Thermal insulation can be defined as the material or a combination of materials that retard the transfer of heat. Heat may be transmitted between materials by the process of convection, conduction or radiation. Insulators reduce the flow of heat.
Heat always flows from warmer to cooler surfaces until the temperatures of both surfaces become equal. Thermal insulation reduces this flow of heat. An insulator is a bad conductor of heat. Materials that are less dense are better insulators. Thus, gases are more effective insulators than liquids, which in turn are better insulators than solids. The primary function of thermal insulation is to protect equipment by controlling the surface temperature.