Bivy cover is a waterproof outer shell for a sleeping bag. Bivy or bivvy, comes from bivouac, which means a temporary encampment for the night under little or no shelter. So, a person without the comfort and added weight of a tent might opt to carry a bivy bag instead. Bivy covers are most often sacks with a breathable and waterproof membrane such as Gore-Tex.

Best bivy sacks are the ones with the best waterproofing and the least weight. This might interest certain people who try to trim down their rucksack weight to the miminum. A tent is probably always heavier than a good bivy cover. Additionally, setting up camp will be very simple, just roll out your bivy, insert your sleeping bag, and go to sleep.

Also, if you prefer shelters such as tarps and lean-tos, a proper bivy bag might save your ass one day if a really nasty weather should take you by surprise. Apart from being waterproof, bivy sacks also act as extra insulation combined with your sleeping bag.