Ultralight Bivy Sacks 101

Ultralight Bivy Sacks 101

Ultralight bivy sacks are used by backpackers for cowboy camping or under floorless shelters, such as tarps or pyramids, to protect their 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 Ascent Bivy, or the Rab Ridge Raider 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
Ultralight bivy sack under a floorless Dyneema DCF Tarp

Most ultralight bivy sacks are sized for use with a sleeping pad and a sleeping bag or quilt. But sleeping in them really 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 a lot easier to get into and out of than ones with front openings where you need to slide in feet first. This is particularly true at night when you’re feeling especially stupid and uncoordinated.

Ultralight Bivy Sack under a floorless pyramid trap
Ultralight Bivy Sack under a floorless pyramid trap

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

Most ultralight 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 sheet for sleeping. Some ultralight bivy sacks are available that are all mesh for use in warmer weather use, but the norm is part mesh and part solid fabric.

Tarping in the Pemigewasset Wilderness
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 that has 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 you take 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 an Ultralight Bivy Sack?

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

Tarp camping with a flat tarp, a catenary cut tarp, or a pyramid tarp is lighter weight than using a tent, even an ultralight Dyneema DCF tent from the likes of 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 an ultralight 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 environment surrounding you.

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