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 only a good insulator when it stays still. When it moves around it carries heat along with itself and stops being a good insulator. So insulation materials are often designed in a way that creates lots of partially or completely isolated air volumes that prevent this air from moving around and thus let it show its insulation capabilities.
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