Enjoy serious warmth without bulk in Seirus` Thermax Skull Liner working to moderate temperature and moisture for you so your head stays warm dry and comfortable. FEATURES: Ultra-Thin Thermax Wicking Quick Drying Breathable Lightweight Low Profile Flatlock Stitching 3 in 1 Uses: Fits Beneath Helmets Layers Under Hats Works Well All AloneThermaxThermax prevents heat loss by trapping a layer of warm air around the body while transporting perspiration outward for evaporation.