For vessels with heat transfer jackets, especially for steam service, it is recommended to insulate and clad or sheath the insulation. The temperature rating of the insulation must be appropriate for the service it will see. Because of the low resistance of stainless steels to chloride induced stress corrosion cracking, all insulation used must have little to no chloride content. Other considerations include the R value (or thermal resistance), and the thickness necessary for the application. Typical insulating materials used in vessel manufacturing are: polystyrene, basalt mineral wool, fiberglass, cork, polyurethane, polyisocyanurate, and foamed glass. For added protection, there are several commercial coatings which can be applied to the exterior of a vessel to provide a chloride barrier. The cladding attached to sanitary vessels are designed so there are no ledges where puddles can form. Penetrations through the cladding, such as heat transfer pipes, and fittings may fully welded, caulked or gasketed. For fully welded penetrations, consideration must be given to thermal expansion of the vessel with respect to the cladding.