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A bivouac sack (also known as a bivy, bivvy, or bivi) is an extremely small, lightweight, waterproof shelter, 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.
Essentially, a bivy 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. They can be made from lightweight silnylon to increase strength and water shedding. 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. Another solution is the use of an inner vapour barrier liner bag, for example a silnylon sack, to prevent body moisture from entering and condensing in the sleeping bag.
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. Gore-Tex Exchange Lite is an air permeable version of Gore-Tex fabric which can be safely zipped up around the user's head in order to shut out the elements completely. This fabric allows moisture and carbon dioxide to escape.
Nowadays there also exists 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.
Many campers gladly accept the increased carrying weight of a bivy shelter for the perceived increase in comfort it affords. However, the traditional bivy sack still holds its place among mountain climbers and backpackers, and is frequently carried on long or dangerous hiking or hill walking expeditions and high mountain climbs as a compact emergency shelter.