Based upon Thermoconductivity (W/m•K) or the transfer of heat and cold, composite materials and other plastics rank as the best insulators at a 0.3 W/m•K. Portland cement, which is often used on the bottom of many vinyl swimming pools is also 0.3 W/m•K. Compare this with other materials used for pools: Concrete at 1.7 W/m•K, Steel at 43 W/m•K, Zinc at 116 W/m•K, and Aluminum at 250 W/m•K.