Thermals & Fleece: Being warm and comfortable is all about wearing the proper clothes for the situation. Dressing in layers is important! Each layer has its own purpose. It all starts with the base layer (also called underwear and sometimes thermals, short for thermal underwear) which is the first layer that is put on. It is worn on the skin. Its purpose is to breath and insulate. Polyester is best because it wicks moisture, offers comfort and warm. The base layer which is next adds ultimate insulation to maximize warmth via heat retention. Polyester fleece (either low or high pile) are the best choice. Browse our selection of thermals and fleece.