Ventair is a breathable fabric much like Gore-Tex. It is completely waterproof & air permeable & is located around the base of the bag. Simply fill the bag with your kit, roll the top closed, then as you compress the contents air is expelled through the breathable membrane retaining the waterproof properties of the whole sack. Simple! Integrated adjustable webbing straps ensure the contents stay compressed & also act as useful attachment points or carrying straps.