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
Because interfaces, where two different materials touch, further reduce the capability to transmit heat, thus making rhe insulator more effective.
All of the materials you mention and most others used to insulate buildings are essentially small pockets of air contained in another insulator.
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