Layer 1 - The Base Layer
This
layer is worn next to your skin and must transfer (wick) sweat away
from your body as fast as possible. Polypropylene and Polyester fabrics
are best for this. Beware of cotton clothing which can absorb 30+% more
than its weight in water and transfers heat away from your body thirty
times quicker than dry air.
A good base layer will wick moisture away from your body to keep you dry, warm & comfortable. The purpose is to keep you dry. A good wicking fabric will attract liquid moisture and use body heat to convert this to liquid vapour.
Layer 2 - The Middle Layer
The
main job for the mid-layer is to hold warm dry, insulating air inside
the clothing system and close to the body. It must also continue the
transmission of water vapour through the clothing system as started by
the base layer.
The mid layer provides the essential layer of insulation. This layer should be highly breathable and preferably not waterproof. Liquid vapour should be free to keep moving to the outer layer.