Woolmark describes wool as a hygroscopic fibre that can absorb moisture vapour within its structure. Its natural three-dimensional crimp also creates spaces where still air can be trapped. Those are related but different features: one concerns moisture exchange, while the other contributes to insulation.

Wool's Crimp and Moisture Uptake Support Different Comfort Functions

The combination helps explain the fibre's use in changing activity conditions. A garment's warmth is not determined by composition alone, however; thickness, knit or weave density and fit alter the result. Understanding both air spaces and vapour uptake is more useful than treating breathable and warm as opposite properties.