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
*Trapped* air is a good insulator.
Air that’s moving around will also move the heat around, which is the opposite of insulation.
Air in terms of conduction is a good insulator, because in conduction it’s the bumping into of individual atoms that transfers the heat/energy from one area to another.This works to our benefit in terms of like dry suits and double pane windows because the air can be kept still and can only transmit energy through conduction.
However because gases/air can move wholly (wind, currents, etc.) They can trasmit heat through convection as well. By taking energy at the surface and those energized particles just being moved somewhere else. In a house with no other insulation, the heat inside would convect onto the walls, conduct through them, and then convect somewhere else outside. This makes it not cost effective to use air to insulate houses, because you would have to make a essentially a double layer house that is perfectly sealed between the two layers.
In the case of jackets, they wouldn’t be very flexible and comfortable to wear if they were insulated with air, because to be effective it would be like wearing a bunch of balloons.
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.
Air is a good insulator because of how few particles exist, which means it’s less likely for them to bounce and pass on heat, whereas a solid is all joined together, so if one end heats up, it moves through the metal easily.
But air is also fluid, which means it’s constantly moving around, meaning it will pass on the heat or take it away frequently.
If you have trapped air in between two solids, you have a very good insulator.
Air that can’t move is a very good insulator, the problem is without something to prevent it from moving it isn’t a insalor, most of the insulators, have very big holes inside of them where the air can savely not move
Air is only a good insulator when not moving or moving very little. When it moves it takes heat away via convention, which is very good at equalising temperature. That’s why an air gap in double glazed windows is quite small, air is the insulator ( sometimes an inert gas ). When air can’t move around, it’s great at stopping heat loss through conduction.
Air expands and contracts with temperature. This makes it less dense when warmer and vice versa. Hot air rises. I’m sure you have heard of that. This creates a convection current. If we trap air and limit its movement, then air will only transfer heat by conduction and some radiation(everything will radiate heat).
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.
Because air moves around, especially when there’s a temperature difference.
You don’t want air moving around if it’s going to carry away all the heat you are trying to keep insulated.
So Insulation is often just some solid but very porous material that will hold a lot of air in it and keep it from drifting away, but its still utilizing the air to help it insulated things.
All the stuff about trapping air is absolutely the primary. It might also be worth noting, air isn’t made of very much. Having a larger thermal mass creates more stable temperatures as it takes more energy to change the object being heated. I dunno how to ELI5 this but I can give it a shot.
ELI5: The more of something there is, the longer it takes to change its temperature. So using insulation adds more stuff to be heated.
Latest Answers