Why do wool and down keep us warm?
For humans, a big part of staying comfortable is keeping the temperature right. Too hot or too cold and our body freaks out. If the body’s core temperature gets just a few degrees above normal for too long, heat exhaustion can develop into heat stroke, producing dehydration, confusion, and eventually even death. Just a few degrees below normal produces severe shivering, poor coordination, and again confusion and even death. Like other mammals, we regulate our body temperature physiologically and behaviorally within a very narrow range, because the machinery of our body works best in that range of temperatures.
Where does the heat come from to keep out bodies around 37 degrees C? That’s almost always warmer than our surroundings, so our bodies must be generating heat or we would cool down. Like all living things, we have enzymes throughout our bodies that are constantly performing chemical reactions, and each chemical reaction releases a little bit of heat (the second law of thermodynamics says they have to, and they obey). So as long as we’re alive, the combination of all those little chemical reactions causes our bodies to keep pumping out heat.
To keep our core temperature constant, we need to let that heat dissipate away fast enough but not too fast. When we get too hot, we look for a cool place to be so there’s a steeper thermal gradient between our bodies and their surroundings, causing faster heat dissipation. Or we point a fan at our body to speed the flow of warm air away from it and to promote evaporative cooling.
When, on the other hand, we get too cool, we either seek shelter in a warmer place or put on warmer clothing. Ah, the snug comfort of slipping on a thick jacket on a cold day, or wrapping oneself in a warm blanket! Like our fear of burning ourselves in fires, our experience of snug warmth and our intuitive knowledge of how to get it date back long before we had the mental tools to understand how jackets and blankets could protect us against the cold. In fact, that experience of snug warmth dates back to before our earliest memories, to when our parents swaddled us lovingly on cold days, and held us close to the warmth of their own bodies.
But now that we’re all grown, we shouldn’t take these things for granted anymore, right? The basic idea of insulation is simple enough: if we let less heat flow away from our body, then more of the new heat that is generated in each second sticks around, which will raise the temperature around us, keeping us warm. We’ve got to let some of the heat flow away, or we’d get hotter and hotter till we died. But the colder it is around us, the less flow of heat we want, so the more insulation we need.
But what is it specifically about materials like wool and down that provide such good thermal insulation? Why is wool warmer than an equivalent amount of leather or rubber? Why is down so light and yet so warm? The key is the air pockets between the fibers of wool and between the tiny feathers in down.
Like all gases, the air around our bodies is a lousy conductor of heat. The molecules in a gas are far away from each other, and conduction of heat only happens when a faster-moving molecule collides with a slower-moving one, transmitting some of its kinetic energy to the other molecule. Since heat is simply molecular kinetic energy, this transfer of kinetic energy between molecules enables heat to flow through a body from where hotter areas to cooler ones. This does happen a bit in a gas, but much less than in liquids and solids, where the molecules are much closer together.
If the air around our bodies were completely still (and if we too kept completely still), then that air would act as a pretty good heat insulator, and we would stay nice and warm. But air being a fluid naturally moves around, and the movement of air carries heat away from our bodies by a process called convection. On a windy day, the atmosphere of warmer air near our bodies is carried away, and it is replaced by cooler air, thus speeding the loss of heat. That’s why the wind chill factor is a relevant measure of how cool the air feels on a cold, windy day.
But when we wrap our bodies in layers of wool or down, we surround ourselves with a huge number of tiny air pockets. The fibers or feathers between those air pockets block the movement of air from pocket to pocket, thus reducing the flow of heat by convection within the material. Convection can still carry heat away from the surface of a sweater or jacket, but the temperature is lower there than at the surface of our bodies, so there is less heat loss by convection than if we were naked or wearing less insulating clothing.
One famed property of wool is that it still insulates when wet, unlike cotton which only insulates well when dry. Why is this? When cotton gets wet, water fills up the tiny air pockets that otherwise would provide insulation. Because at the microscopic level water doesn’t cling as well to wool as it does to cotton, wool can get wet without its tiny air pockets being filled up with water, so it still reduces heat loss by convection.
Synthetic fiber insulation like Thinsulate and PrimaLoft works the same way as natural fiber insulation. The best insulation will pack as many tiny air pockets as possible into the smallest possible space with the least weight of material. There should be enough fiber between the air pockets to minimize convection, and yet enough space between those fibers to let moisture pass through the fabric to wick away from the body. Ideally, the fibers will also be made out of molecules that water does not readily cling to, so the air pockets don’t fill up with water when the fabric gets wet.
© Joel Benington, 2012.