This specification covers the types, physical properties, and dimensions of unfaced, preformed rigid cellular polyurethane modified polyisocyanurate plastic material intended for use as thermal insulation on surfaces. This insulation can be classified into six types according to its compressive resistance: Types I, IV, II, III, V, and VI. Also this insulation can be classified as Grades 1 and 2 according to its service temperature range. The thermal insulation is produced by the polymerization of polymeric polyisocyanates in the presence of polyhydroxyl compounds, catalysts, cell stabilizers, and blowing agents. Different test methods shall be performed in order to determine the thermal insulations following properties: density, compressive resistance, apparent thermal conductivity, hot-surface performance, water absorption, water vapor permeability, dimensional stability, closed-cell content, surface bearing characteristics, tensile strength, and leachable chloride, fluoride, silicate, and sodium ions.