The bivy bags ideally must have zippers, strong body, and tear-resistant construction. These should also include: Robust zippers: The strong the zipper, the longer you can use the sleeping bag. If the zippers are of cheap quality and break, you may not be able to keep the cold out. Straps: These straps keep the sleeping pads in place inside. Multiple Ventilation: Many bivy sacks have mesh windows and flaps on the top to bring in some fresh air into the bag.