Two ultralight shelter options are Bivy Sacks and Bivy Shelters. A Bivy Sack(also known as a bivouac sack, bivy, or bivi bag) is an extremely lightweight alternative to traditional tent shelter. Bivy Sacks and Bivy Shelters are used by climbers, mountaineers, hikers and minimalist campers to reduce the weight of their shelter system. A minimalist bivy sack 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 and rain. Make sure the the bivy you select uses breathable fabric to allow humidity to pass. Bivys made out of non-breathable fabric allow humidity to condense on the inside of the fabric.
The “bivy shelter” is part bivy sack and part single-person tent. Using hoops or poles, a bivy shelter is usually supported along its length just enough to keep the fabric off of the occupant, and most importantly to provide some additional breathing room around the head.
In contrast to a bivy shelter, a traditional bivy sack typically cinches all the way down to the user’s face, leaving only a small hole to breathe or look through. Many campers gladly accept the increased weight of a bivy shelter for the huge increase in comfort it affords. However, the traditional bivy sack certainly still holds its place among climbers and backpackers, and is frequently carried on long or dangerous hikes and climbs as a compact emergency shelter.