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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 10F of insulation and forming an effective barrier against wind chill and 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-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 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.
New Condition
Mil-Spex is a good reproduction of the original.
