD, the first in the US for the Athena Institute.