Fabric & Care
Winter clothing: warmth is trapped air, not thickness
Insulation works by holding still air. Understanding that explains why down beats a thick jumper, why wet insulation fails, and why the wind matters more than the temperature.
Fabric itself is a poor insulator. What insulates is the air trapped within and between fibres, held still enough that it cannot carry heat away by convection.
Every practical fact about cold-weather clothing follows from that.
Why loft matters
Loft is the volume an insulating material occupies — how much air it holds. It is the single best predictor of warmth.
Down has exceptional loft for its weight, which is why a down jacket weighing 400 grams outperforms a wool coat weighing two kilograms. The down itself contributes almost nothing thermally; the structure it creates does everything.
Fill power measures down quality: the volume in cubic inches that an ounce of down occupies. 550 is ordinary, 700 is good, 850 and above is exceptional. Higher fill power means more warmth per gram, not necessarily more warmth overall — fill weight, the actual quantity of down, determines total insulation.
A jacket with 850 fill power and very little down is light and not especially warm.
Down versus synthetic
Down has the best warmth-to-weight ratio available, compresses to almost nothing, and lasts decades with care. Its critical weakness is that it collapses when wet — the clusters clump, loft disappears, and insulation drops to near zero. It also dries very slowly.
Synthetic insulation — PrimaLoft, Coreloft, Thinsulate — is heavier and bulkier for the same warmth, and retains most of its insulating value when damp. It also dries fast and costs less.
The choice follows the conditions: down for dry cold, synthetic for wet cold or high exertion. Hydrophobic-treated down sits between the two and is a genuine improvement, though not a full solution.
Wind strips away the layer of warm air your clothing has trapped and replaces it with cold air. This is why a thin windproof shell over a jumper is dramatically warmer than the jumper alone, and why a very warm but wind-permeable fleece performs badly in exposed conditions.
Adding a windproof layer is usually the cheapest and largest improvement available to someone who is cold.
Wool in the cold
Wool's advantages in winter come from its structure.
The natural crimp creates loft and traps air. The scaled surface is hydrophobic while the fibre interior is hygroscopic, so wool absorbs up to about 30 per cent of its weight in moisture without feeling wet and continues insulating while damp — the property that makes it fundamentally different from cotton.
It also generates a small amount of heat when absorbing moisture, an effect called heat of sorption, though the practical contribution is modest.
Wool is heavier than down for equivalent warmth and considerably better in damp conditions.
The layering system
Three functions, covered more fully elsewhere and worth restating.
Base layer — moisture management. Merino or synthetic. Never cotton, which absorbs sweat and holds it against the skin, conducting heat away roughly 25 times faster than air.
Mid layer — insulation. Wool, fleece, down or synthetic fill.
Shell — wind and water. Even a light one transforms the performance of the layers beneath.
Extremities
Where most people are actually cold, and where the marginal improvement is largest.
In cold conditions the body constricts peripheral blood vessels to preserve core temperature, which means hands and feet go cold first. The counter-intuitive consequence: warming your core makes your hands warmer. Putting on a hat and a warmer jacket frequently does more for cold hands than better gloves.
Hats. The claim that most heat is lost through the head is a myth arising from a flawed military study, but the head is frequently the only uncovered part, and uncovered skin loses heat.
Mittens beat gloves for warmth, because fingers share warmth. Gloves beat mittens for dexterity.
Boots should not be tight. Compressed feet have restricted circulation and no room for the air layer that insulates. Thick socks in a tight boot are colder than thin socks in a roomy one.
Wool socks, not cotton, for the same reasons as base layers.
Care that preserves warmth
Down needs washing occasionally. Body oils and dirt cause clumping and reduce loft. Use a down-specific detergent, wash gently, and — this is the essential part — tumble dry on low with clean tennis balls to break up clumps. It takes several hours. A down jacket dried flat will remain clumped and will never recover its loft.
Store down uncompressed. Long-term compression permanently damages loft. Hang it or store it loose, not in a stuff sack.
Reproof waterproof shells. The durable water repellent finish wears off, and a jacket that has "stopped being waterproof" usually just needs a wash-in or spray-on treatment. When the outer fabric wets out and stops beading, breathability collapses and you get cold and clammy inside a functioning membrane.
Do not use fabric softener on anything technical. It coats fibres and destroys both wicking and water repellency permanently.
