RMAX Thermasheath-3 Insulation is the best of the best. Rmax Thermasheath-3 is an energy-efficient thermal insulation board composed of a closed-cell polyisocyanurate (polyiso) foam core bonded to reinforced aluminum foil facers with clear coating for limited protection against oxidation on each side. It serves as a code prescribed Water-Resistive Barrier (WRB). Custom installation required.