Bivy Tent or sack is derived from the name “bivouac sack” and was invented for climbers who craves for light weather protection for their sleeping bags. The early bivy sacks were only waterproof nylon slipcovers for sleeping bags. These are great for protecting the sleeping bags from rain but in terms of ventilation, it is not as good as our body heat produces ventilating vapon.
The bivy tent designs today has 2 tiers of fabric. The botoom tier is made of durable grade nylon (normally taffeta and sometimes oxford) which is coated with urethane so that the material becomes waterproof. This is same type of material which are used for tent floors. The top tier is made of ripstop nylon which is a lighter fabric and also treated with a waterproof and breathable laminate (Gore Tex Tegraltex). Many layers of this laminate is applied to increase the durability and performance.
Through the years, the original bivy made way for the bivy shelter with tentlike features. The bivy shelter has 2 added characteristics not found in normal bivys. There is an expanded area of shielded headspace plus a full enclosure to counter bad weather and insects. Theses adeed features became very popular with hikers, mountaineers who want ultralight weight protective tents. Moreover, some bivy tents include an opening for your head, so some moisture (if it rains) does enter the tent through the unshielded opening. One can minimize this by pulling the headhole’s drawstring snuggly.
So who uses such bivy tents or sacks? People who travel solo frequently in the backcountry, long-distance bike trips, big-wall climbers, etc… Some versions of the bivy has side armholes that allow you to sort gear or cook while still being protected in the warmth of the bivy. Some have full-length zippers which means better ventilation or straps for securing the sleeping pad in place. Bivys restricted airspace offers no sit-up space and it is tough if there is a storm. But sleeping in a bivy is like sleeping under the stars which can feel rather liberating. At the same time, this lightweight carrier can shield one from insects and bugs and the rain. If you are one who feels uncomfortable in tight spaces, then you may feel uneasy in a bivy in bad weather.
Modern bivys have overlapping material and zippers that can help keep the interior of a bivy to an acceptable humidity level and a breathable waterproof bivy helps keep one cool and dry.