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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Heat can move by traveling through things, and it doesn’t travel very fast through air, making air a good insulator. However, heat can also hitch a ride on moving things, and air moves very easily. So if air heats up and flows, it takes the heat with it. That defeats the insulation property. Air can flow through tiny cracks and holes in a house and take heat with it. It can even move heat inside a sealed wall through convection – the heated air rises, causing it to move and possibly transfer the heat, then when it is cool, more heated air rises to replace it, and so on, creating a loop of moving air that takes heat with it.
To be a good insulator, air has to be still. That’s why insulation like fiberglass or foam holds the air still in small bubbles or gaps.
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