Bivouac sack

A bivouac sack (also known as a bivy, bivvy, or bivi) is an extremely small, lightweight, waterproof , and an alternative to traditional tent systems. It is used by climbers, mountaineers, hikers, ultralight backpackers, soldiers and minimalist campers. The bivouac sack has a larger, similar counterpart, called a bivouac shelter.

The main benefit of a bivouac sack shelter is speed of setup and ability to use in a tiny space as compared to tent-like shelters. A bivouac sack is a good choice for hikers or climbers who have to camp in tight areas or in unknown areas.

A bivouac sack is a thin waterproof fabric shell that is designed to slip over a sleeping bag, providing an additional 5 to 10 °C of insulation and forming an effective barrier against wind chill and rain. A drawback of a simple bivy sack is the humidity that condenses on the inner side leaving the occupant or the sleeping bag moist. This problem has been alleviated somewhat in recent years with the advent of waterproof/breathable fabrics, such as Gore-Tex, which allow some humidity to pass through the fabric while blocking most external water.

A traditional bivy bag typically cinches all the way down to the user's face, leaving only a small hole to breathe or look through. Some modern fabrics are sufficiently gas-permeable that they can be safely zipped up around the user's head in order to shut out the elements completely.

A more recent development is the bivouac shelter or "bivy shelter", a compromise between a bivy sack and a single-person tent. Often employing hoops over the head and feet, a bivy shelter is held sufficiently taut to keep the fabric off the occupant inside in order to prevent condensation from soaking into bedding. This style of shelter also provides some additional breathing room around the head. Its design also enables the occupant to roll over comfortably, thereby allowing comfortable sleeping in any desired position without altering the orientation of the top surface of the bivouac shelter.

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