M Staying Dry in a Bivy Sack: Techniques for Managing the Moisture Balance Between External Precipitation and Internal Condensation

by Mike Martin

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Article Summary:

When most people think of bivy sacks, a picture of a climber clinging to a precarious ledge while anchored into a fully waterproof bivy comes to mind. True enough, bivy sacks have been used as mountaineering shelters for decades due to their small footprint, ease of deployment, and compact stuffed size. On the other end of the spectrum, ultralight backpackers have adopted highly breathable (though not waterproof) bivies to provide wind, spray, and bug protection when a small tarp is used as primary rain defense. The former system is bombproof and simple while the latter is favored by many of our staff and readers for its light weight and versatility. Both systems are susceptible to moisture problems.

In use, a bivy sack is regularly exposed to moisture from both precipitation and the condensation of dew. Indeed, protection from these external elements is often the primary factor that leads a hiker to consider using a bivy sack in the first place. But a bivy's performance in keeping a sleeping bag dry also depends crucially on its ability to dissipate moisture from internal sources such as damp clothing, respiration, and perspiration.

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