High-quality goose down is the warmest insulating material by weight and is highly compressible. Down is a common fill for sleeping bags, which are preferred for winter camping and mountaineering. Typical down-insulated items in a mountaineer's pack, besides the sleeping bag, include a hooded jacket, pants, vest, booties and sometimes down-insulated mitts. High-quality goose down has 700 to 800 loft, which means it fills 700 to 800 cubic centimeters per gram of down. Down is expensive and when damp or wet loses its ability to insulate as effectively.
This natural fiber is much less absorbent than, say, cotton (which is never worn in extreme cold) and thus keeps a person warmer than down when wet. The dead airspace that gets trapped in wool fibers keeps people warm just as it did the sheep it came from. Wool is the preferred material for cold weather socks and certain base layers. Companies such as SmartWool, Ibex, Arc'teryx and Patagonia produce finely spun merino wool and micro wool fibers into moisture-wicking base layers that are naturally antimicrobial and nonitchy.
Synthetic Fibers of the 1990s and 2000s
Polyester, polypropylene, nylon and blends make up the brunt of extreme cold weather layers. Polyester and "polypro," as it is termed, are made to wick moisture away from the body. These are used as base layers, as well as insulating layers, in the form of thick fleece or pile jackets, pants, long-sleeved shirts, hats, mittens, insulating glove liners and balaclavas. In extreme cold, these must be worn with additional insulation, such as a down jacket and pants, as well as a wind-blocking layer often made of some form of nylon.
Insulating base and mid-layers must be topped by an overlayer in windy or wet conditions. Some down expedition jackets are made with a waterproof, windproof nylon-coated shell, but many must be worn with a separate jacket shell and pants. Materials such as Gore-Tex are well known for these layers. Sometimes, if you are in an environment that is so cold that there is no moisture in the air, a WindStopper layer is needed instead of a waterproof layer. Footwear, too, is layered in extreme cold. Overboots are worn over regular insulated boots to create another layer of warmth around the feet.
High-Altitude Mountaineering Suits
One of the warmest items is a mountaineering suit, specifically made for high-altitude expeditions. These are meant to provide the body with the maximum amount of insulation in extreme temperatures, wind chill and exposure. Suits such as Mountain Hardwear's Absolute Zero Down Suit, Marmot's 8000 Meter Down Suit and Feathered Friends Expedition Down Suit all incorporate a full-body design that helps to better lock in warmth and is meant to fit over all other layers. The only thing that goes over a mountaineering suit is a climbing harness. As of August 2009, these all cost around a grand, but are made with 800 loft and have proven to be the best clothing to wear in the most extreme cold.