How does wool keep you warm when it’s wet?

Dear Cecil:

I have a bet with my roommate about why wool is the only kind of material which still keeps you warm when it is wet. I say it's because even when wool is seemingly saturated with water, some dead air space still remains and provides insulation. She says (the dummy) that only wool dries from the inside (skin side) out. I don't know what that has to do with keeping you warm. Who is right?

Cecil replies:

Dear Matthew:

You’re both wrong, so I guess I win the bet. I would prefer a cashier’s check.

Each wool fiber (measuring about a thousandth of an inch in diameter, depending on the grade) consists of a bundle of corticle cells, made up of polypeptide chains arranged in coils. These corticle cells are wrapped up in a scaly outer layer called a cuticle, which in turn is covered by a filmy skin called an epicuticle. The epicuticle actually sheds drops of water.

In addition, raindrops are less likely to break up on the surface of wool and seep through than with other fabrics, since the fuzziness of the fibers cushions the fall. So in a light rain, much of the water runs right off, the fabric hardly getting damp at all.

But the real genius of the wool fiber lies in its ability to cope with the high humidity that you may get during rainstorms or at other times. The sheep (they may look stupid, but it’s all an act) have cleverly equipped the epicuticle with tiny pores that allow water vapor to pass through to the core, where it’s chemically absorbed. A single fibre can slurp up to 30 percent of its own weight in moisture without feeling wet.

Wool does act as a natural insulator, thanks to its built-in crimp. The fibers repel each other, keeping a bit of dead air in between them. But it’s the epicuticle that does the heavy lifting when it comes to keeping you warm despite the wet.

Send questions to Cecil via cecil@straightdope.com.

Comment on this Column