Developed as a proprietary insulating material used exclusively in Spyder brand products, Thermaweb is a synthetic material made from ultra-fine polyester fibers. The construction and palcement of these fibers allows them to effectively trap and retain your body's heat. Their synthetic nature also means they can hold on to their insulating abilities in wet conditions, where other materials, such as down, lose their thermal properties. Garments that have Thermaweb inside are also very packable, as the ultra-fine fibers are able to compress significantly.