Thermal insulators are meant to reduce the rate of heat transfer by conduction, convention and radiation -- the standard methods by which heat transfers. This can be either in order to prevent heat loss or to keep heat out. In order to do this, all insulators share certain properties.
The best thermal insulators have the lowest thermal conductivity; this is the property of a material that measures how well it can conduct heat through its mass. The lower the conductivity measure, the less well a material is able to conduct heat, thus enabling it to trap heat or protect contents from outside heat.
Thermal insulators should also be resistant to heat, since they will likely be subject to heat on their surfaces because of the inability of heat to move through them. A thermal insulator without a high heat resistance quotient runs the risk of melting or burning.
Air permeability is the property of a material to allow air to pass through its weave or pores. It is often attributed to materials such as those used in the manufacture of clothing. High air permeability means a lower level of thermal conductivity.
Based mainly on thermal conductivity, some of the best and most common thermal insulation materials include fiberglass, which is made of spun threads of melted and fluffed glass, and foam, which has pockets of gas that do not conduct heat well.