Bivvy or “bivy” is just a nickname for bivouac sack. It is a kind of shelter that looks like a loose sleeping bag. They’re quick to rig up, smaller than any other type of tent, very light and waterproof. They’re favored by solo backpackers, mountaineers and soldiers.
A simple bivvy is made of one layer of waterproof material. Early versions of the bivvy bag employ silnylon as the outer shell. It is a very light fabric that provide about another 10 degree fahrenheit extra protection from weather.
One big problem with the earlier versions of the bivy is the humidity that condenses on the inside renders sleeping bags to be wet. This still happens in some tents today. The use of the waterproof/breathable fabric, Gore-Tex, helps to remove some of the humidity while keeping out water from the outside.
Another way that this problem is addressed is by using an inner vapor barrier liner bag or VBL. VBLs prevent body moisture from entering the condensing in the sleeping bag.
In the past, bivvy bags come with a small hole above the user’s face for breathing. Thanks to Gore-Tex Exchange Lite, an air permeable version of the fabric, the bivvy can be safely zipped up to cover the whole body including the head thus shutting out the elements completely. This new fabric allows both carbon dioxide exhaled by the user and moisture to escape.
In biological terms, the bivy is a macroinvertebrate, meaning it has no spine or rigid structure. Bivouac shelters on the other hand employs hoops over the head and feet so that the fabric is kept off the user resulting in more breathing room around the head. This also prevents the condensation from soaking into the sleeping bag.
Bivvy shelters are a bit heavier than bivvy sacks but the increased comfort justifies the additional weight. However, bivvy sacks are still favored among mountain climbers and backpackers or adventurers on dangerous expeditions as a compact emergeny shelter.
Improvement in bivvy shelters have given rise to the number of experienced hikers who favor them. Some like the Outdoor Research Deluxe Bivy have anticlaustrophobic features like a white interior with a shock-corded Delrin hoop for breathing room and 5-foot zippers that allow you to ventilate while keeping you dry.
Bivvies are also popular among carp fishermen who fish the whole night. This solo shelter is becoming more advanced and user friendly as manufacturers scramble to make better bivvies in response to the increasing demands.