How Fabrics Insulate: Trapped Air, Loft and Warmth-to-Weight
Fabrics keep you warm by trapping a layer of still air, not because the fibre itself is "warm". The more motionless air a material holds per gram, the better it insulates, which is why lofty, low-density structures like wool, down and brushed fleece outperform dense, flat ones. This guide explains loft, warmth-to-weight ratio, and why several thin layers beat one thick layer.
Key takeaways
- Fabrics insulate by trapping still air; the fibre matters far less than the structure that immobilises that air.
- Loft (recovered thickness) is the best predictor of warmth, and warmth-to-weight ratio is loft divided by mass.
- Wool holds air through natural crimp and stays useful when damp; down gives the best warmth-to-weight when dry but collapses wet.
- Brushed fabrics gain warmth from a raised nap of trapped air, not from extra mass.
- Layering beats one thick layer because it traps more total air and lets you regulate heat as your activity changes.
Still air is the insulator, not the fibre
Air is one of the poorest conductors of heat available, far worse at moving heat than any textile fibre. So the job of a cold-weather fabric is to hold a thick blanket of air close to your skin and stop it moving. Once air can circulate freely, it carries body heat away by convection and the insulation collapses.
This is why two garments of the same fibre and weight can feel completely different. A tightly woven worsted wool suiting and a loose lambswool knit are both wool, but the knit traps far more air in the gaps between yarns and fibres. The fibre matters less than the structure that immobilises the air.
It also explains wind. A knit that insulates well indoors loses much of its value in a stiff breeze, because wind pushes through the open structure and flushes out the trapped air. That is why a thin, near-windproof shell over a lofty mid-layer works so well, even when the shell adds almost no thickness of its own.
Loft and warmth-to-weight ratio
Loft is the thickness a material springs back to after being compressed, and it is the best single predictor of warmth. A fabric that lofts to 20mm of mostly-air will out-insulate a denser fabric half that thickness, whatever the fibres are made of. More trapped air per millimetre means more resistance to heat flow.
Warmth-to-weight ratio is that insulating power divided by mass. Down is the benchmark: high-quality goose down lofts enormously for very little weight, which is why a packable down jacket can outperform a heavy wool overcoat at a fraction of the mass. The trade-off is that down loses most of its loft when wet, because the clusters mat together.
Synthetic down alternatives, typically polyester engineered into fine, crimped or hollow filaments, chase the same trick: maximise air-trapping surface for minimal mass. They loft slightly less than top-grade down for a given weight, but they keep more of their structure when damp and dry faster, which matters in wet climates.
Why wool, down alternatives and brushed fabrics work
Wool fibres are naturally crimped and scaled, so they resist packing flat and hold air between them even under load. That crimp is why a wool jumper recovers its loft after you take a rucksack off, where a flatter fibre would stay crushed. Wool also has an unusually high moisture regain, roughly 16-18% under ISO 6741-1 conditions, against about 8.5% for cotton and only 0.4% for polyester, so it can absorb damp without feeling cold and clammy against the skin.
Down alternatives and true down work by sheer structure: countless fine filaments or clusters create thousands of tiny air pockets, none big enough to let air circulate. The finer and more three-dimensional the fill, the more still air it holds per gram, and the warmer it feels for its weight.
Brushed fabrics, such as brushed cotton flannel, moleskin or fleece, are mechanically raised so that fibre ends stand proud of the surface. That nap creates a shallow but dense layer of trapped air right against the skin, which is why a brushed flannel feels warmer than a plain woven shirt of identical GSM. The fabric did not gain mass; it gained loft.
Why layering beats one thick layer
Several thin layers trap air within each fabric and also in the gaps between them, so the total still-air thickness is greater than one slab of equal weight. Each boundary is another barrier to convection. It is the same principle as double glazing: the air gap, not the glass, does the insulating.
Layering also lets you regulate. You generate far more heat walking uphill than standing at a bus stop, and a single thick coat gives you one setting. With a base layer, a lofty mid-layer and a shell, you can shed or add insulation to match your activity and keep sweat from soaking the system, which is what actually makes you cold once you stop moving.
The classic structure is a moisture-moving base layer next to the skin, a lofty insulating mid-layer such as fleece or a light down or synthetic piece, and a wind- and water-resistant shell on top. The shell adds little warmth itself but protects the trapped air beneath from being stripped away by wind.
Matching fabric to real conditions
For cold, dry, still conditions, raw loft wins, so a down or high-loft synthetic mid-layer gives the most warmth for the least weight and bulk. WMO and NOAA climate normals are a sensible starting point for judging what a season actually delivers: a region whose winter means sit around 0 to 5C with low wind asks less of a system than one combining sub-zero temperatures with persistent wind.
For wet or variable weather, prioritise materials that keep working when damp. Wool and synthetic fills hold useful warmth wet, where down flattens and stops insulating until it dries. If you regularly get rained on, a synthetic mid-layer under a proper shell is more forgiving than premium down.
For stop-start activity, lean on layering and breathability over a single heavy garment. The goal is to stay dry from the inside: a base layer that moves moisture, a mid-layer you can vent or remove, and a shell that blocks wind. A garment that is warm when you are still and soaked with sweat when you are not has failed at the job.
Frequently asked questions
Is wool or down warmer for the same weight?
For pure warmth-to-weight in dry conditions, good down wins clearly: it lofts more air per gram than wool can. Wool's advantage is that it keeps insulating when damp (moisture regain around 16-18%) and resists being crushed flat, so for wet weather or as a hard-wearing mid-layer, wool is often the more practical choice even though it is heavier.
Why does a thin brushed flannel feel warmer than a thicker plain shirt?
Brushing mechanically raises fibre ends to form a soft nap, and that nap traps a dense layer of still air right against your skin. Warmth comes from trapped air, not fabric mass, so a brushed shirt can feel warmer than a heavier plain weave of the same or greater GSM because it holds more motionless air at the surface.
Does down stop working when it gets wet?
Largely, yes. When down gets soaked the clusters mat together and lose loft, and without loft there is no trapped air and little insulation until it dries, which can take a long time. This is the main reason synthetic alternatives exist: polyester fills keep more of their structure when damp and dry faster, at a small cost in warmth-to-weight when dry.
Why is layering better than one thick coat?
Layers trap air inside each fabric and in the gaps between them, so total still-air thickness, and therefore warmth, is greater than a single layer of equal weight. Layering also lets you add or remove insulation to match your activity, which prevents the sweat build-up that makes you cold once you stop moving. One thick coat gives you only one setting.