How a fabric holds warmth

Short answer: It is not the fabric that keeps you warm, but the air trapped inside it. The more small air pockets a garment has, the better it insulates. The fabric simply holds the air in place close to the body.

Air is the insulation

Still air conducts heat poorly, and that is exactly what makes it good insulation. A knit with many small air spaces therefore holds warmth better than a dense, flat fabric. The fibres themselves matter less than the way they are knitted and how much air they hold. See women's bamboo underwear.

Why structure matters

Two fabrics of the same material can hold warmth differently simply because they are knitted differently. A soft knit with some body traps more air than a smooth, thin fabric. That is why a sweatshirt with a lightly brushed inside feels warmer than a T-shirt, even when both are made of the same fibre.

Why several thin layers work

Two thin layers trap air between them and insulate better than one thick layer of the same weight. The extra air layer works as another layer of insulation. It is also more practical, because you can take a layer off when you come inside and put it back on when you go out. See bamboo panties.

The innermost layer

The layer against the skin has two jobs: holding air and keeping the skin dry. A soft, close-fitting inner layer that does not squeeze holds a thin layer of air against the body. If it sits too loosely, the air shifts every time you move, and the warmth escapes faster.

Moisture ruins insulation

When the air pockets fill with moisture, heat is carried away instead of held. Water conducts heat far better than air, so even a slightly damp layer insulates worse than a dry one. That is why you can feel cold after a walk where you got a little sweaty. See women's bamboo sweatshirts.

Keep the layers dry

Choose an inner layer that moves moisture away from the skin and a middle layer with room for air. Change the inner layer if it gets wet from sweat or rain. Take a layer off before you start to sweat rather than after. The insulation then lasts longer, and you avoid getting cold when you stand still.

Washing affects the air pockets

A knit that felts or gets pressed flat in the wash loses some of its body and with it some of its air. Wash at a low temperature with a mild detergent. Let the garment dry flat or on a line, and avoid wringing it hard. The fabric then keeps its structure and its warmth.

Thickness is not everything

A thick garment is not necessarily warmer than two thin ones. What counts is how much still air sits in and between the layers, and whether that air stays dry. Think in layers rather than thickness when you dress for cold days, and start with a soft layer closest to the body.

Frequently asked questions

What keeps you warm, the fabric or the air

The air. The fabric only holds the air in place, and it is the still air that insulates against the cold. That is why the knit matters more than the fibre.

Why are two thin layers warmer than one thick

Because an extra layer of air forms between them, and that air insulates as well as the fabric itself. You can also easily take a layer off.

Why do you feel cold in wet clothes

Because water pushes the air out of the fabric, and water carries heat away from the body far faster than air. A dry inner layer makes a big difference.