Why does wool heat up when wet?
Taking a break in a sweaty cotton sweater is a cold and unpleasant experience. With a wool sweater, it's a completely different matter. That the wool warms even when it is wet is an old truth, but what is this due to?
Wool could be called a smart insulation. The very small air pockets in each wool fiber provide both insulation and breathability. Wool keeps you warm when it's cold and when it's very hot it helps keep you cool. It is no coincidence that Bedouins use thin woolen garments in the scorching heat.
The fiber can absorb as much as 30 percent of its own weight in moisture without feeling wet. This makes wool a hygroscopic insulation (a material that can absorb and release water vapour). The ripple of the merino fiber keeps the small air pockets intact even when it gets wet. At the same time, the ripple helps transport moisture away from the body. The very small air pockets mean that the air cannot circulate as easily and the heat is retained. The same applies to cold. Compare with styrofoam. Frigolit has small air pockets and works both to keep something warm and to keep it cold.
The shrinkage in the wool fiber forces each strand to bump against each other, as opposed to laying side by side or laying flat together. This keeps the small air pockets intact and acts as little insulators - the key to being able to keep you both warm and cool. Air has the ability to move heat through convection - in other words, by moving and circulating. Through convection, air can transport heat from one place to another. When air is in very small pockets, it cannot circulate easily, so heat is retained. The same applies to cold.
Merino wool also has another built-in property. A natural water-repellent ability, the epicuticle, which is due to the outermost thin layer of the fibers. At high humidity, the small pores of the fiber draw the vapor into the center of the fiber where it is absorbed via a chemical process. A chemical reaction, where the chain of the water molecule is actually broken, creates a chemical reaction with the wool fiber molecules and generates heat when it has absorbed a lot of moisture. But the air pockets in the wool cause the moisture to evaporate so you won't overheat when you sweat.
As described above, wool has a combination of properties that actually make it the best natural insulating material on earth all year round. In addition, it is a material that naturally breaks down into soil when it can no longer be used.
