For many experienced mountaineers and alpinists, there is no question about it: A bivy sack is a mandatory piece of equipment in the mountains! Though originally conceived as a precautionary safety measure, these days bivy sacks are not only used as emergency shelters but also by minimalists who wish to enhance the simple pleasure of a night under the stars.
But what does this piece of the fabric have to offer that is not already provided by expensive high-tech clothing? How can a simple fabric sack make a difference between life and death in emergencies? What are the differences between the different models and materials? The Bergzeit buyer’s guide provides some insight.
Essentially a bivy sack is just wind- and waterproof cover that is roughly the same size as a sleeping bag. The bivy provides shelter from wind and moisture in the event of an emergency, when waiting out poor visibility conditions, or if the weather unexpectedly turns bad.
High mountains are especially prone to sudden and sometimes unpredictable weather changes. Being ill-equipped and failing to read the signs correctly, or being unable to get off the mountain in time, can quickly lead to life-threatening situations – as has happened all too often. Those caught on a mountain in a storm are usually exposed to a combination of heavy precipitation, low temperatures, and high winds, which can lead to hypothermia and death in a matter of hours.
Wind Chill
Strong winds, especially when temperatures are around freezing, and humidity is high, have a severe cooling effect on the upper skin layers. The body tries to compensate and quickly loses the necessary heat in the process. Any moisture on the skin exacerbates the effect because of the warmth that is lost during evaporation. This means that at an air temperature of 0°C and a wind speed of 50 km/h, a person has a perceived skin temperature of about -8°C.
When there is no possibility of quick retreat a bivy sack may be the only available option to find shelter from the elements. Even solid waterproof clothing can provide limited protection since even the best clothing will become wet eventually and much body heat is lost through the relatively large surface area. Bivy sacks provide a space for warm air to accumulate without being immediately blown away.
Bivy Sack Materials
When researching different options for bivy sacks you will quickly notice significant price differences. The main reason for these differences is the material chosen for the shell.
Cheap bivy sacks are often made using a membrane that does not let moisture exit from inside. Such models are usually either made of durable plastic, polyurethane-coated membranes, or silicone-coated cloth. When a person spends long periods of time inside this type of bivy sack condensation tends to collect in the interior and soak into clothing and sleeping bags. Such models are therefore only suited for emergency situations. Their greatest advantage is their affordable price. They also usually are comparatively light and take up little space. The Mountain Equipment Ultralite Bivy Sack is a great model in this category.
If the bivy sack is going to be used for a planned overnight stay in combination with a sleeping bag, it is highly recommended to buy a breathable bivy sack. However, you should be aware that even these more sophisticated membranes only mitigate the condensation problem to a certain extent. High-end Carinthia bivy sacks use the extremely breathable GORE-TEX® Arid Lite Air membrane; Black Diamond uses the ToddTex™ membrane known from their tent line; Rab and Exped use the highly breathable eVent® fabric. Extremely light bivy sacks may also use the water-repellant Pertex Endurance membrane. Exped’s bivy bags, such as the Uno UL sacks have a reputation of being extraordinarily good, but their quality is reflected in the price.
Construction of a Bivy Bag
Often the shape of a bivy sack is just what the name implies: sack-shaped. There are however also some models that can be tied off in various ways, or that incorporate a small rod system to provide more headroom, for example, the models produced by Vaude and Pieps. Some of these bivy sacks might more aptly be called bivy tents.
If you tend to travel in pairs or small groups, it is also a good idea to consider a wind shelter. These can fit multiple people with all their gear, a good way to conserve heat. Wind shelters also offer some room for activities like cooking, whereas bivy sack’s main purpose is to provide shelter and a place to sleep. Brands such as Hilleberg, Ortik and Terra Nova offer some interesting wind, shelter models. These types of shelters are especially popular for winter treks in northern Scandinavia since they are often the only way to get out of the wind on the treeless Fjäll.
Uses
If you do end up having to use a bivy sack in an emergency, you should still try to find a spot protected from the wind. On a large snow-covered area, the best possibility is often to dig a small hollow or snow cave. When using a bivy sack in snowy conditions it is especially important to have an insulating pad of some kind. The foam padding in the carrying and back support systems of backpacks are well-suited for this purpose. Many climbing backpacks even have an additional pad for sitting incorporated as a standard feature. And don’t forget, a breathing hole is crucial!
Whenever possible you should try breathing externally from the bivy sack, since inside exhalation will increase condensation dramatically. Realistically, a bivy sack will increase a sleeping bag’s warmth by at least 5-8°C.