Air and Only Air Insulates
Trapped air is what insulates. When dressing for winter you want clothing with “loft” to hold air. Fabrics include down, synthetic fleece, and wool. This is another advantage to the layering system, as several layers will trap more air than a single thick one.
Wind Protection is a Must
A tightly woven outer layer of nylon or other material will block the wind. It is important that this layer be a compromise between windproof and breathable since you want any perspiration to be able to escape. Many winter jackets have an integral wind shell. During the trip your most important item will be your wind shell, which rarely will you not be wearing. It is imperative that your wind shell has a hood for wind protection and snowfall.