Dow Building Solutions Styrofoam SIS is a three-in-one structural insulated sheathing. It provides structural bracing, insulation, and weather protection in one convenient package. The system has R-values that range from 3 to 5.5, and it eliminates the need for OSB, T-bracing, or wood let-in bracing, the maker says. Each panel is made from 80% post-consumer recycled content. A 4- by 8-foot panel weighs 16 pounds versus 45 pounds for OSB, the maker says. 866.583.2583.