More than 25% of body heat is lost through the head and extremities of the body. The vest with hood design provides the best insulation and protection for colder dives. The hood protects against chaffing from mask straps, keeps your head and ears toasty warm. The fullzip design allows you to zip up for 100% warmth and protection, or zip completely down when the sun comes out, your work rate goes up or just for ease of getting on and off.