boiler insulation

Tests by live steamers have indicated (although inconclusively) that insulation is of negligible value as an economizer either in improved steaming or fuel savings on live steam locos although it does appear to make some difference in the external jacket temperature. Several materials, including a simple air space, have been used over the years, all with advantages and drawbacks. Fiberglass, wool felt, sheet cork, ceramic cloth, balsa wood and others have all been tried with approximately equal results. One factor that is often a consideration is moisture retention, the tendency of the material to trap and retain moisture between the jacket and boiler, particularly if either one of those is steel. My current choice of insulation material is called Fiberfrax, an inert ceramic cloth material capable of withstanding up to around 1900°F and available in several thicknesses.