Bivouac sack

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

Design
A bivouac sack, at its barest, is a thin waterproof fabric shell (for example, made from lightweight silnylon) designed to slip over a sleeping bag, providing an additional 5 to 10 °F) of insulation and forming an effective barrier against wind chill and rain. A drawback of a simple bivouac sack is the humidity that can condense 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 or Pertex Quantum, which allow some humidity to pass through the fabric while blocking most external water. Another solution is the use of an inner vapor barrier liner bag, for example a silnylon sack, to prevent body moisture from entering and condensing in the sleeping bag.

A traditional bivouac bag typically cinches all the way down to the user's face, leaving only a small hole to breathe or look through. More recent developments in high-tech breathable fabrics have resulted in the creation of Gore-Tex Exchange Lite, an air permeable version of the fabric which can be safely zipped up around the user's head in order to shut out the elements completely. The fabric not only allows moisture to escape, but also the carbon dioxide exhaled by the user.

Bivouac shelters
Nowadays there also exists the bivouac shelter or "bivy shelter", a compromise between a bivouac sack and a single-person tent. Often employing hoops over the head and feet, a bivouac 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 bivouac shelter for the claimed increase in comfort it affords. However, the traditional bivouac sack still holds its place among mountain climbers and backpackers, and is frequently carried on long or dangerous hiking expeditions and high mountain climbs as a compact emergency shelter.

Popularity
In the UK, bivouac shelters have become very popular amongst coarse fishing and large carp fisherman, who, in pursuit of their quarry, fish throughout the night. This upsurge in popularity has increased competition amongst manufacturers and designs have become more advanced.