Wool clothing is often chosen for cold weather, while cotton is favored for its comfort in warmer climates. This difference is not about the thickness of the fabric, but rather the natural architecture of the materials themselves. Understanding the physical and chemical distinctions between these two common textile fibers reveals why one excels at insulation and the other is prone to causing a chilling effect. The disparity in warmth is determined by how each fiber handles air and moisture.
The Core Difference in Fiber Structure
Wool fibers are natural protein structures, primarily composed of keratin, that possess a complex, three-dimensional shape. Each wool fiber is naturally crimped, meaning it has a distinct, permanent wave or kink that is absent in cotton. This crimp arises from the internal structure of the fiber’s cortex, which is split into two halves that expand differently when moisture is absorbed, causing the fiber to bend.
In contrast, cotton fibers are composed almost entirely of cellulose, forming structures that are smooth and flat. The cotton fiber’s shape is straighter and ribbon-like, lacking the inherent springiness and wave of wool. The wool fiber’s surface is also covered in tiny, overlapping scales, similar to roof tiles. This fundamental difference in shape and surface texture is the foundational reason for the thermal performance disparity between the two fabrics.
The Mechanism of Air Trapping and Insulation
The natural crimp of wool fibers causes them to resist being packed together tightly when spun into yarn and woven into fabric. This creates a high-volume, low-density material that is full of millions of microscopic air pockets. Air is a poor conductor of heat, and when trapped and held stationary within these pockets, it forms a highly effective thermal barrier. This layer of trapped, stationary air prevents the body’s warmth from escaping into the colder surrounding environment.
Cotton fabric, however, is made from smooth, straight fibers that pack down densely when compressed or woven. This leaves very little space to trap air, meaning cotton has a high thermal conductivity, allowing body heat to pass through the material easily. Since the primary source of insulation in clothing is the air held within the fabric, wool’s inherent loft and crimped structure make it a far superior insulator than its smooth, flat counterpart.
Moisture Management and Thermal Regulation
The way wool and cotton interact with moisture is the second major factor contributing to wool’s superior warmth, especially in variable conditions. Wool fibers are unique because they have a hydrophobic, waxy exterior coating but a hydrophilic core. This structure allows the wool fiber to repel liquid water on its surface while simultaneously absorbing a significant amount of water vapor—up to 30% of its dry weight—into its internal structure without feeling damp.
When wool absorbs this water vapor, a chemical process called heat of sorption occurs, where the moisture molecules bond with the fiber’s protein structure and release a small amount of heat, which can contribute to warmth. Even when the fiber is wet, the moisture is locked away inside the core, and the crimped structure continues to hold insulating air pockets. Cotton, conversely, is highly hydrophilic and absorbs liquid sweat directly into the fiber structure, holding it close to the skin. This saturated fabric conducts heat away from the body very quickly, causing rapid evaporative cooling that leads to a chilling effect.