Durasheath-3 is a rigid foam plastic thermal insulation board composed of environmentally sound, closed cell, polyisocyanurate foam core bonded to specially coated glass fiber mat facers on each side. This sheathing is suitable for use in walls, in new and retrofit commercial, residential, agricultural and industrial buildings. ItâÇ™s ideal for stud or cavity wall installations on either the exterior or interior wall surface. When used on the interior face of a wall, Durasheath-3 must be covered by an approved thermal barrier. Durasheath-3 is available in 1" to 3" thicknesses as well as several different lengths.