How do heat insulators work?

Question

Polystyrene (styrofoam) is a good insulator, but air is an even worse insulator. Why then, do people keep items in polystyrene boxes instead of leaving them exposed to air? How exactly do these insulators like styrofoam work? I would appreciate if you make references to the molecules of the materials.

Answer 1

First, I think you must review the definition of insulation. For heat (thermal) the definition is as follows;

Thermal insulation, materials used to reduce the rate of heat transfer

Insulators increase the path heat must travel to get past the border. Air does not increase the path the heat must travel, so it is poor for insulation.

Another part of it is the ability to conduct heat through the material itself. Polystyrene is not able to conduct heat well. Air is able to conduct heat well. That is also why it is able to be a good insulator.

If you want to see the mechanics, here is a quote from the second source;

First, let me explain why metals generally conduct heat better than other solids do. In metals, some of the electrons (often one per atom) are not stuck to individual atoms but flow freely among the atoms. Of course that’s why metals are such good conductors of electricity. Now if one end of a bar is hot, and the other is cold, the electrons on the hot end have a little more thermal energy- random jiggling- than the ones on the cold end. So as the electrons wander around, they carry energy from the hot end to the cold end, which is another way of saying they conduct heat.

Of course how fast they conduct heat depends a lot on things like how many free electrons are around, and especially, how fast they move, and especially on how far they usually go before they bump into something and change direction. Those are the same factors that determine how well the metal conducts electricity. So there’s a rule that works very well, saying that the thermal conductivity (at some temperature) is proportional to the electrical conductivity. That’s convenient because it’s much easier to measure electrical conductivity than thermal conductivity.

If you have any more questions email me.

Answer 2

There are three basic mechanisms for heat transfer: conduction, convection, and radiation. Air is a worse thermal conductor than styrofoam, but convection dominates if it is allowed to flow (hot air rises, for example). Porous solids such as stryofoam are more effective than air alone because they entrain the air, preventing convective heat transfer.