The insulation layer is vital to your hiking apparel system. The function of this layer is to retain a protective layer of air. In general, these garments are made of polyesters that are designed to make fibers stand up, trapping air between the fibers. The most popular material used to create insulation layer garments -- such as vests and pullovers -- is fleece. Fleece can be bunted (napped to make a fluffy layer) or piled (layered to create a thicker layer).