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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This is a great question.
Stationary air is indeed an excellent insulator. Moving air, however, is a terrible insulator. It can be hard to keep air from moving. Note that the “effective temperature” in winter depends on wind, because when the air moves you lose heat very quickly.
The best insulators are teeny-tiny pockets of air sandwiched between layers of lightweight, thin material. That air can’t move much. The best insulator (in an atmospheric environment) is aerogel. The best flexible insulator isn’t even synthetic: it’s Arctic duck down.
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