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 not moving or moving very little. When it moves it takes heat away via convention, which is very good at equalising temperature. That’s why an air gap in double glazed windows is quite small, air is the insulator ( sometimes an inert gas ). When air can’t move around, it’s great at stopping heat loss through conduction.
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