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Heat can be transferred in any one of three ways, radiation, convection and conduction. Radiation which is the transfer of energy through electromagnetic waves, does not require material to travel. In this way, heat can be transferred through a vacuum as the sun's heat does to warm the Earth.

 Convection is the transfer of heat through fluids, which include gasses like air. Conduction is the transfer of heat through material in contact with other material. Conduction operates by the energizing of molecules in close proximity to other molecules. The denser the material, the easier it is for heat to be transferred through conduction. With regard to insulation, it is the conductive mode of heat transfer that we are concerned with.

Because conduction requires the close proximity of molecules, dense materials such as solids are much better conductors of heat than sparse materials. Solids conduct much better than liquids which conduct much better than gasses. Since the molecules in gasses like air are spread far apart, they are poor conductors and hence, excellent insulators.

Dense solids such as gold, silver or copper conduct much better than materials such as fiberglass. Conduction cannot occur at all where there are no atoms present to excite. Consequently, the best prevention of heat transfer through conduction is a vacuum. This is why thermoses generally have a layer where the air has been evacuated.Thermal insulation works by creating a barrier of poorly conductive material between two environments.