Backpackers use ultralight bivy bags to protect their sleeping bags and quilts from water. These are basically sleeping bag covers that have mosquito nets over their faces.
The majority of ultralight bivy bags have enough space inside to fit a mattress, a pad, or a quilt. However, sleeping in them isn’t as cramped as you might think.
To keep the bug netting from getting in your eyes while you sleep, many ultralight bivy bags have a cord loop. This cord can be tied to the rafters of a shelter for trail use and your bivy can protect you from mice and bugs. If you need to change clothes, your bivy bag can provide a little more privacy in a shelter.
A bivy bag protects your sleep system from splashback when you are sleeping under a roof. This happens when rain bounces off the ground, under your tarp, and onto your sleeping gear. If you have a rectangular tarp, this can pose a problem. However, it is less likely to happen if your tarp has a wider or more shaped shape.
Although they won’t add much warmth to your quilt or sleeping bag at night, most ultralight bivy bags don’t provide any extra warmth. Many are made with a top fabric which is very breathable to vent condensation at night. You can use the top fabric as a sheet in hot weather and place it on top of your quilt or sleeping bag to prevent bugs from biting.
Ultralight bivy bags are delicate gear that were handcrafted and should be treated with care. To prevent mildew, make sure you lubricate any zippered bivy bags you purchase. You can remove black mildew from the bivy fabric by washing it with diluted Mirazyme.
Other types of bivy sacks
Many of the bivy bags you will find at REI and other outdoor retailers are heavier that the ultralight hand-made bivy bags that you can purchase from smaller gear makers. These sacks are designed for winter use in a shelter or snow cave, but some of them have bug netting over the face so they can be used in warmer conditions such as the REI Superlight Bivy Sack and Outdoor Research Interstellar Bivy.
Some bivy sacks can be classified as tents, such as the Outdoor Research Alpine Bivy Sack or the NEMO GoGo Elite 1 Bivy Sack.
Ultralight Bivy bags are not meant to be used with floorless shelters. These bivy types can be used as standalone shelters. They are all called bivy bags, which can be confusing. I hope this helps clarify the differences.