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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This is a great question.
Stationary air is indeed an excellent insulator. Moving air, however, is a terrible insulator. It can be hard to keep air from moving. Note that the “effective temperature” in winter depends on wind, because when the air moves you lose heat very quickly.
The best insulators are teeny-tiny pockets of air sandwiched between layers of lightweight, thin material. That air can’t move much. The best insulator (in an atmospheric environment) is aerogel. The best flexible insulator isn’t even synthetic: it’s Arctic duck down.
I am pretty sure the phrase “Air is a good insulator” refers to air as an insulator in regards to electricity.
Air is a great insulator, water suspended in air is an amazing container and conductor of heat.
Humidity is why air can transfer and hold heat.
Air as insulation only works when it’s sealed and moisture free, that’s very hard to do so closed cell foams and fiber matting out performs.
Insulation is just a material to hold air. It keeps it from moving around too much, and heat transfer also slows from medium to medium,so having many small pockets of air instead of one large one slows the transfer even further
Air is a good insulator as long as it doesn’t move.
An easy example are thermopanel windows, they are just two glasses with trapped air in between.
Still air is a very good insulator. Moving air is very good at transferring heat through convection. Insulation stops air from moving.
Heat doesn’t move through air very well. However, air with heat in it tends to move through other air rather rapidly because gas density decreases with temperature.
That quote should be updated to “stationary air is a very good insulator”. Free air moves around too much, so insulation was created to hold it stationary so it can… you know, insulate.
Air is shit at conducting, but kicks ass at convecting. Insulation makes many little pockets of air to keep the lack of conducting and also stop the convection.
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