It’s true… The bivouac sack (or “bivy” sack) is merely a weatherproof cover for your sleeping bag with a breathing hole—the perfect bear burrito, filled with your ambitions to complete those ultralight objectives. Some call bivies 'cramped'. Others call them 'miserable nights' spent in the harsh wilderness when the car is far beyond your physical and mental fortitude. To many, they’re simply insurance against unplanned overnights, ideally never to be used. But the devout know better: indeed, there’s an elegant and sophisticated simplicity to the bivy sack. When used under the right conditions, a bivy sack is key to mastering the fine art of fast and light travel on big backcountry missions.

First, what is a bivy sack?

Bivy sacks are minimalist shelters for one that offer an additional layer of protection against the environment for you and your sleeping bag—think cowboy camping with a full-body shell. Just larger than your sleeping bag and mattress, bivies are typically constructed of waterproof or water-resistant materials to shield against wind and precipitation. Originally developed as emergency shelters, today bivy sacks and bivy shelters come in a range of styles. Traditional, ultra-simple bivy bags—like the MSR Pro Bivy and E-Bivy—weigh less than a one-person tent and offer a utilitarian approach for traveling as light as possible. On the other end of the spectrum, full-featured bivy shelters add a pound or so more to your pack, but include an internal pole for expanded headspace and full-length zippers, offering a small-tent-like experience.

Who is the bivy adventurer?

The choice to bivy is one of priorities and goals. Adventurers moving fast with low-volume packs choose this supremely efficient shelter for the freedom to travel unencumbered. These adventurers are driven to trade the comfort of a tent for the luxury of a lighter, smaller load on their backs or in their haul bags. Their mindset is on successfully completing an objective, rather than lounging at camp. Traditionally, alpine and big-wall climbers relied on bivy sacks for hunkering down during multi-night objectives in technical terrain, where sleeping ledges are too cramped to set up a tent. But these days, bikepackers, fastpackers, thru-hikers and minimalist solo adventurers also choose bivies to shed every possible gram while maintaining a degree of weather protection.

Why choose a bivy sack over a tent?

Under the right conditions, bivies equal freedom. They enhance your connection to the outside world and they demand a true adherence to the minimalist mindset, which is an achievement all its own. Here's why a bivy may be your ultimate ally on big mountain endeavors. Less Weight The most pared-down of bivies weigh just a few ounces. MSR’s Pro Bivy, engineered to meet the needs of professional alpinists and serious adventurers, weighs just 283g (10-oz). Even lighter still, MSR’s E-Bivy weighs a mere 170g (6-oz.), perfect for tossing in a pack lid as an emergency shelter on everything from all-day hikes to backcountry ski tours. Packability The ultra-compact nature of bivies makes them a valuable asset when shedding weight. Some stuff down to the size of a burrito, disappearing into bikepacking bags and small alpine packs. With a smaller pack, you can cover more miles and more easily traverse technical terrain. Sleeping Spaces Technical terrain is where the bivy shines. Requiring only the length and width of your sleep system, bivies open up new campsite possibilities. Rock ledges, climbers’ nooks on craggy scrambles, snow caves and narrow backcountry spaces too tight for staking out a tent all become available real estate for a night under the stars. Warmth Like a light rain shell jacket, bivies trap in a few degrees of warmth and hold that heat better than a tent; this takes an open night on the ground up a notch, allowing you to get the rest you need. MSR’s bivies feature water-resistant, breathable ripstop nylon for the top layer to reduce condensation and keep you drier and more comfortable inside. By their nature, bivies are far more enjoyable when the weather is fair—most will offer little protection in a nasty Cascades or Rockies storm blitz. But during high-pressure systems in the shoulder seasons or on summer alpine traverses, bivies add an extra layer of warmth to minimalist nights. Pairing a bivy with a tarp expands your forecast options while keeping weight often below or similar to a one-person tent, and gives you the flexibility to take both or just one component depending on the trip. Efficiency No setup. No pitching. No hassle. With nothing to assemble after a long day, nor to break down the next morning, bivies get you back on your route faster. All that time and energy saved may be devoted to the day ahead. Additionally, these ultralight shelters offer an exceptional weight-to-protection ratio and efficiency. Immersion in Your Environment While tents cut off your sense of place when you're inside, bivy camping—out in the open, with the enormousness of the mountains surrounding you—is an experience all its own. The peaks of the Picket Range or the red rocks of the southwest feel closer, larger and more rugged when you have an immersive front-row view.

The Art of the Bivy: 14 Tips for a Comfortable Bivouac

Interested in bivy camping on your next adventure? Follow these tips for an optimal experience.

1. Find a sheltered zone or nook; often popular climbs will feature bivy sites surrounded by rock walls.
2. Ensure you're not setting down in a small water channel (pay attention to the contours around you).
3. Place the gear you need to keep warm at the foot of your sleeping bag.
4. Change out of wet layers before bed.
5. On cold nights, place a Nalgene® bottle of hot water in a core zone—crotch or armpits—for instant warmth.
6. Just like camping in a tent, cook a good distance away from your sleeping site.
7. Cinch the bivy's hood down around your face, but avoid breathing inside the bivy which can create condensation.
8. A buff or balaclava makes a great lightweight face mask.
9. If possible, during the day turn the bivy inside out and air it out.
10. In snow, consider using an ultralight tarp underneath and two sleeping pads—a closed-cell pad and an air pad—for greater insulation from the ground. See these tips for snow camping.
11. Because the majority of your gear won't be protected, consider a backpack liner/cover to guard against precip.
12. Consider choosing a water-resistant sleeping bag as extra insurance.
13. Use these tips to trade the austere bivy experience for one with a little luxury.
14. Check the forecast! And enjoy the lighter pack on your back.

Other Factors to Consider

Like other types of fast and light equipment, bivies are specialized pieces of gear. As great enablers of big pursuits, they require you to set your priorities. Any features added to the bivy sack to combat a greater range of environments also add weight. When choosing whether a bivy is right for your adventure, consider the impact of factors like condensation, bugs, the restrictive space and the forecast. Determining the types of adventures you wish to tackle—fast overnights to tag a peak vs. a month-long thru-hike—and your comfort tolerances will help you choose the right balance of features and performance.

MSR’s Bivy Offerings

MSR offers two high-performance bivies for minimalist travelers.

Pro Bivy

Our Pro Bivy was designed to meet the needs of professional alpinists seeking an ultra-simple shelter for demanding endeavors. It features a rugged ripstop nylon floor coated with our advanced Xtreme Shield™ waterproof coating for long-term durability. On top, the 2-ply breathable ripstop nylon helps manage condensation, providing a more comfortable night in technical terrain.

E-Bivy

Our E-Bivy is a longtime favorite of backcountry ski tourers. At just 170g (6-oz.), it makes a great reusable emergency shelter for your backcountry safety kit. It also features our Xtreme Shield coating on the floor, and its silicone-coated fabric top blocks wind, dew, spindrift and precipitation—insurance for a single-day adventure that may become an overnighter. Related Posts:

• Tent? No Thanks. I Prefer Tarps.
• MSR Staff Picks: 5 Fast-and-Light Pieces
• 3-Season vs. 4-Season Tent: Which is Right for You?

Updated. Originally Published September 18, 2019.