The Science of Keeping Warm
Dressing to stay warm is all about slowing the transfer of heat from your body to the outside environment. Basically you’re trying to put your warm body in the best thermos possible. The process of heat transfer can be described quantitatively using the law of heat conduction: H=kAdT/dx.
In the equation above, H is the amount of heat energy per unit time that moves from your body to the outside. A is the surface area of your body. dT is the difference in temperature between your body and the outside. And dx is the distance from your skin to the outside. The final element, k* is a constant determined by the insulating material.
The k, or thermal conductivity, of water is .6. The thermal conductivity of air is .023. From this you can see that the conductivity of heat through water is (.6/.023 times) or about 26 times greater than through air.
Dry fleece is mostly trapped air and has a thermal conductivity of about .08. Cotton saturated in water is mostly water and will have a thermal conductivity close to that of water. The thermal conductivity of rubber is .2. It’s pretty easy to see that dry fleece is the way to go to maintain your body heat.
And this is why a skin out wetsuit is warmer than an open cell wetsuit – they both get wet, but the skin out suit slows evaporation and cooling. Not as cozy as a drysuit though.