Wool, Flannel, Tweed: Which Winter Cloth Is Warmest?

Every winter the same three wool cloths come back into rotation: wool itself, flannel, and tweed. All three have a reputation for warmth, and all three earn it, but they do not earn it the same way. One relies on the fiber, one on the finish, and one on the weave. Which is actually the warmest depends on what you are asking the cloth to do, and on whether you are sitting still indoors or walking into the wind.

What actually makes a cloth warm

Warmth in cloth is mostly about trapped air. Still air is one of the best insulators there is, so a fabric keeps you warm to the degree that it holds a layer of it against your body and stops it moving. Every warm cloth is really just a different way of trapping air. The fiber can do it, if its shape builds in tiny pockets. The finish can do it, if the surface is brushed up into a soft fuzz. The weave can do it, if the cloth is dense enough to block the wind that would otherwise strip that warm layer away. Wool, flannel, and tweed each lean on a different one of those three, which is why they feel warm in different situations.

Wool: the fiber does the work

Wool is warm before it is ever woven, because of the fiber itself. A wool fiber is naturally crimped, a built-in waviness that keeps the fibers from packing down flat, so a wool yarn is full of small air spaces. As Wikipedia's account puts it, "because of the crimp, wool fabrics have greater bulk than other textiles, and they hold air, which causes the fabric to retain heat." That trapped air is the warmth. Wool also manages moisture unusually well, absorbing close to a third of its own weight in water before it feels wet, so a damp day does not immediately collapse the insulation the way a soaked cotton would. That fiber advantage is why wool gives the most warmth for its weight, and why a thin wool layer can outperform a much thicker one made of something else.

Flannel: the finish does the work

Flannel starts as an ordinary soft cloth, wool or cotton, and gets its warmth from what is done to it after weaving. It is brushed: a fine metal brush rubs the surface to raise loose fibers into a soft nap on one or both sides. That raised fuzz is not just for softness. It holds a shallow layer of still air right against the skin, which is exactly the layer that keeps you warm, and it is why a flannel shirt or a flannel sheet feels warm the instant it touches you in a way a smooth cloth of the same weight does not. The trade is that flannel is usually a lighter cloth built for comfort next to the skin, not a barrier against weather, so its warmth is the cozy indoor kind rather than the walk-into-the-wind kind.

Tweed: the weave does the work

Tweed is a rough woollen cloth, usually woven as a twill or herringbone, and closely enough that it earns its living outdoors. It was made to withstand harsh climates, a staple of Scottish, Irish, and Welsh country clothing, and it is prized for being moisture-resistant and durable. Its warmth is structural: a dense woollen weave is hard for wind to push through, and blocking the wind is what keeps the warm air near your body from being carried off, which is the main way you lose heat outside. So tweed is not necessarily warmer to the touch than a soft wool knit, but it is the one of the three built to keep you warm while you are moving through cold, damp, blowing weather rather than sitting by a fire.

So which is warmest?

For raw warmth relative to weight, wool the fiber wins: its crimp traps more air per gram than either of the others can, which is why a wool base layer or a wool knit is the efficient choice. For soft, immediate, indoor warmth, flannel is the pick, because the brushed nap puts an insulating layer right where you feel it, against the skin. For real weather, tweed is the answer, since a dense woollen weave blocks the wind that would otherwise carry your warmth away, and it shrugs off damp while it does so. The honest answer to which is warmest is that it depends on the job: wool for warmth you can carry lightly, flannel for comfort by the fire, tweed for the walk back in the cold.