Abstract: Structural insulation sheathing (SIS), preferably in the form of a panel, comprises a structural facer and an insulation member. The structural facer and insulation members are in intimate, planar contact with one another, both are plastic, and the SIS structure meets both the structural (i.e., ASTM E72) and insulation (R>2) requirements for the North American residential market. The structural facer member of the SIS structure is high-density polyolefin foam. The structural facer comprises a skin/core structure with the skins of a higher density than the core, and the structural facer has a flexural modulus typically of at least about 30,000 pounds per square inch (psi). The insulation member of the SIS is a foamed sheet of thermoplastic, e.g., polyisocyanurate.