So one of our readers asked us this question the other day: Why does wool retain its insulating properties when wet, but other fibers (such as cotton) don’t?
There are a couple of reasons for this. The foremost reason is that cotton soaks up and holds on to more water than wool. Most of this water only leaves the clothing by evaporation and evaporation takes a lot of heat away from your body. Cotton will get wet easily and stay wet for much longer than most other fabrics. More water equals more heat loss.
But why does it hold more water? Wool fibers have a scaly coating that repels water while cotton fibers do not have this. Also, cotton is made of cellulose, the same stuff that paper is made from, which is very effective at absorbing water. Weight for weight, cotton will absorb more water than wool. Cotton “holds on to” the water more than wool. When knitted together into a fabric, cotton effectively acts as a sponge. So, when a cotton garment gets wet it will be holding more water for longer compared to a woolen garment. When soaking, some of this water can drip off, but after a point the main way in which the clothing dries is by evaporation.
Synthetic fibers are even better in this regard than wool, they absorb very little water, especially when wrung out after soaking.
One more method by which a clothing keeps you warm is by trapping the air between your skin and itself that your body’s metabolism has already warmed. When cotton gets wet, the fibers collapse and there is no longer any dead air space to hold that warmed air. Wool and other synthetic fibers such have much sturdier fibers that hold their shape even when wet. Thus, even when wet, a wool shirt is going to continue trapping a lot of your body heat while cotton will just release it back to the environment.