Fleece is a great material for insulating and keeping you warm in the coldest of conditions. Fleece is an extremely popular material for use in outdoor clothing, especially for lining coats, jackets or even socks and other underwear. Fleece has been used for centuries by many civilastions as a material for keeping warm, and Patagonia's fleeces are great quality, warm garments. They can be worn underneath a coat or jacket or can be worn as an outer layer themselves, with great windproofing qualities and comfort they will be extremely warm to wear whilst hiking in the mountains or encountering sub-zero temperatures. Fleece is extremely warm, but it also gets very heavy when wet so it should be avoided as an outer layer if you will exposed to wet conditions, as this will ruin it's insulation qualities.
Fleece is a great material for insulating and keeping you warm in the coldest of conditions.