What fabrics are warmest for winter clothes?
Down for insulation, wool for tailored warmth, merino for base layers, fleece for mid layers. Cotton is the one to avoid next to skin.
Warmth is trapped air, and fibers differ in how they trap it and how they handle moisture. Down traps the most air per ounce - nothing beats it for outer layers in dry cold. Wool insulates even when damp and blocks wind in tight weaves - the reason it has owned winter for centuries; a wool coat over a sweater covers most city winters. Merino next to skin regulates and stays warm when you sweat, which cotton fatally does not: cotton absorbs sweat, holds it against you, and conducts heat away - 'cotton kills' is the hikers' rule for a reason. Fleece is the value mid-layer: warm, light, dries fast, but wind passes straight through, so it always needs a shell. Cashmere is luxury warmth - fine for dry indoor-outdoor life, too delicate for rough daily rotation.