BackcountryForum.com: Clothing Science and Folklore

Wicking - Why is Wool warm and Cotton cool?

I gained some new insight the other day. I think it was from that hot brain book. We understand that wool can hold 35% of its weight in moisture before it feels wet, but why does it sometimes 'feel' warm, even warmer than fleece or down? Also, what's up with the base layer? Is is just a wick, or is it more?

First, why is wool warm?

It turns out that even when we are wearing the right amount of cool weather clothing so that we are not sweating our body is still giving off moisture. This moisture evaporates from our skin, which takes heat in the form of latent heat of vapourization. Water vapour doesn't need to be 212degF. It can exist as partial pressure at cooler temperatures. The skin surface is about 85degF when wearing a comfortable amount of clothing. This latent heat comes from the sensible heat of the skin and the moisture it is giving off. If we are wearing a wool sweater or wool mitts or wool socks this water vapour re-condenses as it passes through the the wool, because it cooler than the body the further you go out. This condensing releases the latent heat, which makes the wool warmer than it otherwise would have been. So the wool gains both latent heat and sensible heat from the moisture. If you are wearing wool mitts and you get a gust of wind your hands will first feel cool as more moisture evaporates off the skin, but then they will feel warm again as they gain some heat coming back from the wool. The wool doesn't produce any heat, everything it gains from the body must eventually be released to the environment, but it acts like a thermal capacitor, absorbing and releasing both latent and sensible heat. There is also some additional heat released when moisture freezes on the outside of your clothing, and the heat is taken up when the ice melts or sublimates, unless you brush the ice crystals off now and then. Fur is particulary good at managing this heat of fusion, by allowing ice crystals to be shed.

Why is cotton cool?

Cotton can also absorb water, and thus can store both latent and sensible heat the same as wool. The difference is that wool still acts as both insulator and thermal capacitor when it is wet, whereas the cotton is more likely to be saturated, which effectively short circuits heat directly away from the body, and so the body loses heat as though it were naked and soaking wet, even colder because they can effectively increase the surface area for evaporation while providing a negligible amount of insulation.

What about polyester, silk, cotton flannel, etc?
Depending on how they are put together, most insulating materials, even cotton, can provided varying degrees of thermal insulation and thermal capacitance before short circuiting. In hot weather you want something more like thin cotton. In cold weather you want behaviour more like wool, but the total amount of capacity for holding moisture without short-circuiting varies depending on the climate and application. Wool itself varies in its porperties. Some wool is better for very wet and cold weather. Some wool is more like down, not as good in wet weather but lighter for its warmth in dry weather.

So what's up with base layers?

So a base layer can have many functions.
1. Thermal insulation against thermal conduction.
2. Thermal capacitor of both latent and sensible heat.
3. Allowing transport of both water vapour and liquid moisture.
4. Providing increased surface area for both evaporation and condensing.
5. Short-circuiting its thermal insulation when saturated, for hot weather.
6. Or retaining some thermal insulation when near saturated, for cold wet weather.

I am still not entirely sure of the distinction between base layer and mid-layer, or whether skin fitting base-layers are always needed for comfort. Genrally speaking on a long trip you only need to optimized performance for the worst possible conditions. The rest of the time is largely a matter of comfort and convenience. Most of the time I just wear a wool sweater without any skin layer underneath or shell layer over it, but I have those extra layers in the pack to switch it into a parka or foul weather jacket.

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