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Commercially offered bivouac sacks are made of a '''lightweight waterproof (plastic) material'''. Their disadvantage is that humidity will condense on the inner wall of the bivouac sack over time. This effect is stronger when the outside temperature is low. That's why this type of sack is not recommended to use in winter. You and at least your [[sleeping bag]] will get moist and if you don't have the possibility to dry your sleeping bag during the day, its insulation effect will permanently decrease, especially if it is a down sleeping bag.
The condensation effect is minimized by choosing '''breathable materials''' for the bivouac sack, e.g. Gore-Tex, Sympatex, Hyvent. These materials make it possible for humidity to pass through the fabric while water from the outside can not come in. Unfortunately breathable sacks are usally pricey.
Some bivouac sacks are '''aluminised''' from the inside and reflect the biggest part of your own body warmth. Sophisticated bivouac sacks are even '''covering your face''' with an air permeable fabric which also allows breathing air to pass through it. This way you're completely safe from rain and water (and mosquitoes). There are also '''bivouac sleeping bags''' which are like normal [[sleeping bag]]s but have a waterproof outer shell.
Another variation is a so-called '''bivouac shelter''', which is more comfortable than a bivouac sack but not yet a real [[tent]]. Here, the waterproof fabric is tautened by some hoops over head and feet. This way, the condensing liquid is not soaking into your [[sleeping bag]]. There is also more space for breathing. Thus, lying in a sort of tent might feel more private than just lying around in a sleeping bag. The increased comfort of a bivouac shelter is paid by a higher pack size and slightly more weight to carry (max. 1kg). A disadvantage in comparison to a [[tent]] (which doesn't matter if you only want to use it for sleeping) is that you don't have the possibility to sit up in a bivouac shelter.Also the bivouac shelter doesn't provide enough space for you backpack.
[[Category:Equipment]]
