Quick Chimney Insulation Facts Wood, Wood Pellet, & Coal applications require insulation. Oil & Gas appliances are recommended to have insulation, but not required. A minimum 1” clearance is needed from the exterior of the masonry structure to combustibles. Two types of insulation are available: TOP-MIX is a Vermiculite based poured insulation and contains NO asbestos. TOP-WRAP is 1/2" thick Foil-Faced Ceramic Wool Blanket that provides the zero clearance requirement.