Small and lightweight, waterproof compression sac from Lowe Alpine (size extra large). It lets air out and keeps the contents dry. Its base is made using eVent waterproof yet air permeable fabric, which allows you to force the air from the dry bag, as the top is rolled down. The air can only pass through the fabric in one direction, keeping the bag and it's contents compressed. To maintain compression, the Airstream has two additional buckled straps running the length of the bag. Once the top is rolled down, then these keep the bag compact.