A middle layer of clothes that provides extra insulation and insulates body heat. If it’s cold, or if the activity level is low, a thicker layer is needed. The aim is to create a layer of air that insulates; the more air there is in the clothes the better the insulation. Woolpower’s thicker middle layer garments are made of Ullfrotté Original, 400-600 gram. The temperature and activity level determine how thick the middle layer needs to be.