Why do wool knitted pullovers stay warm when wet, while cotton ones lose their insulation?

Wool knitted pullovers retain warmth even when wet due to the unique properties of wool fibers. Wool is composed of keratin, a complex protein that naturally forms crimped, coiled fibers. This crimp creates tiny pockets of air within the fabric, acting as an insulating layer that traps heat close to the body. Even when wool absorbs water, these air pockets remain intact, preserving its insulating properties. This ability to retain warmth is further enhanced by wool’s hygroscopic nature, meaning it can absorb moisture (up to 30% of its own weight) without feeling damp to the touch. The moisture gets absorbed into the core of the fiber, while the outer layer remains relatively dry, allowing the wool to keep trapping air and insulating effectively.

Another fascinating aspect of wool is its ability to generate heat through an exothermic reaction. When wool absorbs moisture, it releases a small amount of heat in the process, which can help keep the wearer warm even under wet conditions. This makes wool an excellent material for outdoor wear, especially in cold, damp environments.

On the other hand, cotton behaves quite differently. Cotton fibers are smoother and lack the crimp structure found in wool. When cotton absorbs water, the fibers swell and lose their ability to trap air, significantly reducing their insulating capacity. Wet cotton also conducts heat more easily, allowing body warmth to escape. As a result, a wet cotton pullover feels cold, heavy, and clings to the skin, making it much less effective at keeping the wearer warm compared to wool.

This is why wool is a preferred material for clothing in cold or wet conditions, while cotton is better suited for dry, warmer environments. Wool’s natural insulation, moisture-wicking properties, and ability to generate heat make it a superior fabric when warmth and comfort are needed in challenging weather.