Keeping the insulation dry reduces radiation loss and thus saves fuel. Heat loss by radiation from steam pipe to water or wet insulation can be 30 times greater than that to air due to higher thermal conductivity.
The effectiveness of insulating materials depends on the density and composition of raw material like mineral wool, ceramic wool, silica, aerogel and refractory. A combination of one or more of the insulating materials enhances the overall insulating effectiveness.
When insulation gets wet, the air cells are filled with water, which is a better conductor of heat when compared with air. Depending on the temperature, the thermal conductivity of water can be 25-50 times that of air. The water entrained in the insulation will also get heated by absorbing the heat from the hot pipe surfaces. Thus, more amount of heat will be lost through wet insulation, thereby decreasing the effectiveness of the insulation.
As seen in the graph the thermal conductivity of water at ambient temperature is 0.6 W/m0C (4.16 BTU inch/hour/foot2/oF) and that of air is 0.025 W/m0C (0.17 BTU inch/hour/foot2/oF), 24 times that of air.
Steam lines passing through flooded ducts or waterlogged grounds should be adequately protected to prevent them from getting wet. Sufficient precaution should also be taken to prevent the rain water from entering into the insulation.