A bivouac sack, bivy, bivy sack, bivi bag or sometimes, in the UK, Emergency Sack or Survival bag, is an extremely lightweight, waterproof alternative to traitional tent systems. It is used by climbers, mountaineers, hikers and minimalist campers. A bivy sack at its barest is a thin waterproof fabric shell designed to slip over a sleeping bag, providing an additional 5 to 10 °F of insulation and forming an effective barrier against wind chill and rain. A drawback of a simple bivy sack is the humidity that condenses at the inner side leaving the occupant or the sleeping bag moist. Better bivy sacks consist of Gore-Tex (or a similar breathable fabric) to allow the humidity to pass.