eli5 | Why does Insulation exist if “air is a very good insulator”?

Question

This has bothered me ever since I first heard the phrase as a kid.

If air is a good insulator, why do we fill things with insulating material? (Ex: walls with fiberglass, coats with cotton)

I realize these things are very porous, so hold a lot of air. But by them being used at all, must mean air isn’t that great on its own.

Is it just a matter of air is only “good” and other stuff is just even better? Or is it just considered good by being a bad conductor?

In: 3423

Answer 1

Still air is a good insulator, moving air is a conductor of heat, the purpose of the insulation is to hold the air in place and prevent it from moving, which is why the insulation creates “pockets” of air.