RIE Cyclops bivy sack, Gortex, used
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A bivy sack, or bivouac sack, is a personal-sized waterproof shelter. The basic purpose of a bivy sack is to add a thin waterproof fabric over you in your sleeping bag to protect you while you sleep. It provides insulation and prevents wind chill and rain. The most basic bivy sacks have an opening for your eyes and for breathing. Though, recently, with the introduction of waterproof and breathable fabrics such as Gore-Tex, bivy sacks are able to close without any opening, completely shutting out all the elements.