A bivouac sack is an extremely small, lightweight, waterproof shelter, and an alternative to traditional tent

Tent

A tent is a shelter consisting of sheets of fabric or other material draped over or attached to a frame of poles or attached to a supporting rope. While smaller tents may be free-standing or attached to the ground, large tents are usually anchored using guy ropes tied to stakes or tent pegs...

 systems. It is used by climbers

Climbing

Climbing is the activity of using one's hands and feet to ascend a steep object. It is done both for recreation and professionally, as part of activities such as maintenance of a structure, or military operations.Climbing activities include:* Bouldering: Ascending boulders or small...

, mountaineers

Mountaineering

Mountaineering or mountain climbing is the sport, hobby or profession of hiking, skiing, and climbing mountains. While mountaineering began as attempts to reach the highest point of unclimbed mountains it has branched into specialisations that address different aspects of the mountain and consists...

, hikers

Hiking

Hiking is an outdoor activity which consists of walking in natural environments, often in mountainous or other scenic terrain. People often hike on hiking trails. It is such a popular activity that there are numerous hiking organizations worldwide. The health benefits of different types of hiking...

, ultralight backpackers

Ultralight backpacking

Ultralight backpacking is a style of backpacking that emphasizes carrying the lightest and simplest kit safely possible for a given trip. Base pack weight is reduced as much as safely possible, though reduction of the weight of...

, soldiers and minimalist campers. The bivouac sack has a larger, similar counterpart, called a bivouac shelter

Bivouac shelter

A bivouac traditionally refers to a military encampment made with tents or improvised shelters, usually without shelter or protection from enemy fire or such a site where a camp may be built. It is also commonly used to describe a variety of improvised camp sites such as those used in scouting and...

.

Design

A bivy sack at its barest is a thin waterproof fabric shell (for example, made from lightweight silnylon

Silnylon

Silnylon, a portmanteau of "silicone" and "nylon", is a synthetic fabric used mainly in lightweight outdoor gear. It is made by impregnating a thin woven nylon fabric with liquid silicone from both sides. This makes it strong for its weight, as the silicone substantially improves the tear strength....

) designed to slip over a sleeping bag

Sleeping bag

A sleeping bag is a protective "bag" for a person to sleep in, essentially a blanket that can be closed with a zipper or similar means, and functions as a bed in situations where a bed is unavailable . Its primary purpose is to provide warmth and thermal insulation...

, providing an additional 5 to 10 °F (2 to 5 °C) of insulation and forming an effective barrier against wind chill

Wind chill

Wind chill is the felt air temperature on exposed skin due to wind. The wind chill temperature is always lower than the air temperature, and the windchill is undefined at the higher temps...

 and rain

Rain

Rain is liquid precipitation, as opposed to non-liquid kinds of precipitation such as snow, hail and sleet. Rain requires the presence of a thick layer of the atmosphere to have temperatures above the melting point of water near and above the Earth's surface...

. 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

Waterproof fabric

Waterproof fabrics are fabrics that are inherently, or have been treated to become, resistant to penetration by water and wetting. They are usually natural or synthetic fabrics that are laminated to or coated with a waterproofing material such as rubber, polyvinyl chloride , polyurethane , silicone...

, such as Gore-Tex

Gore-Tex

Gore-Tex is a waterproof/breathable fabric, and a registered trademark of W. L. Gore and Associates. It was co-invented by Wilbert L. Gore, Rowena Taylor, and Gore's son, Robert W. Gore. Robert Gore was granted on April 27, 1976, for a porous form of polytetrafluoroethylene with a...

, 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

Silnylon

Silnylon, a portmanteau of "silicone" and "nylon", is a synthetic fabric used mainly in lightweight outdoor gear. It is made by impregnating a thin woven nylon fabric with liquid silicone from both sides. This makes it strong for its weight, as the silicone substantially improves the tear strength....

 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. 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

Bivouac shelter

A bivouac traditionally refers to a military encampment made with tents or improvised shelters, usually without shelter or protection from enemy fire or such a site where a camp may be built. It is also commonly used to describe a variety of improvised camp sites such as those used in scouting and...

 or "bivy shelter", a compromise between a bivy sack and a single-person tent

Tent

A tent is a shelter consisting of sheets of fabric or other material draped over or attached to a frame of poles or attached to a supporting rope. While smaller tents may be free-standing or attached to the ground, large tents are usually anchored using guy ropes tied to stakes or tent pegs...

. 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

Backpacking (wilderness)

Backpacking combines the activities of hiking and camping for an overnight stay in backcountry wilderness...

, and is frequently carried on long or dangerous hiking

Hiking

Hiking is an outdoor activity which consists of walking in natural environments, often in mountainous or other scenic terrain. People often hike on hiking trails. It is such a popular activity that there are numerous hiking organizations worldwide. The health benefits of different types of hiking...

 or hill walking expeditions and high mountain

Mountain

Image:Himalaya_annotated.jpg|thumb|right|The Himalayan mountain range with Mount Everestrect 58 14 160 49 Chomo Lonzorect 200 28 335 52 Makalurect 378 24 566 45 Mount Everestrect 188 581 920 656 Tibetan Plateaurect 250 406 340 427 Rong River...

 climbs as a compact emergency shelter.

Popularity

In the UK, bivy shelters have become very popular among coarse fishing

Coarse fishing

Coarse fishing is a term used in the United Kingdom and Ireland for angling for coarse fish, which are those types of freshwater fish other than game fish . The sport and the techniques used are particularly popular in the United Kingdom and mainland Europe.-History:The term "coarse fishing"...

 and large carp

Carp

Carp are various species of oily freshwater fish of the family Cyprinidae, a very large group of fish native to Europe and Asia. The cypriniformes are traditionally grouped with the Characiformes, Siluriformes and Gymnotiformes to create the superorder Ostariophysi, since these groups have certain...

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.