eVac Dry Bags and Compression Dry Sacks use an eVent fabric base. It is waterproof and air permeable: water will not pass through the laminate, but air passes through the eVent laminate in both directions equally easily.
The eVent fabric does not work as a 'one way valve'. If an item (like a sleeping bag or puffy jacket) is packed down tightly inside an eVac Dry Bag, it will expand, drawing air into the dry bag.
Here's how an eVac Dry Bag https://seatosummit.com/products/the-evac-dry-bag should work:
Pack your compressible item into the eVac Dry Bag, roll and click the closure, then sit on the dry bag. You will be able to squeeze almost all of the excess air out. Then pack the dry sack in a tight space before it draws itself full of air again to achieve the best utilization of available volume inside a backpack.
eVac Compression Dry Bags https://seatosummit.com/products/the-evac-compression-dry-bag have compression straps on the sides. Cinching them down will force the air out through the base of the dry bag, and the straps will ensure that the dry bag remains compressed. An eVac Compression Dry Bag weighs a little more than an eVac Dry Bag
Please note: eVent laminates are sometimes described as "breathable": they allow moisture-vapor to pass through while remaining waterproof. Some end-users have misconstrued this, believing that wet/damp items packed inside an eVac Dry Bag/Compression Dry Bag will dry out. They will not - moisture vapor will only pass through the laminate if the inside of the dry bag is warmer than the outside. A 'waterproof-breathable' jacket works because the wearer is (usually) warmer than the outside air. An item of wet gear is not warmer than the outside air, so if packed in an eVac Dry Bag it will remain wet.
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