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.

But air is a gas. It starts to move when there are temperature gradients, creating convection. That greatly reduces its insulative properties.

This is why so many insulating products are foams or fibers. They create barriers for the air pockets inside them to prevent large convection currents.