1.1 This specification covers vapor retarders for thermal insulation, specifically, flexible materials with permeance of 0.10 perm or lower and surface burning characteristics of 25 flame spread/50 smoke or lower. These materials are inte This specification covers vapor retarders for thermal insulation, specifically, flexible materials with permeance and surface burning characteristics. Vapor retarders are classified based on vapor retardance and strength properties: Types I, II, III, I