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?
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Air is great at insulation *if air doesn’t move*. Each piece of air has a lot of space between it and the next piece, so it takes a lot of heat to bridge each gap and spread that heat.
However, if air is allowed to move, then the heat doesn’t need to spread itself; instead the air moves through all that space and brings the heat with it. And because of how heat works, any air that is heated will be attempting to move more and more. So if air is left by itself, it’s kind of a terrible insulator.
So quite a few of the best insulating materials are actually just a way for air to be held in place. Things like foams benefit from air’s insulating properties while also preventing its movement.
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