It is mostly trapped air
Why Is Wool So Warm?
Wool fibres are crimped and springy, so they hold a huge amount of still air — and still air is a poor conductor of heat.
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Step 1 of 5
Air is the real insulator.
Still air carries heat very badly. Almost every warm thing you own — wool, down, a duvet, a wetsuit — works by holding air where it cannot move.
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Step 2 of 5
Wool fibres are kinked, not straight.
Each fibre has a natural wave in it. Kinked fibres cannot pack flat, so a woollen cloth is mostly gaps — and those gaps are full of air.
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Step 3 of 5
It still works when it is damp.
Wool can hold a lot of water inside the fibre while the outside stays dry to the touch, so the air pockets between fibres survive a soaking.
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Step 4 of 5
The fibres are covered in tiny scales.
Under a microscope a wool fibre looks like overlapping roof tiles, all pointing the same way down its length.
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Step 5 of 5
Those scales are why a jumper shrinks in a hot wash.
Heat and tumbling let the fibres creep, and the scales let them creep one way only. They ratchet past each other and tangle for good. That is felting.
A single sheep gives roughly 2 to 5 kg of wool a year — enough for about six jumpers, from an animal that regrows the lot by the next shearing.
The short version
Wool is warm because its crimped fibres trap a great deal of still air, and its one-way scales are why a hot wash shrinks it for good.
Try it yourself
Hold a loose woollen jumper up to a light. The gaps you can see through are the air pockets doing the actual insulating.
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