Concepts of Insulation

Dressing for being active in winter is simply a matter of wearing the clothes which will allow you to retain enough (but not too much!) warmth close to your body. To best understand how to dress for being active in cold weather it is useful to know a little bit about how heat gets trapped and how heat gets lost. Heat is lost from our bodies by three main processes: convection, conduction and radiation. These are:

Convection

This is the same process which causes thunderclouds to form, which cause wind, and which cause the top floor of your house to be warmer than the ground floor. The basic idea with convection is that warm air rises relative to cold air. In the framework of being outdoors in cold weather, this means that air that is warmed by your body will tend to want to escape up through openings near the top of your body like necklines or armpit openings, which will be replaced by colder air flowing in from openings near the bottom of your body like the bottom of your shirt/jacket.

Conduction

Conduction is the process which transports heat through a substance from one place to another. If you’ve ever put a cast iron frying pan on the stove and gone to grab the handle, you are well acquainted with the act of heat conduction! For winter clothing the key points are that air is a poor conductor of heat whereas water is an excellent conductor of heat. Thus to keep warm you want to maximize the air layers while minimizing the amount of water in those layers.

Radiation

Radiative heat losses are usually a relatively small percentage of one’s total heat loss. To understand radiative losses one only has to look up in the sky. That big shining yellow object is one big radiative source (it can’t be conduction -- if you thought air was a good insulator, you should try the vacuum in space!). Generally radiative losses are minimized by using reflective materials around the source of loss. Emergency space blankets that look like a large sheet of heavy duty foil helps minimize radiative losses. However, as a rule, radiative losses are not a major factor in choosing clothing for being active outdoors in the winter.

What This Means For Clothes

So how does convection, conduction and radiation fit into the practical picture? Really it is a matter of applying these simple concepts to what we know about the body and its heat production. The body’s core temperature sits at about 37oC. Radiation and conduction causes some of this heat to make its way to the skin and away from the body. One should note that in the context of the above discussion, fat makes a poor conductor because as tissues go it actually has a very low water content. Thus a nice fat layer actually makes a good insulative layer for the body core. However, what we are interested in doing is keeping that warm layer of air that is close to the skin and that has been warmed by the body. The way to do this is to physically restrict that air from moving and hence by exiting due to convection, and also to limit the heat movement from that air to the cold air outside via conduction.

The way to do this is to wear clothing in which the fabric contains many small air pockets. Fabrics such as wool or fleece are great examples of this type of fabric. Both of these fabrics have many small, randomly woven threads which create many small air spaces. These air spaces do not transmit much heat by conduction and hence insulate well. Another important consideration for warmth is the fit of the clothes. From the description of convection above, one can see how it is important to have clothes that fit well, and that do not have huge gaps at the neckline etc. If they do then convective losses will be higher. Also if clothes are too baggy then when we move around we tend to pump the air out from our clothes like a bellows.

So, it’s simple, you just wear a well fitted wool sweater or fleece top and you’re fine ... right? Not so fast!

Evil Sweat

In the ideal world, we would just have to worry about keeping that warm layer of air beside us and minimize conductive losses by blocking gaps and we’d be all toasty warm. However there are a host of other concerns such as the fact that some parts of the body require more insulation than others, and that we tend to perspire when we move, and we just plain generate more heat when we move but then we stop generating that heat when we stop. Let’s consider each of these issues in turn.

An important process which carries heat from one part of our body to the other is blood circulation. Logically then, regions that receive more blood are more susceptible to losing heat if not well insulated. The prime example of this is the head. You may have heard the saying "if you’re cold put on a hat". This is in part because upwards of 40% of the blood circulation is sent to the head, primarily for the brain to use. Keeping your head warm helps this portion of the circulatory system retain its heat so that when the blood returns to the body’s core, it is still at a toasty temperature. On the other hand, if you are generating too much heat, taking off that hat can provide a huge cooling effect as well.

By far the biggest issue with dressing for winter activities is maintaining a careful balance between being warm enough and being too warm. The body’s natural response to being too warm, which can result if you start generating too much heat and your insulative layers do a good job at keeping that heat close to your body, is to perspire. Evaporation of water (e.g. sweat) is an endothermic reaction (??? might be exothermic, can’t remember my physics quite well enough...) meaning that it requires energy. Since evaporation uses energy your skin actually feels colder as your sweat evaporates (if the reaction was the other way, it would give off heat making you feel warmer). So normally this is good, as the body gets too warm, it perspires and the evaporation of this perspiration cools us off and we stay thermally regulated. Unfortunately, when dressed for winter this is not really the case. When we are wearing multiple layers of clothing, this perspiration soaks into the layer closest to the skin and stays there. This now means that there is an insulating layer whose air pockets which were providing the insulation are now at least partly filled with water. But water is a good conductor so suddenly that insulating layer has become a conducting layer, transferring heat away from you. This is why when you’ve been exercising hard and stop you suddenly feel very cold. When you stop you quit producing extra heat and the residual heat around your body is transported away by the wet layer beside it.

What then is the solution? The classic answer is: dress in layers, don’t wear a cotton inner layer if you can help it, and use wool or fleece as an outer layer. The rationale behind this answer is as follows. If you dress in layers, you have the flexibility to adjust your insulation to your heat production. As you become more active you can remove layers until you reach a comfortable equilibrium and you minimize the amount of overheating and hence perspiring that will occur. Cotton gets a bad reputation in active clothing (some people say "cotton kills") because it is a very popular fabric because it feels nice to wear, however it is very good at trapping and holding onto moisture. So any little bit of perspiration that you emit will get sucked into a cotton inner layer and held there to act as a heat sucking garment when you stop perspiring. A far better option is to wear one of the many synthetic fabrics which are designed to transport water away as an inner layer. Fabrics such as polypropolene, bi-polar, etc. are specially designed so that they wick water away from the skin to the outer reaches of the fabric. At this point the water (i.e. perspiration) can evaporate or continue to wick into outer layers. The important aspect of this, is that the highly conductive layer of water is moved away from the skin preventing the transport of heat when the body stops being so active. The benefits of an outer layer of wool or fleece were mentioned above, but basically both fabrics have a large number of small air pockets which provide good insulation. As a final note, on cold and windy days, it is useful to have a windproof outer layer so that the cold wind does not literally blow right through your clothes, removing your warm air inner layer.

Synopsis

In summary:

1. Dress in layers so that you can adjust the amount of insulation.
2. Continually adjust layers to prevent perspiration.
3. Avoid cottons as inner layers, lean towards wools or fleeces as outer layers.
4. Use a windproof outer layer on windy days.
5. Keep the head and trunk area well insulated for maximum warmth.