A bivouac sack (synonyms: bivy sack, bivi bag or just bivy) is a very lightweight and minimalist alternative to a tent. The most simple version of a bivouac sack would be a plastic bag pulled over your sleeping bag.
Origin of the word
Bivouac is originally a term for a (military) camp consisting of very simple or improvised shelters. The term is also used with scouting, trekking and mountain climbing trips.
If you are on the road for several days and sleep outside, you might –depending on climate and weather– need a shelter from wind and rain to stay warm and dry while sleeping in your sleeping bag. Bivouac sacks can increase the insulation effect by up to 15 °C by forming a barrier against wind and water. They also protect your sleeping bag against dirt and sparks e.g. from a camp fire.
Unlike a tent, they are very much lighter (about 500g) and smaller and can be carried along for cases where you unexpectedly need to sleep outside.
If you don't have access to a bivouac sack for the moment or don't want to spend money for one, you might use a big plastic trash bag pulled over your sleeping bag. Otherwise, you might consider making one yourself out of recycled or cheaply bought material.
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 bags 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.