Winter bivy sacks were first developed as solo shelters for mountaineering and climbing where participants were interested in lightweight and highly compressible camping gear. Bivy Sacks are also an attractive option for backpacking hunters, backcountry skiers, and winter backpackers who want the lightest and smallest shelter they can carry for overnight or emergency use.
Winter bivy sacks are different from three-season bivy sacks which are often used in conjunction with a tarp for insect and splashback protection, where rain hitting the ground can bounce back and wet your quilt or sleeping bag. In contrast, a winter bivy sack is intended as a standalone shelter by itself or in conjunction with a snow shelter such as a trench or snow cave. Winter bivy sacks tend to be much higher in volume to hold winter-weight sleeping bags without compressing the insulation and work best in dry and cold conditions where precipitation is in the form of snow.
Winter Bivy Sacks – Advantages
The biggest advantage of a winter bivy sack is the ability to use it anywhere you want to stop and sleep. Most models don’t require 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. Insect netting is also available in some cold weather bivy sacks and can be useful when they’re used in spring conditions when bugs begin to emerge,
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 pad and sleeping bag. Simply unroll your bivy sack, slip your sleeping pad and sleeping bag inside, and crawl in. This makes them suitable for sleeping on narrow mountain ledges or in forested glades where there’s just enough space to lie between two trees.
There are no 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 and make great emergency shelters for solo winter hikers, provided you carry a sleeping pad and enough insulation to get through the night.
Winter Bivy Sacks – Disadvantages
Bivy sacks are much more confining than tents, with only enough space for you and a few small personal items. You can’t really do anything inside one except listen to podcasts or go to sleep. It’s awkward to change out of your wet clothes or put on dry clothes inside depending on the type of bivy sack you have, it can be damn hard to put on your boots, even to go for a pee at night. There’s no place to store 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 reduce the temperature differential between the inside of the bivy and the outside which causes condensation to form. Tent condensation occurs for the same reason but tents have better ventilation and more volume, so it’s easier to keep your sleeping bag dry when it occurs.
Despite these disadvantages, I have friends who swear by their winter bivy sacks because they’re so fast to deploy compared to a tent or tarp. I’ve used winter bivy sacks and can appreciate their strengths, but winter nights last a very long time and it’s nice to have a little extra space to change your clothes or sleep with your gear inside your tent. Whichever you choose depends on what you’re trying to accomplish.
Comparison Table
Types of Winter Bivy Sacks
There are three types of winter bivy sacks to choose from:
- Hooped front entry bivy sacks
- Hooped side entry bivy sacks
- Sleeping bag covers
There are two types of hooped bivy sacks: those with front entrances that you slide into feet first and those that you can enter from the side. Both come with one more short fiberglass or carbon fiber rods that give shape to the hood and prevent it from collapsing onto your head or torso.
The head-end of a hooped front entrance bivy is usually screened with a solid cover that can be zippered closed for warmth or to block wind or rain. This design provides good ventilation for your head and helps to expel the water vapor in your breath. The Outdoor Research Helium Bivy is a good example of the front entry hooped design.
Side-entry hooped bivy sacks usually have a zipper along one side so you can easily get in and out of them. This design is also easier to vent because you can roll up the top cover in dry weather to reduce internal condensation. The Outdoor Research Alpine Ascentshell Bivy is a good example of a hooped bivy with a side entrance.
The simplest and lightest weight winter bivy sacks are little more than sleeping bag covers. Some have drawstrings that you can pull closed over your head, some have shaped mummy-style hoods or simple slits above the face for ventilation. While not as livable or well-ventilated as the hooped bivy sacks designs described above, they do the job if you want a highly compressible, ultralight weather protection for your sleep system. The MSR Pro Bivy and the Black Diamond Twilight Bivy are representative examples of the sleeping bag cover design.
How to Choose a Winter Bivy Sack
There are multiple features and trade-offs to consider when choosing a winter bivy sack.
Waterproof/breathable top fabric
Having a waterproof/breathable top fabric is important to vent water vapor and help reduce 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 performance comparisons between different bivy sacks difficult.
More headroom
If headroom is important, consider getting a bivy shelter with a hooped design. These bivy sacks come with a flexible pole that slides into the hood to create more volume around your face and shoulders. The tradeoff is that they tend to be heavier than more minimal bivy sacks.
More volume
Winter bivy sacks tend to be high volume to fit bulky winter sleeping bags and inflatable pads with a high R-value. When choosing a bivy sack you want to ensure that there is enough room for your sleeping bag insulation to fully loft and that it’s not compressed by having too tight a fit.
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 a “hatch” at the head end, to crawl in and out of. You’ll stay drier if the ground is covered in snow and it’s much easier to put on and take off your shoes.
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 three-season 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, and that the foot box is large enough for your feet.
Emergency Bivy Sacks
There is a class of emergency bivy sacks that are only intended for emergency use such as the SOL Escape Lite Bivy. These are made with thin reflective aluminized fabric and lack the durability or breathability of the multiuse bivy sacks listed above.
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