Why does a good insulator have to be thin and porous??

[geniusjohn5]

I always assumed a good insulator had to be thick b/c the more material there is the more "stuff" the hampers the heat flow (ya, stupid flow of logic). But I'm wrong, specifically why does a thin box filled with pockets of air contribute to heat insulation?

[MattChina]

I always assumed a good insulator had to be thick b/c the more material there is the more "stuff" the hampers the heat flow (ya, stupid flow of logic). But I'm wrong, specifically why does a thin box filled with pockets of air contribute to heat insulation?

I think it has more do to with the material used.

[UTF-8 U+6211 U+662F]

I always assumed a good insulator had to be thick b/c the more material there is the more "stuff" the hampers the heat flow (ya, stupid flow of logic). But I'm wrong, specifically why does a thin box filled with pockets of air contribute to heat insulation?

I think it has more do to with the material used.

Air is a poor conductor. It has a thermal conductivity of around 0.025 W/(m*K). For comparison, wood has a thermal conductivity of around 0.1 W/(m*K). This is why materials like foam are good insulators, because they contain many pockets of trapped air. Transfer of heat through air usually takes place through convenction instead of conduction. Think about going outside on a chilly day. If there is wind outside, you feel much colder, but if there's no wind, you don't feel so cold. The air in the foam is secured so that heat cannot convect through the material. This helps prevent the heat from flowing from the inside to the outside of the box.

[satvik03]

If you look at the formula for conductivity, you'll see that the material used is very important. The conductivity of materials can range very widely, so although thickness and surface area are factors, the material used is probably to most important part.