The conduction of heat through a material depends on two things: the temperature difference across the material, and a property of the material called the thermal conductivity. Heat flows more quickly through a material with a high thermal conductivity, so a material with a low thermal conductivity is good for insulating to prevent heat loss (or for preventing something cold from warming up). Some plastic materials have low thermal conductivities and make good insulators.

However, another way to make an insulator is to have a low-density material or porous material, with lots of small internal spaces or cavities. These materials incorporate lots of air into them, and air is also an effective insulator, as long as it can’t flow. As soon as air can flow, it carries off heat with it, which causes the object to quickly cool. An example of a low-density insulating material is fiberglass insulation. This material is made of glass that has been spun into fibers.

Fiberglass insulation has a thermal conductivity that is 20 times lower than solid glass, and is therefore a much better insulator, just due to its low density!

Covalently bonded materials tend not to conduct electricity very effectively, while metals and salts dissolved in water conduct electricity extremely well. This means that electrical insulators are often made from organic compounds: plastic, rubber, and so forth.