To most people, the very thought of bivy sacks brings to mind an image of a mountaineer sheltering in a waterproof bivy perched precariously on a ledge. It is true, for several decades bivy sacks have served as mountain shelters because of their compact size, thick padding construction, and ease of set up. On the other hand, ultra-light bivies are preferred by backpackers as they are made with breathable tarp material which, although not completely waterproof, provides basic protection from light rain, wind, snow, and even bugs.
While the former option is simpler in design and offers better protection, the latter is the preferred choice by many of our readers and staff members owing to their versatility and comparatively lighter weight. However, both options are vulnerable to moisture.
A bivy sack is continuously exposed to moist conditions caused by dew condensation and precipitation. In fact, protection against harsh outdoor environments is the primary reason why hikers choose bivies over other options. Lastly, a bivy’s capacity to keep things dry, such as sleeping bags, can also be judged by its ability to eliminate moisture and perspiration from damp clothing and other moisture-laden items.