A thermal insulator is something that prevents heat from moving from one place to another. There are 3 main ways that heat can travel: convection, conduction, and radiation. Typically the phrase 'thermal insulator' refers to a material that blocks conduction.
Conduction is what happens when something hot physically touches something cold. Heat moves from the hot surface into the cold one, warming it up. To keep this from happening, you use a material that heat can't move through very easily (a thermal insulator).
But what sort of material is that? Try some experiments. Wrap up a cup of really hot water in a material that you think might be an insulator (i.e. a blanket, perhaps). Set your hand on the outside of the insulator and see how hot it feels. Try it with different materials. The hotter it feels on the outside, the more heat is escaping from the inside, and the poorer the insulator is. Try the same thing with an ice cube. Which one feels the coldest from the outside?
You may also be wondering how a thermal insulator actually works.
The key point in getting rid of thermal conduction is to have very few ways in which thermal energy can travel easily. One of the best ways for thermal energy to travel is as energy in the electrons which conduct electricity in metals, so you want to avoid metals. Another is as tiny sound waves, so you don't want a good crystal (like sapphire) in which sound waves travel a long way before bouncing in a new direction. Gases (like air) have low thermal conductivity, but they're prone to thermal convection, in which big currents of flowing material (driven by gravity) carry the heat around. Those currents can be interrupted with very thin walls of plastic, as in styrofoam, an excellent thermal insulator. Blankets use cloth fibers to partially stop convection.
To get the best thermal insulation, sometimes it's necessary to also suppress thermal radiation (mainly infrared light). That can be done with very thin reflecting metal layers, like ones you see on a glass thermos bottle. But didn't we say metal is to be avoided? You certainly don't want metal reaching between the parts that are supposed to be at different temperatures, but it's ok to have metal layers all at one temperature- there's nowhere for them to take the heat.
(republished on 07/27/06)