- Bivouac sack
A bivouac sack (also known as a "bivy sack", "bivi bag" or just "bivy") is an extremely lightweight, waterproof alternative to traditional
tentsystems. It is used by climbers, mountaineers, hikers, ultralight backpackers, soldiers and minimalist campers.
A bivy 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 chilland 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-Texor "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 silnylonsack, 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 breathed out by the user.
Nowadays there also exists the
bivouac shelteror "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 claimed 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
hikingor hill walkingexpeditions and high mountainclimbs as a compact emergency shelter.
In the UK, bivy shelters have become very popular amongst
coarse fishingand large Carpfisherman, 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.
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