Design
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. 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.
Read more about this topic: Bivouac Sack
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—Ralph Waldo Emerson (1803–1882)