by Michael Hodgson

Bivouac sacks offer a compromise shelter for the minimalist adventurer or an additional line of protection for the winter traveler who wishes to ensure his or her bag stays dry. In addition to offering protection from the elements, a bivi can extend the temperature rating of a bag by up to five degrees.

Bivy's are an important component in the sleeping bag system because no matter what material the sleeping bag shell is made of, a sleeping bag is not an outer garment. A GTX bivy sack is the "outer garment" for a sleeping bag and becomes at times a weatherproof tent for the individual mountaineer.

It is important to realize that a bivy is not a fool-proof piece of gear. It is the user and the user's metabolism that is responsible for how a bivy will perform. This performance is often judged by how well the bivy manages moisture and by how well water vapor is pumped through the waterproof/breathable membrane. Those who sleep "wet" and churn out the perspiration may find that they overload the membrane and, in cold climates, end up soaking their bags quickly-in really cold conditions that can lead to ice build-up inside a bag, a potentially dangerous combination.

Vapor Barrier Liners can help to prevent this build up of moisture, but VBL's come with their own set of usage guidelines and challenges. For one, sleeping in a VBL is akin to napping inside a giant plastic baggie and, oftentimes, about as pleasant. Managing air-flow, however minimal, and minimizing excessive heat build-up to reduce perspiration generation is essential and an art in itself.

Drawbacks? As an addition to a sleeping bag system during the winter when used in conjunction with a tent or snow cave, there are few. As a minimalist shelter, however, the drawbacks mount up. For one, it's darn tough to cook inside a bivy sack should the need arise. Also a bivy provides absolutely no shelter for gear and can become a "nylon prison" if a long storm requires waiting out.

© 1999 Michael Hodgson; All Rights Reserved