Winter bivy sacks were first developed as solo shelters for mountaineering and climbing where participants were interested in lightweight and highly compressible gear. The biggest advantage of a winter bivy sack is the ability to use it anywhere you want to stop and sleep, be it a rock ledge on the side of a mountain, a snow cave, or a shallow ditch you stomp out in the snow to protect yourself from the wind. They require no tent stakes or tent poles and they pack up very small which is an advantage when you want to keep your winter gear lightweight and compact.
Most winter bivy sacks are made with waterproof/breathable top fabrics to help reduce internal condensation and waterproof base fabrics to keep your sleeping bag and pad dry. In some cases, hooped tent poles have been added to winter bivy sacks to making them more livable in stormy or inclement conditions when you need to stay in them for a longer period of time. Insect netting is also available in some cold weather bivy sacks and can be useful when they’re used in spring conditions.
Advantages of Bivy Sacks
There are many advantages to sleeping in a waterproof bivy sack over a tent. It is easy to find a place to put a bivy sack at night since it only requires as much space as your sleeping bag and sleeping bag or quilt. Simply unroll your bivy sack, slip your sleeping pad and sleeping bag inside.
There are no guylines to set up and tent stakes to freeze into the ground so you can get inside without delay and get warm. Being waterproof, you don’t have to lie on top of a groundsheet either, since the bottom of a waterproof bivy sack is designed to keep you dry. Bivy sacks also add a few degrees of insulation to your sleep system.
But two of the biggest advantages of waterproof bivy sacks are their lightweight and packability, compared to a tent. These two properties are important for climbers, cyclists, adventure racers, and fast and light hikers who want don’t want to be weighed down and want to bring as little gear as possible. Bivy sacks also make great emergency shelters for solo winter hikers, provided you carry a sleeping pad and enough insulation to get through the night.
Disadvantages of Bivy Sacks
Bivy sacks are much more confining than tents, with only enough space for you and a few small personal items. Your backpack and the rest of your gear will be fully exposed a night without any cover. Bivy sacks are also more prone to internal condensation than a tent, even when manufactured with waterproof breathable materials. You’re best off keeping them open or unzipped at night to maximize air circulation and ventilation in order to keep your sleeping bag/quilt dry and condensation free.
How to Choose a Winter Bivy Sack
Here are the key considerations to weigh when choosing a winter bivy sack.
Waterproof/breathable top fabric
Having a waterproof/breathable top fabric is important to vent water vapor and help minimize internal condensation that can make your sleeping bag wet. However, unlike rain jackets, most bivy sack manufacturers do not list the laboratory measurements used to rate waterproofing (hydrostatic head, abbreviated “HH”) or breathability (movable water transmission rate, abbreviated “MVTR”). That can make expected performance comparisons between different bivy sacks difficult.
That said, winter bivy sacks made with Gore-tex or Event are significantly more breathable (often by a factor of 2 or more) than bivy sacks made with the proprietary waterproof/breathable knock-offs like Black Diamond’s Nanoshield or the North Face’s Futurelight fabrics. They generally cost more, though.
If headroom is important to you, consider getting a bivy shelter with an interior pole. These bivy sacks come with a flexible fiberglass pole that slides into the hood area to create more volume around your face and shoulders. The tradeoff is that they tend to be heavier than more minimal bivy sacks.
Entrance and exit
It’s much easier to get in and out of a bivy sack that has a zipper along the side than one that only has one at the head end.
Insect netting for warmer temperatures
If you plan to bivy in early spring when biting insects emerge, make sure to get a winter bivy sack with a mesh panel over the face so you can sleep without insects biting your face at night. If you only plan to sleep in a bivy sack in winter or in snow caves, a mesh panel will be less important. For warmer weather use, I’d encourage you to invest in an ultralight bivy sack or bug shelter rather than a winter bivy sack, since they’re much cooler to use. See our 10 Best Backpacking Bug Shelters for our recommended warm weather bivy sacks.
Weight and Packed Size
Don’t forget to consider the weight of the bivy sack and its packed size, since one of the chief benefits of using a bivy sack is gear weight and size reduction. There’s often a tradeoff between features and weight/size, but some winter bivy sacks are surprisingly lightweight and compressible.
Sizing and Fit
When in doubt, order a bivy sack (check the retailer’s return policy) and lie in it at home to see if it fits. This will probably tell you a lot more about whether it will work for you than comparing the specs of multiple models listed online. Check to make sure that you can fit your sleeping pad and sleeping bag inside, that the foot box is large enough for your feet.
More Winter FAQs
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