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If you are unfamiliar with the expanded number of bivy options in circulation, take a look. You'll be impressed by the variety of choices and intelligent designs available to independent adventurers who are determined to keep their loads as light as possible.
Who uses bivy sacks? People who:
Bivy sack is short for "bivouac sack." It originated as an invention to serve the needs of climbers who wanted lightweight emergency weather protection for sleeping bags during multiple-day ascents, particularly on big walls.
Early bivy sacks were little more than waterproofed nylon slipcovers for sleeping bags--good for keeping sleeping bags shielded from rain, not so good when ventilating vapor produced by body heat.
Bivy design today involves 2 tiers of fabric. The bottom tier typically consists of a durable grade of nylon (usually taffeta, sometimes oxford) that is coated with urethane to make it waterproof. This is the same material used for most tent floors.
The top tier is usually made of ripstop nylon (a lighter fabric) and treated with a waterproof/breathable laminate such as Gore-Tex®, Tegraltex or REI Elements®. Multiple layers of a laminate are often applied for durability and performance.
Over time, the original bivy spawned a sister product with tentlike characteristics, the bivy shelter. For an extra pound or so, a bivy shelter adds 2 features not available with traditional bivies— an expanded area of shielded headspace and a full enclosure to block out bad weather and insects. These extras have helped bivy shelters grow in popularity with nonclimbers, particularly ultralight hikers.
In addition, a bivy-inspired subcategory of 2-walled tents has emerged. The average weight of these streamlined tents (about 4 pounds) is heavier than a standard bivy (2 pounds or less), yet they offer more interior wiggle room at a modest weight—a comforting fact for soloists who like a little sit-up space in their shelter but want to travel light. Examples: REI Sololite; Kelty Clark Tent; Walrus Micro Swift; Sierra Designs Clip Flashlight.
Even when designed with modern fabrics, a traditional bivy sack is intended primarily for mountaineers or committed minimalists—people who drill holes in their spoons to save a few fragments of an ounce.
A basic bivy performs 2 basic functions: It keeps a camper's sleeping bag dry and increases its warming capacity by approximately 10 degrees.
A bivy sack includes an opening for your head. When rain falls, some moisture can potentially find its way inside via the unshielded head opening. A camper can minimize that risk by pulling the headhole's drawstring very snuggly. Doing so, of course, turns the headhole into more of a nosehole, which some people find far too restrictive. Yet this is a small sacrifice to ultralight travelers who prize a bivy's next-to-nothing weight.
Additional bivy considerations:
Bivy sack evolution has led to a category of low-rise tents known as bivy shelters. These models include mesh panels attached to the head opening, plus small suspension systems (poles, hoops or stiffened wires) that lift fabric off a camper's face. In a bivy shelter, it is possible to achieve full enclosure and shut out bugs and rain. This requires a little resourceful venting during a downpour, but it can be done.
For many go-light long-haul backpackers, a bivy shelter's fortified wedge of head space provides just enough of a comfort zone to make a bivy's restricted air space seem acceptable. In areas of persistent rain, bivy shelters lose some of their appeal; it can be tough to wait out a storm inside a shelter that offers no sit-up space. Yet bivy shelters make a lot of sense in areas blessed with benign weather, such as the Sierra Nevada.
Is a bivy too tight for you? You might initially think so. But don't dismiss this style of shelter too quickly. A tent offers campers a roomier, secure, roof-over-your-head sensation, no question. A bivy, though, minimizes any sensory/emotional separation between you and the outdoors. It's almost like sleeping under the stars—a very liberating experience. At the same time, you are protected by an adequate—and very lightweight—barrier that shields you from nature's less desirable elements, like bugs and raindrops. However, if tight spaces make you uneasy, you are likely to feel uncomfortable inside a bivy, particularly during bad weather. In that case, move up to a tent.
How does air circulate inside a bivy? Breathable/waterproof laminates such as Gore-Tex fabric make it possible for vapor produced by body heat to be pushed through (and out of) the fabric. Raindrops, meanwhile, are repelled. A breathable/waterproof bivy works best in situations where a warm, humid body is resting somewhere cool and dry. In rainy conditions, though, modern bivies are designed with enough overlapping material and zippers that it is unlikely you will have to completely zip them shut. Manually venting a zipper or flap helps maintain an acceptable interior humidity level.
Can condensation be a factor with a bivy? Potentially, yes. A bivy is basically a single-wall tent. When warmed vapor escapes from your body or lungs, it rises to meet colder air. When the vapor makes contact with the laminated bivy fabric, air can no long carry all the moisture, so some collects on the inside of the treated fabric. In 2-walled tents, this moisture passes through the breathable tent canopy and settles on the rainfly. With a bivy, though, this can produce a slight amount of dampness on its inner wall. In icy conditions, this could lead to a thin layer of frost on the inside.
Will a bivy really keep a sleeping bag dry? When wet, Gore-Tex fabric sometimes produces a clammy feeling when it touches your skin, but it's just that—a sensation, not a soak-through. Good ventilation helps minimize this condition, and many wilderness travelers regard this as an acceptable inconvenience when measured against a bivy's minimal weight.
Bivy sacks: Well-suited for mountaineers and minimalist-minded adventurers who take short-term (1- and 2-day) trips. Requires a mindset that adapts well to Spartan situations.
Bivy shelters: Popular with ultralight long-distance backpackers and touring cyclists. A good choice for people who explore in areas of infrequent rain. Extra headroom and full enclosure make them more acceptable to recreational explorers who can mentally adapt to spending nights in a compact space. Snug but light.
Compact tents: Some models in the 4-pound range offer a blend of spaciousness and modest weight; in many cases, the preferred choice for recreational explorers.