A bivy sack, short for “bivouac sack,” is a very lightweight sleeping bag cover. It protects the bag and user primarily from moisture and insects. Bivy sacks are very tight enclosures designed for the minimalist. Think of a bivy as a bag for your sleeping bag. There are many options for bivy sacks ranging from low priced emergency bivies to more elaborate bivies.
Types of Bivies
A bivy sack can be a simple emergency bivy made from space blanket material to more elaborate bivies that resemble mini tents. The smallest and lightest versions use only space blanket material (about $15). Other versions are two to three times as large, heavier, and have a liner, and are larger but more comfortable.A basic Emergency Bivy is made from space blanket material and the edges are sealed to give it the shape of a sleeping bag. These bivies are completely water and wind proof and do NOT breath at all. Crawl into the bivy for warmth and protection from the elements. They are very small, inexpensive ($15-$25), lightweight, and effective. I recommend including an emergency bivy in every emergency kit. You can get very low cost bivies at retail outlets such as Walmart. These bivies are typically made from silver mylar material and will be effective in reflecting heat. I prefer the SOL bivy because (quoted from their instructions), “The Heatsheets material used in your emergency bivy represents a more durable, quieter reflective material than the traditional mylar polyester, space blanket products. Most importantly, the Heatsheets polyethylene material is not subject to catastrophic rips that would render a mylar, polyester blanket or bivy useless, exposing you to the elements and potentially much more.” A Breathable Bivy is more comfortable and water resistant rather than waterproof. They usually have a more comfortable lining and will reduce the amount of condensation inside the bivy. I talked with one minimalist who used a breathable bivy inside of a basic bivy. This layered approach kept most of the condensation away from him, added warmth, and created a waterproof shelter. Expect to spend between $25 and $50 for a breathable bivy.
A Lined and Waterproof Bivy is a more comfortable and larger version of the basic emergency bivy and generally costs between $25 and $50. This bivy is waterproof and does NOT breath. It is larger and slightly heavier than the basic emergency bivy. This bivy would also be a very good option for a vehicle kit.
For hiking kits and Bugout Bags I recommend the ultra small and light small space blanket bivy. For home and vehicle kits I would consider using one of the larger, lined versions.