“Bivy" (short for Bivouac) sacks are a lightweight option to provide shelter to solo adventurers. Unlike tents, Bivy sacks do not use poles. They work like a waterproof sleeve to your sleeping bag. Because sleeping bags are (mostly) not waterproof, bivys can provide this protection and tend to be much lighter than tents. Most Bivy sacks are made out of a waterproof and breathable material. In addition to protections from the elements, Bivy sacks can add up to 10 degrees of insulation to your sleeping bag.
For the minimalist or lightweight adventurer, Bivy sacks provide the answer for emergency shelters. These sacks, however, have some drawbacks. When deciding to purchase a bivy, you need to weigh the benefits of a pole-less lightweight shelter with comfort. These shelters or sacks have a hard time dealing with the moisture created by your body. If you tend to sleep warmly or sweat a lot, you will be likely to wake up wet. Bivys provide no protection for your gear. They also are impossible to cook in and aren’t the most pleasant or comfortable place to wait out a wet storm.