It is any material that, interposed between two environments at temperatures
different temperatures, appreciably slows the transfer of heat from the ambient
warmer to cooler. Disregarding the transfer of heat by radiation, the perfect insulator is the vacuum. However, the use of cold rooms with vacuum insulation is not common, due to the technical difficulties and the high cost.
Thermal insulators are materials formed by an infinity of hollow cells filled with air or other gas. The cells must be small, in order to prevent the movement of the gas enclosed. The larger the number of cells (pores), the better the insulating materials, implying a small density.
