Saturday, 29 December 2012

Keeping Warm Part 3: Dryness

Staying Warm 99In my summary post on how to keep warm, I mentioned how critical is was to keep your insulator dry. In this post, I will look at this issue in more detail.

Dryness
If you check the termal properties of water, you will see that water is not as good an insulator as cotton, wool, polyester, or polypropylene. Water is between 5 and 10 times worse at insulating than they are. When an insulator becomes wet, the thermal properties swing towards that of water and away from that of the insulator. The wetter the insulator becomes, the more like water it behaves when it comes to losing heat.

Think sweat: we sweat (and dogs pant) because our sweat conducts heat away from our skin and the evaporation process also absorbs heat to cool us. These processes are good enough to cool us when we overheat. Likewise, a wet insulator will also transport heat both through conduction and the evaporation process and in quantities enough that it will start to cool you instead of keeping you warm. While you may not notice the reduction in insulating properties too much during a high level of activity, you will probably notice it when you stop!

Back to the three common types of insulators: cotton, wool, and poly (either polyester or polypropylene, and they are sometimes called "technical fabrics"). How much water they can absorb is what sets these materials apart. Cotton will absorb up to 60%, wool up to 40%, polyester up to 5%, and polypropylene up to 0.02%. There is a nice summary table of water absorption here. Polypropylene is not listed in that table, but can be found in "Polypropylene: The Definitive Users Guide" on page 128.

As you can see, both wool and cotton can absorb tremendous amounts of water. When they do, their thermal properties become more like water and you can lose a lot of heat. A wool fabric can perform a bit better when wet than cotton, because it can absorb some water inside its wool fibers, but it will still not perform as well as a poly fabric. The poly's absorb almost no water and tend to dry quickly as well, and are often my first choice for dealing with cold weather.

However, poly's have other downsides not related to staying warm and because of this, some people reach for wool or cotton before reaching for poly. For example, when I am cozying up to a campfire, I will always pull on my jeans and a cotton jacket. A stray flanker or spark from the fire will melt right through any poly fabric, whereas the cotton is much more resistant to the spark.

A note of caution is important here: any wet insulator can contribute to exposure and lead to bodily harm or even death. Please learn the signs of frostbite and hypothermia and how to deal with them. Always have a back-up plan in place when outside in the cold. Carry extra clothing when possible. I always carry a cellphone with me when I run in the winter and I always run close to a road. If I injure myself, I can call for a ride.

While using an insulator like a poly means you have something that is going to perform well when wet and also dry quickly, you should protect your insulator from getting wet in the first place. Keep rain or snow off by wearing a waterproof shell on top of your insulator. This can be anything from a garbage bag to a piece of clothing with Gore-Tex or some other high-tech water barrier. These shells often double as wind barriers, so you can get a "two-for-one" with one piece of clothing. With Gore-Tex, you actually get a "three-for-one", as I am about to explain.

Perspiration is one source of wetness that many people forget about. Maybe because they cannot see it, but the effect of sweating on an insulator is the same as the effect of snow or rain.

One solution is to follow the total "vapor barrier" method. This is like wearing a garbage bag right next to your skin and under your insulator. It allows you to sweat all you want, but the "garbage bag" prevents the sweat from contaminating your insulator. If you put another "garbage bag" on top of your insulator, you will have an insulator that is probably not going to get wet nor going to cool from the wind. Just don't expect to get many fashion awards!

A much more common method to keep your insulator dry is to use poly insulators with a Gore-tex shell. The poly's can transport sweat away from your skin, not absorb a lot of the water in the process, and then let the sweat evaporate through the Gore-Tex (Gore-Tex is like a one-way mirror: it lets sweat pass through one side but blocks rain on the other).

So in summary: you need to keep your insulator dry from sweat and from rain. Technical fabrics like polyester, polypropylene, and Gore-Tex are great for this. Natural fabrics like cotton and wool are not.


The complete Keeping Warm series is here:

Keeping Warm: Summary
Keeping Warm: Insulation Thickness
Keeping Warm: Dryness
Keeping Warm: Wind

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