The bivouac sack (or “bivy” sack) is essentially a weatherproof sleeping bag cover with a breathing hole—the perfect bear burrito filled with your ambitions to complete those ultralight objectives.
Some call them cramped. Others, “miserable nights” spent unexpectedly in the harsh wilderness, when the car is far beyond your physical and mental fortitude. To many, they’re merely insurance against unplanned nights in the hills, ideally never to be used.
But the devout know better: There’s an elegant and sophisticated simplicity to the bivy sack.
When used under the right conditions, it’s key to mastering the fine art of fast and light travel on big backcountry missions.
First, what is a bivy sack?
Bivy sacks are single-person, minimalist shelters that offer an additional layer of protection for you and your sleeping bag against the environment—think cowboy camping with a full-body shell.
Just larger than your sleeping bag and mattress, they’re 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 MSR’s Pro Bivy and E-Bivy—weigh much less than a one-person tent and offer a utilitarian, pure 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 small-liter packs choose this supremely efficient shelter for the freedom to travel unencumbered. They’re 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, and the bivy is their ally.
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 for a tent.
But these days, bikepackers, fastpackers, thru-hikers, and minimalist solo adventurers choose bivies in order shed every possible gram while maintaining an element of weather protection.
Why should I choose to bivy 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 can be your ultimate ally on big mountain endeavors.
The most pared-down of bivies weigh just a few grams. MSR’s Pro Bivy, engineered to meet the needs of professional alpinists and serious adventurers, weighs just 283 g (10 oz). Even lighter still, MSR’s E-Bivy weighs a mere 170 grams (6 oz), perfect for tossing in a pack lid as an emergency shelter on everything from all-day hikes to backcountry ski tours.
The ultra-compact nature of bivies is one of their greatest assets. Some stuff down to the size of a burrito (the food kind), disappearing in bikepacking bags and small alpine packs. With a smaller pack you can cover more miles and more easily traverse through technical terrain.
And technical terrain is where the bivy shines. Requiring only the length and width of your sleep system, bivies open up site 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.
Like a light rainshell jacket, bivies trap in a few degrees of warmth and hold that heat better than a tent—taking an open night on the ground up a notch to gain the rest you need. MSR’s bivies feature a water-resistant, breathable ripstop nylon for the top layer to reduce condensation to keep you drier and more comfortable inside. By their nature, bivies are far more enjoyable when the weather is fair—most 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 your nights beneath the peaks.
Pairing a minimalist bivy with a tarp expands your forecast options while still keeping weight often below or similar to a one-person tent, and gives you the flexibility to take both or just one component.
No pitching. No hassle. With nothing to set up after a long day or to take down the next morning, bivies get you back on your route faster. All that time and energy saved can be devoted to the day ahead. Indeed, bivies as convenient as they are compact. And then, of course, there’s the weight-to-protection efficiency that these ultralight shelters offer.
Immersion in your environment
While tents can cut off your sense of place when inside, camping out in the open, with the enormousness of the mountains surrounding you, makes bivy camping an experience all its own. The peaks of the Pickets Range or the red rocks of the southwest feel closer, bigger and more rugged when you have an immersive, front-row view.
The art of the bivy: Tips for a comfortable bivouac:
- Find a sheltered zone or nook; often popular climbs will feature bivy sites surrounded by rock walls.
- Ensure you’re not plopping down in small water channel (pay attention to the contours around you).
- Place gear you want kept warm at the foot of your sleeping bag.
- Be extra diligent about changing out of wet layers.
- On cold nights, place a Nalgene® bottle of hot water in a core zone–crotch or arm pits–for instant warmth.
- Like camping in a tent, cook away from your sleeping site.
- Cinch the bivy’s hood down around your face, but avoid breathing inside the bivy.
- A buff or balaclava makes a great lightweight face mask.
- If possible, during the day, turn the bivy inside out and air it out between sleeps.
- 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.
- Because the majority of your gear won’t be protected, consider a backpack liner/cover to guard against precip.
- Consider choosing a “water-resistant” sleeping bag as extra assurance.
- Use these tips to trade the austere bivy experience for one with a little luxury.
- Check the forecast! And enjoy the lighter pack on your back.
What other factors do I need 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. Features that are added to combat a greater range of environments will come with added weight. When choosing whether a bivy is right for your adventure, you’ll need to consider the impact of factors like condensation, bugs, the restrictive space, and forecast, and whether any or all apply. Determining the types of adventures you wish to tackle—fast overnights to tag a peak vs. a month-long thru-hike in the forest—as well as conditions, 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.
Our Pro Bivy was designed to meet the needs 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 to get the rest you need.
Our E-Bivy is a longtime favorite of backcountry ski tourers. Just 170 grams (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 becomes an overnighter.