Backpacking bivy sacks are often used for cowboy camping or under floorless shelters, such as tarps or pyramids, to protect sleeping bags/quilts and sleeping pads from wind, insects, mice, and other creepy crawlers. Most are not waterproof and function more like sleeping bag covers encapsulating a sleeping pad and sleeping bag or quilt, with mosquito netting over the head and face for insect protection. They're very different from heavier-duty waterproof bivy sacks used for winter camping or mountaineering, like the Outdoor Research Alpine XT Bivy or the Big Agnes Three Hooped Bivy, which resemble mini-tents with collapsible rods to make them more livable in harsh weather.

Ultralight bivy sack under a floorless Dyneema DCF Tarp

Most bivy sacks are sized to fit a sleeping pad and a sleeping bag or quilt. But sleeping in them isn't as claustrophobic as it might look. There's still plenty of room to sleep on your side or prop your head up on your arm to read at night. Bivies with side zipper access are also much easier to get into and out of than ones with front openings where you must slide in feet first. This is particularly true at night when you feel especially stupid and uncoordinated.

Bivy Sack under a floorless pyramid trap

Many bivy sacks have a cord loop that you can tie to the underside of your shelter to hold the bug netting off 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 into your long underwear.

When sleeping under a flat tarp or catenary-cut 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 onto your sleeping gear. This can be an issue if you have a narrow rectangular or catenary-cut tarp but is less of a concern if you use a wider one whose sides are staked close to the ground in bad weather.

Using a bivy sack in a shelter. Cabot Cabin – White Mountains

Most bivy sacks don't impart much extra warmth to your sleeping bag or quilt at night, maybe 5 degrees tops, 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. In very hot weather, you can use the top fabric of a bivy, like a bed for sleeping. Some bivy sacks are available with all mesh tops for use in warmer weather, but the norm is part mesh and part solid fabric.

Tarping in the Pemigewasset Wilderness

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 with a zipper, it's good to lubricate it periodically with Gear-Aid Zipper Lubricant so it doesn't snag, and to dry your bivy sack out after every trip to avoid mildew. If you see black mildew spots begin to form on the bivy fabric, washing your bivy sack out with Revivex Odor Eliminator will eliminate the mildew and prevent damage to the fabric.

Recommended UL Bivy Sacks

Why Would You Use a Bivy Sack?

So far we've discussed what a bivy sack is, but not why you'd want to use one. There are two reasons: tarp camping (for gear weight reduction) and wilderness immersion.

Tarp camping with a flat, catenary-cut, or pyramid tarp is lighter than using a tent, even an ultralight Dyneema DCF tent from Zpacks or Tarptent. You still need some sort of insect protection under a tarp, and at 5-8 oz, a bivy sack weighs significantly less than an inner tent or net tent, making it possible to carry a complete shelter system that weighs under a pound.

In addition, sleeping in a bivy sack is a much more immersive way to experience the wilderness than sleeping inside a tent, with walls and a floor. If you cowboy camp in a bivy sack or sleep under a tarp, you'll feel much closer to the surrounding environment.

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