A bivy sack provides protection from the elements for a night spent out under the stars, as well as serves as an emergency shelter in extreme conditions, and is perfect for camping without a tent for those who like to keep their loads light.
A bivy sack is a robust, waterproof and windproof cover made of materials that are also used for jackets or tents. You can either lie, sit, or even stand in it, depending on your situation, as well as close it down tight until just a small breathing hole is left open.
In cold temperatures, you should always use a bivy sack in a location that’s as sheltered from the wind as possible in order to avoid excessive cooling. A sleeping pad provides additional protection from cold seeping upward from the ground. In winter, you should usually dig a snow cave under a closed blanket of snow to provide additional protection from cold temperatures.
Vapor Barrier Liner
The VAUDE Bivouac I.2 is designed as a vapor barrier liner (VBL) for additional protection when bivouacking. This vapor-proof, lightweight inner bag can be used as a sleeping bag inlet for an additional layer of insulation. This barrier prevents body moisture from collecting inside the sleeping bag and possibly freezing. Moisture inside the bag can reduce the insulation capacity of the sleeping bag considerably and damage both the fill and the inner structure when frozen. A VBL is therefore ideal for winter conditions.
When you are lying still and resting in an environment with extremely high vapor saturation, your body automatically regulates its vapor output. So the next morning you don’t wake up bathed in sweat, but just with a slightly clammy feeling if you’re wearing functional base layer clothing. Under normal circumstances this will disappear quickly once you get up and start moving, due to your increased body heat.