What is the term for the layer of insulating material used in boiler components?

The term "refractory material" specifically refers to heat-resistant materials that are used to line boilers and other high-temperature systems. These materials are designed to withstand extreme temperatures and prevent heat loss, which helps to maintain efficiency in the boiler's operation. Refractory materials also have the necessary structural integrity to bear the weight and pressures associated with high-temperature conditions.

Other choices do not accurately describe this specific insulating layer in the context of boiler components. Insulation generally refers to any material used to reduce heat transfer but doesn’t specifically emphasize the high-temperature resistance aspect of refractory materials. Reflective coatings are typically used to reflect heat rather than insulate against it. Similarly, a thermal barrier can refer to various forms of insulation, but it lacks the specific high-temperature context and structural characteristics associated with refractory materials. Thus, "refractory material" is the most precise term for the insulating layer designed for the demanding conditions found in boiler applications.