1.1 ,This specification covers two types of thermal insulating brick for industrial or marine boiler furnaces. Type I is a special, 2500°,F (1371°,C) maximum service temperature, insulating firebrick that is used as backup insulation for refractory furnace linings.2 Type II is a standard insulating brick that, in general, is used where there may be direct contact with combustion gases, such as forge and stress relieving furnaces.3