Nicknamed bivy sacks, bivouac sacks were designed to assist climbers on big-wall routes. They provide lightweight weather protection, which is certainly useful in the event of a sudden rain or snowstorm. Utilized by hikers, backpackers, and mountaineers in addition to climbers, bivy sacks are waterproof fabric shells that easily glide over your sleeping bag.
Whether you love to travel by yourself through backcountry, or are simply a hiking and camping enthusiast, a bivy sack is a fantastic item to have with you. They fully protect against wind and rain, and work out wonderfully on long-distance trips. So lightweight you’ll probably forget you’re carrying it, bivy sacks keep you dry no matter how extreme the weather.
These so-handy sacks typically feature two fabric components, which are found on the top and bottom tiers. The top tier consists of ripstop nylon, a lightweight fabric usually treated so it’s both waterproof and breathable. Previous incarnations of bivy sacks caused interior condensation following several hours of use, however most of today’s options are designed to eliminate this issue.
If you plan on winter or snow camping using a bivy sack, from my experience – these shelters are not my first choice. A few years ago, my father and I went backpacking up in the Great Naturalist Basin in the Uintas mountains early summer. My father and younger brother were both using their own bivys and it snowed that first night. The good news is that they stayed dry, they had to curl down in their sleeping bag to keep the melting snow from dripping on their faces. But the bad news, when my dad woke up during the night to go to the bathroom, he didn’t realize how much it snowed and when he opened his bivy sack to crawl out, a lot of snow fell in his sleeping bag which made for a very cool rest of the night. I slept in my two man tent, stayed warm and slept like a baby a night long.
The bottom tier generally features waterproof, more durable nylon similar to the fabric used in tents.
Try a bivy sack on your next cold weather camping excursion and increase bedtime warmth by as much as 10 degrees Fahrenheit!