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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There are three mechanisms for heat transfer
* radiation
* conduction
* convection
radiation is pretty much like the sun warming your face. air is irrelevant in that regard as this effect will also occur in a vacuum.
conduction is when you heat up a pan and burn your fingers when touching it in the wrong place. air is very very bad at that. there is no efficient mechanism between the gas molecules.
convection is when warm air moves around. this is what happens when you use a hair dryer. so you see, air is very good at transferring heat that way.
so when you want to insulate something you have a couple of things to do:
* shield from radiation. that can easily be done with aluminum foil. any metal will be excellent for shielding
* limit heat bridges. the smaller the cross-section, the worse the transfer becomes. thin fibers and spokes do good job at that. ideally non metallic
* limit convection. stop the air from moving using bubbles or pockets. if you do the third well, you get two effects for one. less conduction and no convection. tada! you have created a good insulator
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