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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It has to do with convection.
Air is free to move and its conductivity increase with flow. When air gets in contact with hot surface, it heats and float up, creating an updraft. Updraft creates more contact which leads to more draft, and it quickly snowballs.
Insulators have the purpose to deny air contact to the hot surface, or trap air so it cannot move (block draft)
Heat transfer happen by contact, draft(convection) and radiation.
You need to cut down all three. Make contact to materials or gas that are bad conductors, stop the convection(draft) and reduce radiation (or IR rays, so you need mirror-like or white surface). For example, styrofoam is white, traps air, and its contained air and the styro-plastic are both bad conductors.
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