What is an outer covering for anything, esp. a covering of some nonconducting material such as wood or felt, used to prevent radiation of heat, as from a steam boiler, cylinder, pipe, etc called?

jacket — An outer covering for anything, esp. a covering of some nonconducting material such as wood or felt, used to prevent radiation of heat, as from a steam boiler, cylinder, pipe, etc. (Webster's 1913.)