A steam pipe is to be insulated by two insulating materials put over each other. For best results
Steam pipe: It is used to transfer the steam from the boiler to the working component (turbine). To prevent temperature losses, we use insulation over the pipe.
Insulation: The products (material) which reduce the heat losses to the atmosphere. The conductivity value of insulating material is very less.
If we put inferior insulating material over the pipe then it conducts more heat from steam than superior insulating material to the intermediate surface between two insulating materials and steam losses heat to the atmosphere. Therefore, we use superior insulation put over the pipe and inferior over it.