Thermodynamics of fur

Question

I know from experience that fur (and fuzzy blankets) trap heat, but I can't​ figure out the physics of it. It seems to me that heat transfer would still occur, even through the fur/fuzz.

What are the physics behind the heat-trapping nature of fur?

Answer 1

The theromodynamics of fur is actually more based on the thermodynamics of gasses than of the fur itself. You're right that the heat transfers along the fibers of the fur just like it would in any other case. However, it is worth noting that these fibers are very thin, compared to the total airspace within the fur as a whole. Thus, they can't transmit too much heat.

The real power of fur is that it traps an insulating blanket of gas. Gases could be very good insulators in that they don't transmit heat well. However, they have a fatal flaw: they move. Gases are very good at mixing, and this means any molecule you heat with your warm body can quickly escape out into the cold, taking the heat with it. Then new cold air takes its place.

Worse, gasses (and liquids) can form convection cells. These are volumes of fluid where hot fluid rises on one side and falls on the other. This creates a tremendously efficient heat transfer -- the exact opposite of what you want.

Fur resolves this. The many many individual hairs disrupt the formation of convection cells. They also make it harder for wind to move warm air away from you. In short, they permit you to take advantage of all of the positive insulation benefits of air without having to suffer the drawbacks.

Styrofoam also works in the same way. A fully expanded Styrofoam bead, like those used in Styrofoam packing is mostly air, trapped in tiny little spaces, with very few conductive paths straight through the plastic.