Bivy Tent or sack is derived from the name “bivouac sack” and was invented for climbers who crave lightweight weather protection for their sleeping bags. The first bivy bags were just waterproof nylon sleeping bag covers. These are great for protecting sleeping bags from the rain, but in terms of ventilation, it’s not as good as the heat from our body that vent steam produces.
Today’s bivy tent designs have 2 levels of fabric. The lower tier is made of durable grade nylon (usually taffeta and sometimes oxford) that is coated with urethane to make the material waterproof. This is the same type of material that is used for store floors. The upper level 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 are applied to increase durability and performance.
Over the years, the original bivouac gave way to the bivouac shelter with tent features. The bivy shelter has 2 added features not found in normal bivys. There is an enlarged area of sheltered headspace plus a full enclosure to counter bad weather and insects. These features have in fact become very popular with hikers and mountaineers who want ultralight weight protective tents. Also, some bivy tents include a head opening, so some moisture (if it rains) enters the tent through the unprotected opening. One can minimize this by pulling the cord from the entry hole comfortably.
So who uses such bivy bags or tents? Frequent solo travelers in the countryside, long-distance cycling trips, big-wall climbers, etc. Some versions of the bivy have side armholes that allow you to tidy up gear or cook while still sheltered in the bivy’s heat. . Some have full-length zippers, which means better ventilation, or straps to secure the mat in place. Bivy’s restricted airspace offers no room to sit and is difficult if there is a storm. But sleeping in a bivy is like sleeping under the stars, which can be quite liberating. At the same time, this lightweight baby carrier can protect you from insects, bugs and rain. If you’re one of those uncomfortable in tight spaces, you might feel uncomfortable in a bivy in bad weather.
Modern bivys have overlay material and zippers that can help keep the inside of a bivy at an acceptable moisture level and a waterproof breathable bivy helps keep you cool and dry.