Why a throw is really a layer of air

Pick up a wool throw and a linen one of the same size, and the wool one feels warmer almost at once. It is tempting to say wool is "warmer" the way copper is "conductive". The truth is a little more interesting: almost all the warmth of a textile comes from still air trapped inside it. The fibres mostly just hold that air in place so it can't drift away with your body heat.

That changes how you shop and how you cosy up a room. If trapped air is the point, then thickness and fluffiness matter a lot, and the fibre name matters less than the label suggests.

Try it: build a throw and watch the heat

The picture below is a slice through a throw lying on you, with skin at about 33 °C on the left and the room on the right. Pick a fabric, then change the thickness, the number of layers and the room temperature.

Notice what happens. Switching fabric nudges the numbers a little. Doubling the thickness, or adding a second layer, moves them much more. That is the pattern you see in real clothing and blankets too: insulation tracks the thickness of trapped air far more closely than it tracks the fibre.

What a tog actually is

You may know "tog" from duvets. It is a unit of thermal resistance: one tog is 0.1 m²·K/W, so a higher number means less heat escapes. Duvets are commonly sold at around 3–4.5 tog for summer and 12–13.5 tog for winter. A throw on its own sits far below that, which is why it adds a gentle layer of comfort on a sofa but is not a replacement for a duvet.

So what does the fibre do?

It still matters, just in different ways than raw warmth:

• Wool fibres are naturally crimped (wavy), which makes the fabric springy and bulky so it traps lots of air. Wool can also take up to about 30% of its weight in moisture without feeling wet, which is why it stays comfortable if the room is a bit damp.
• Linen is smooth and dense. It lies flat against skin and feels cool at first touch, which is lovely in summer and a little brisk on a cold evening.
• Cotton sits between the two: soft, easy to wash, and as warm as you make it by weave and thickness.
• Fleece is a fluffy synthetic pile. It traps air well for its weight but does not absorb moisture the way wool does, and it can feel clammy if you run warm.
• Velvet and other pile fabrics get their depth from short upright fibres. They look rich and add a little loft, but the backing fabric decides most of the warmth.

Cosy tricks that cost nothing

Because warmth is about trapped air, layering works. Two thin blankets with a small gap between them can feel warmer than a single blanket of the same weight, and you likely own them already. Fold a throw over the arm of the sofa so it is within reach, and the room feels more inviting before you have used it. A thick throw laid across the foot of a bed adds a layer exactly where you can pull it up on a cold night.

Second-hand wool blankets are worth a look: wool is durable, and a well-made one can last for decades. Give any secondhand textile a good wash or airing first.

What to look for, if you do buy

• Tight budget: choose by feel and loft. Press it: if it springs back and feels thick for its weight, it holds more air.
• Middle budget: check the fibre label (wool, wool blend, cotton) and the weave. A loose, chunky weave holds air; a thin tight one holds little.
• Higher budget: look for natural fibres, finished edges and a size that suits your use.

Measure before you buy. For a sofa throw, measure the length of the seat or arm you want it to cover; a throw about 130 cm wide by 170 cm long is a common size, but the right size is the one that reaches over the part of you that gets cold. For a bed, measure the width of the mattress and decide how much hang you want on each side. Check the care label too, so a favourite throw does not end up shrunk in the wash.