Bivouac sack

From WildernessWiki.org

A bivouac sack, often called a bivy sack, is a lightweight, waterproof shell for a sleeping bag, designed to be an alternative to a traditional tent. It originated as an invention to serve the needs of climbers who wanted weather protection for sleeping bags during multiple-day ascents, particularly on big walls, and is also used today by mountaineers, hikers and minimalist campers.

The bivy sack at its barest is a thin waterproof fabric shell designed to slip over a sleeping bag, providing an additional 5-10°F of insulation and forming a barrier against wind chill and rain. It typically cinches all the way down to the user's face, leaving only a small hole to breathe or look through. Early bivy sacks were little more than waterproofed nylon slipcovers for sleeping bags, which kept out rain but didn't ventilate vapor produced by body heat, resulting in condensation and a damp sleeping bag. Most modern bivy sacks are made of breathable fabrics to allow the humidity to pass.

These modern bivy sacks consist of a two-layered construction. The bottom layer typically consists of a durable grade of nylon that is coated with urethane to make it waterproof. The top layer is usually made of ripstop nylon (a lighter fabric) and treated with a waterproof/breathable laminate such as Gore-Tex, Tegraltex or a similar fabric. Multiple layers of laminate are often applied for durability and performance.

A direct descendent of the bivy sack is the bivy shelter, a cross between bivy sack and single-person tent. For an extra pound or two, the bivy shelter uses hoops or poles to provide a full enclosure with additional headroom and insect protection. Bivy shelters are often geared towards ultra-light hikers or minimalist campers, but the traditional bivy sack still holds its place among mountain climbers and backpackers.

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