Mid Insulating Layer
main job for the mid-layer is to trap and hold dry warm air in order to prevent heat loss. It must also continue the
transmission of water vapor through the clothing system as started by
the base layer.
This layer should fit snugly; otherwise you will lose warm air each time you move - otherwise known as ventilated heat loss.
Good insulating options are Synthetic or woolen fabrics. Because they soak up little moisture, and they retain much of their insulating ability when exposed to water and sweat.