Ultralight backpackers and climbers designed the first bivy sacks to use as an emergency shelter. In essence, the bivy sack is a waterproof and breathable piece of fabric that can be wrapped around the body. It is also designed to trap the body's heat, insulating the person inside. However, for a bivy sack to be completely effective, outdoorsmen must bring along a few other essential pieces of gear.
Choosing the correct bivy sack is the first item on any lightweight gear list. The sack should be relatively snug, but should comfortably fit over the sleeping bag. A person inside should not have a full range of motion--too much extra space inside would compromise the insulation inside the bag and the bivy sack. Most bivy sacks come with a Gore-Tex shell, a proprietary design that claims to be waterproof and breathable. Other features of bivy sacks include straps inside to secure a sleeping bag in one position, long zippers for easy set-up, and arm holes, so a person can perform tasks while inside.
Both a sleeping bag and a bivy sack will fail at insulation if they are not accompanied by an effective sleeping pad. These pads can vary. The most common sleeping pad (and the lightest, too) is the foam pad. The most common foam sleeping pad brand is Therm-a-Rest. These pads can be rolled or folded (like the Z-Rest). While these offer the least amount of comfort, they will insulate the person from underneath--a crucial element in keeping warm while inside a bivy sack. Other sleeping pads are insulated and inflatable. While these offer superior comfort, set-up is more involved and they tend to weigh considerably more than foam pads.
A ground sheet must also accompany a bivy sack set-up. One of the most popular ground sheets is Tyvek, a brand of insulating material usually used to protect the exterior of a house. This is popular because it is lightweight and waterproof. Other options include a sheet of nylon (like a ground sheet for a tent) or a lightweight sheet of plastic tarp (1 to 5 mm thick). Ground sheets protect the sleeping bag and the bivy sack. It will similarly keep these two items dry from dew.
Article Written By Duncan Jenkins
Based in Eugene, Ore., Duncan Jenkins has been writing finance-related articles since 2008. His specialties include personal finance advice, mortgage/equity loans and credit management. Jenkins obtained his bachelor's degree in English from Clark University.