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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Because air is very fluid and tends to move around. Insulation in a wall holds the air in place so it can do its job better.
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Stationary air does not transfer temperature well, but a large volume of air is also quite mobile, and moving air changes how we perceive temperature – think of how a breeze on a hot day cools you down. We fill walls/coats with things that trap the air so it’s more stationary.
*Still* air is a good insulator.
But air is a gas. It starts to move when there are temperature gradients, creating convection. That greatly reduces its insulative properties.
This is why so many insulating products are foams or fibers. They create barriers for the air pockets inside them to prevent large convection currents.
Air is great at insulation *if air doesn’t move*. Each piece of air has a lot of space between it and the next piece, so it takes a lot of heat to bridge each gap and spread that heat.
However, if air is allowed to move, then the heat doesn’t need to spread itself; instead the air moves through all that space and brings the heat with it. And because of how heat works, any air that is heated will be attempting to move more and more. So if air is left by itself, it’s kind of a terrible insulator.
So quite a few of the best insulating materials are actually just a way for air to be held in place. Things like foams benefit from air’s insulating properties while also preventing its movement.
Air is only a good insulator when it stays still. When it moves around it carries heat along with itself and stops being a good insulator. So insulation materials are often designed in a way that creates lots of partially or completely isolated air volumes that prevent this air from moving around and thus let it show its insulation capabilities.
Just to add on what everyone else says – insulation also helps to reduce noise and the spread of a fire.
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
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.
Still air is a good insulator, moving air is a conductor of heat, the purpose of the insulation is to hold the air in place and prevent it from moving, which is why the insulation creates “pockets” of air.
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