OUR LIGHTEST WATERPROOF BIVY A true minimalist bivy for fast and light solo expeditions, the 18-ounce Helium Bivy is the lightest bivy we make. Credit the feathery waterproof, breathable Pertex® Shield+ fabric that doesn’t compromise on durability—the floor is 70D. This bivy’s no-nonsense clamshell design will handle seasons of adventuring to come.