A Bivouac (bivy, bivvy) sack is one of the simplest types of complete shelter. They are generally nothing more than a weatherproof bag big enough to hold you, your sleeping bag, and perhaps some of your stuff. While they do not offer the comforts or space of a full tent they are usually lighter, more compact, and offer almost zero setup time. Typically, they will be constructed out of a waterproof/breathable fabric, such as Gore-Tex, to help negate condensation. Most will have a fully waterproof 'tub' bottom, to keep any ground water out.
Bivy sack types
Bivy sacks come in two main styles, unsupported, and supported.
- Unsupported bivvies are an extremely simple design, just a big sack that your sleeping bag fits into. Generally, an unsupported sack doesn't have any material over your face, and is designed exactly like a mummy bag, with a hood, and is thus leaves you more vulnerable to the elements. Some designs have bug netting that can fully zip over your face.
- Supported bivvies are designed a bit like an extremely small tents, with one or sometimes two poles, to keep the sack off your face when closed up, and sometimes with a pole at your feet to give you some additional room to move. Supported sacks generally have to be staked out, negating some of their simplicity.
A bivy sack is commonly partnered with a tarp pitched over the head of the sack, allowing you to be partially sheltered while getting in and out, cooking, or doing other stuff. It can also provide a nice spot out of the rain to pile your gear that would otherwise be unsheltered.
As with anything designed to provide protection from precipitation, it is sometimes a good idea to use a seam-sealer product on all the seams, to make them waterproof. Some higher-end sacks might come with this already done, or be constructed in such a way as to not require seam sealing.