Ultralight bivy sacks are used by backpackers using floorless shelters to protect their sleeping bag or quilt from moisture and their head from biting insects, as a kind of substitute for the inner tent that you find in more conventional double wall tent setups. They’re basically sleeping bag covers with a waterproof floor that have mosquito netting over the face.
Most ultralight bivy sacks have just enough room inside for a sleeping pad and a sleeping bag or sleeping quilt, but sleeping in them really isn’t as claustrophobic as it might look and there’s still plenty of room to sleep on your side or prop your head up on your arm to read at night.
Many ultralight bivy sacks have a cord loop that you can tie to the underside of your shelter to hold the bug netting up off of your face when you sleep. You can also tie this cord to the rafters in a trail shelter and use your bivy to protect you from bugs and mice. Your bivy sack also provides a wee bit more privacy inside a shelter if you want to change your clothes.
When sleeping under a tarp, a bivy sack helps protect your sleep system from splash-back, which occurs when rain bounces off the ground and under your tarp and on your sleeping gear. This can be an issue if you have a narrow rectangular tarp, but is less of a concern if you use a wide or shaped tarp, whose sides are tied close to the ground in bad weather.
Most ultralight bivy sacks don’t impart much extra warmth to your sleeping bag or quilt at night, although they will block a breeze from chilling you. They are often made with a top fabric that is quite breathable to help vent any condensation build-up at night. The top fabric is so light weight that you can sleep on top of your sleeping bag or quilt and use like a sheet in very hot weather to keep the bugs from biting your legs and arms.
Ultralight bivy sacks are quite delicate pieces of gear made by hand and must be treated gently if you want them to last. If you buy a bivy sack that has a zipper, it’s good to lubricate it periodically with McNet’s Zip Tech so it doesn’t snag, and to dry your bivy sack out after every trip you take to avoid mildew. If you see black mildew spots begin to form on the bivy fabric, washing your bivy sack out with diluted Mirazyme will eliminate it and prevent damage to the fabric.
Other Types of Bivy Sacks
Most of the bivy sacks that you’ll find at REI or other outdoor retailers are two or three times heavier than the hand-made ultralight bivy sacks that can buy from smaller gear manufacturers. While many of these are intended primarily for winter use in a snow cave or shelter, some have bug netting over the face and can also be used in warmer weather, such as the: Black Diamond Twilight Bivy, the Sierra Designs Backcountry Bivy, or the MSR AC Bivy.
Some bivy sacks are so bombproof that they can even be classified as tents because they include their own poles to pitch, including the Outdoor Research Advanced Bivy Sack and the Black Diamond Bipod Bivy Bag.
Unlike Ultralight Bivy sacks, none of these other bivy types are intended to be used with floorless shelters, but are meant to be standalone shelters in their own right. It is confusing that they’re all called bivy sacks, so I hope this post has helped clarify the differences between them.
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