In the depths of winter, not every fabric holds in heat the same way. Down, merino wool, cashmere and fleece insulate through different mechanisms, with very different weight and upkeep. Here is how to tell which fabric keeps you warmest, depending on use, climate and budget.
Key facts
Down (goose or duck feathers) offers the best warmth-to-weight ratio, but loses its insulating power once wet. Against the skin, merino wool handles moisture best while still keeping you warm. Cashmere and alpaca insulate strongly for a thin layer, with alpaca holding up better in damp conditions. For sport, fleece dries fast and stays warm even wet, while heat-tech synthetic fibres rely on a technical base layer.
- Down traps the largest volume of still air for its weight, the physical principle behind its insulating power
- Wool, whether merino, alpaca or cashmere, keeps you warm even slightly damp, unlike down
- Synthetic fleece dries within hours and keeps most of its warmth once wet
- Fibre diameter, measured in microns, determines softness: under 19.5 microns for fine merino, under 19 for cashmere
- The three-layer method, breathable, insulating, windproof, often keeps you warmer than one thick fabric alone
How a fabric holds in heat
A fabric does not produce heat, it holds in the heat your body already gives off. What insulates is not the fibre itself but the still air trapped between the fibres: air is a very poor conductor, and the more of it a fabric can hold, the warmer it keeps you for a given weight. This is what is known as loft, or trapped air volume.
Down and wool do not insulate the same way. Down is made of small three-dimensional clusters that fluff out into a very airy structure, trapping a large volume of air for a tiny mass. Wool owes its warmth to the natural crimp of its fibre, which creates tiny air pockets even when slightly damp. This difference in mechanism is why down wins in dry conditions, and why wool keeps the edge in rain or wet snow.
Six fabrics compared
The table below ranks the most common winter fabrics by insulating power, behaviour in damp conditions and ease of care.
| Fabric | Insulation for the weight | Behaviour in damp conditions | Care | Best for |
|---|---|---|---|---|
| Down (feathers) | Very high, the best warmth-to-weight ratio | Loses insulating power when wet, clumps together | Delicate wash, dry with tennis balls | Dry, intense cold, puffer jackets and parkas |
| Merino wool | High, manages moisture well | Keeps some warmth when wet | Wool cycle at 30°C at most | Base layer, worn against the skin |
| Cashmere | High for a very thin layer | Sensitive to moisture, dries slowly | Hand wash, pills quickly | Dressy jumpers, moderate dry cold |
| Alpaca and mohair | High, hollow and lightweight fibre | Holds up well in damp conditions | Delicate wool wash | Those who feel the cold most, bulky jumpers |
| Fleece | Medium to high, dries very fast | Keeps warmth when wet, breathable | Easy machine wash | Mid layer, outdoor sport |
| Heat-tech synthetic fibres | Medium, generates heat through friction | Dries quickly | Easy machine wash | Base layer underwear, technical layering |
Down comes out on top for warmth-to-weight, as long as it stays dry. Merino wool follows closely, with the advantage of still warming you when damp. Cashmere and alpaca insulate strongly for a thin layer but need more care. Fleece and heat-tech fibres rank lowest on pure insulating power, made up for by fast drying and easy care.
Down, the warmest insulator for its weight
Used in puffer jackets and some parkas, down is made of goose or duck feathers with no stiff quill. Its fill power, measured in cuin, gauges the volume an ounce of down takes up: the higher the number, the more air the material traps for a given weight. A fill of 600 cuin and above signals a good warmth-to-weight ratio, useful when comparing two jackets of similar weight.
Its main weakness is water. When wet, down clumps into small lumps and loses much of its volume, and so its insulating power. That is why it suits dry, intense cold best, and less so rainy winters, where synthetic fill or wool take the lead. Textile Exchange's Responsible Down Standard covers animal welfare across the supply chain, a useful marker to look for in shops.
Merino wool, cashmere, alpaca and mohair: the wool family
Merino wool, the most versatile
Its fine fibre, generally under 19.5 microns for fine merino, does not itch and keeps insulating even damp, since it absorbs water vapour without feeling wet to the touch. It is the reference fabric for a base layer worn against the skin, for hiking as much as for everyday wear.
Cashmere, the warmest for a thin layer
Sourced from the cashmere goat, its very fine fibre, under 19 microns, gives exceptional warmth for a low weight, which allows thin jumpers worn under a coat. It needs more care, though: it pills faster than merino and dries slowly once wet.
Alpaca and mohair, for those who feel the cold most
Bulkier than sheep's wool, these fibres are partly hollow, which gives them excellent insulating power for a moderate weight. They also hold up better in damp conditions than cashmere, an advantage in a changeable winter.
Fleece and heat-tech fibres, the technical options
Fleece, usually knitted from recycled polyester, insulates decently for its weight while drying within hours. Worn as a mid layer under a windproof shell or a parka, it keeps warming you even when damp, which makes it a safe choice for outdoor sport or cycling to work. Faguo, for instance, makes fleeces from recycled polyester, in keeping with the recycled materials found across much of its range.
Heat-tech synthetic fibres, a well-known example being Damart's Thermolactyl, rely on a different principle: they generate and hold heat through friction between the fibres as you move. Less warm than good wool when standing still, they work well as base-layer underwear, under trousers or a jumper, for days spent outdoors without intense activity.
The three-layer method for never feeling cold
Stacking two thick jumpers often warms you less than three thin layers combined. The three-layer method remains the most effective way to handle severe cold:
- Base layer, close to the skin and breathable: fine merino wool or silk, to wick away sweat without chilling you.
- Mid layer, insulating: a wool, alpaca or fleece jumper, depending on activity and budget.
- Outer layer, windproof and waterproof if needed: a wool coat, parka or down jacket depending on climate and how long you are exposed to the cold.
Our guide to choosing between a parka, a puffer jacket or a wool coat covers this outer layer in detail, for city wear or a full day outdoors.
Which fabric to choose for your use
Dry, severe cold with little movement
Down remains the warmest fabric for its weight, in a puffer jacket or parka with a high fill power. Our guide on choosing a warm parka covers the fill weight and fill power to look for depending on how cold it gets.
Outdoor sport and physical activity
Fleece or heat-tech fibres, which dry fast and keep insulating even damp with sweat. Our ranking of the best men's fleece compares five brands on fabric weight, material and price.
Worn against the skin, sensitive skin included
Fine merino wool or alpaca, which do not itch and manage moisture well. Our guide to a wool jumper that doesn't itch covers the fibre diameter to look for.
A dressy jumper for a night out
Cashmere, for its warmth under a very thin layer, best kept for days without heavy dampness given its more delicate care.
Our advice
For maximum warmth when dry, down wins by a wide margin on weight. In damp conditions, or worn against the skin, merino wool remains the most versatile fabric.
For sport or technical use, choose fleece or heat-tech fibres, which dry fast. And always combine several thin layers rather than one thick fabric: it is the still air between the layers that keeps you warm, not thickness alone.