We sleep outside for all sorts of different reasons. Sometimes, it’s because we just want to get away and look at the stars. Sometimes, we want to disappear into the wilderness and let civilization drop away. Sometimes, our objectives take longer than a day, and we have to sleep out by necessity. In all of these scenarios, the things we take with us are different.
A car camping weekend might include a tempurpedic pillow, whereas on a two-day alpine mission your pillow will look a lot like your climbing rope. Sometimes, and this isn’t always, the right tool for the job is a bivy sack.
A bivy sack is an ultralight, one-person shelter with minimal structure. It resembles a waterproof over-sleeping bag.
While they certainly aren’t as comfortable as tents, they are a heck of a lot lighter. So what’s the use? When should you carry one? Why should you? Will you hate yourself if you do?
When To Use an Ultralight Bivy Sack
I carry bivy sacks for two reasons primarily: emergencies or fast-and-light missions in the alpine, and in both cases, the tools I use are different.
Especially when guiding in snowy, cold conditions, I always carry a little emergency shelter. Usually it’s quite simple: a sheet of reflective material sewn into a pocket. Think three-sided human space blanket burrito. They’re light (½ a pound), small (fist sized) and can really help bail you out of a pinch.
I certainly never plan to sleep in one, but they have made a huge difference, especially for clients, sitting out a storm or trapping some extra heat while waiting for the group. They are very delicate and often very hard to get back into their initial packing, but a darn useful emergency tool. I pull one out a few times a season, but carry one every time I’m in big, cold terrain.
They’re a great insurance policy any time you’re travelling somewhere that a long, enforced break might turn unpleasant or deadly. I would, especially in alpine terrain, plan on using one as part of a bivy system, but maybe that means I’m soft.
There are also larger, more substantial bivy systems that users take out planning to sleep inside of. The bivy sacks in this category are more substantial, waterproof, breathable and fully enclosed. I would describe them, generally, as a claustrophobic nightmare of a tent.
They have all of the same basic features and durability, but instead of being large enough to sit inside of, function as a weather-proof exoskeleton for your sleeping bag. Some have small pole systems to keep fabric off of your face, and zipper systems differ substantially, but they’re the kind of thing you could sit out a fearsome storm in.
My most common use for these is on an overnight trip where I expect some weather (but not colossal amounts) and weight is at an absolute premium. These weather-proof bivys, such as Outdoor Research’s Helium and Alpine models, work well, and are surprisingly comfortable. While I wouldn’t plan on hiking the PCT with one, they work well.
Remember, There’s No Enclosed Space For Your Pack Etc.
The biggest drawback versus ultralight tents is that there is no enclosed space. Apart from while you’re sleeping, you, and all your stuff, is totally exposed to what’s going on outside. So if you live in a stormy range or are going out in the winter, the weight penalty of a tent might well be worth it (roughly ½ a pound between a nice bivy sack and an ultralight single-wall tent).
Summer alpine endeavors in the Sierra or Tetons lend themselves well to these sorts of setups. These bivy shelters shine best where the tent sites are poor and unprotected. Rather than chop a whole tent platform into a snowy slope, you only have to make one large enough for you! And you won’t have to worry about snapped tent poles and flattened, ripped tents.
Bivy sacks are low-profile enough and simple enough that they outlast most weather. You may not be the most comfortable inside though.
If you’re thinking of carrying one of these ‘cushier’ bivy sacks as an emergency shelter, think twice. A friend’s advice, which is rather extreme, is: “If you don’t want to get benighted, don’t bring bivy gear”, meaning that the weight and possibility of a lay-down will only de-motivate you from getting back to the trailhead. These bivies are heavy, especially as emergency tools.
Carrying an extra 1-1.5 pounds everywhere may get you into better shape, but it might also ensure that you use the gear more frequently.
Editor’s note: I reached out to Elijah Santos, a professional climbing guide, for his thoughts on utilizing an ultralight bivy sack:
“Bivy sacks hold a special place in my heart, primarily because I often travel solo through the backcountry and want a compact shelter option. They’re a phenomenal option if you need a compact shelter option for fast and light trips, or solo trips where there’s a possibility of variable weather. I spend a lot of time bikepacking, which I think is where bivy sacks truly shine.
They offer good protection from the elements, while taking up relatively little space among your gear. However, if you’re traveling with other people, you’re better off bringing a lightweight tent, which can be split up amongst your team. To me, a tent is the more comfortable option in the backcountry, but a bivy offers ultimate speed and performance when trying to move quickly.”
Pros and Cons of an Ultralight Bivy Sack
As I mentioned, bivy bags can be sweet. They are super light and will keep you dry and happy(ish) in unexpected weather. They might even save your life, depending on the circumstance. But they don’t always outshine tents and tarps. Like other specialty-use outdoor gear, their range of maximum effectiveness is relatively narrow.
I would not, for example, buy a bivy sack as my only backcountry shelter. It’s a niche tool.
However, they have many advantages over tents in particular circumstances. Generally, they are easier to set up, will work in more marginal sites, are lighter and are less likely to be torn up in a big wind event.
These shelters are dead easy to set up, even the more complicated models with little poles. With a bivy sack, you won’t struggle with oddly tensioned nylon panels and flapping tent flies. Its simplicity as s system makes setup in good weather a breeze: just unroll and stuff everything inside. However, if there’s precipitation, you’re in a race from the moment you open the sack until everything is situated.
They go up Anywhere:
Bivy sacks are universally free-standing, though some might scoff at ‘free standing’ being used to describe nylon sacks. Regardless, you can lay a bivy sack just about anywhere. No looking for the perfect, flat platform with good staking turf for your three person tent, just a slot big enough for your sleeping pad and maybe a rock or two to tie down your bag.
Tents can go wrong in any number of ways, especially the exotically designed ones. Bend one pole? Tent’s not going up. Destroy a zipper pull? You’ve compromised the integrity of the tent. Not so with bivy sacks. Though most are clam-shell design and include zippers, a zipper failure rarely means a total failure, as the whole design is so simple that you can always improvise.
Your Ultralight Bivy Sack Purchase Options
As I’ve alluded to, there is a galaxy of choice with bivy sacks, and it all hinges on likely use and the environment you’ll be using your new sack. A winter mountaineering bag won’t really do well as your summer emergency shelter and vice versa. Having slept in a number of different options across widely varying conditions, here are some of my thoughts.
Outdoor Research Helium Bivy (Lightest Weight On-Purpose)
Having used the Outdoor Research Helium for a few nights in relatively comfortable conditions, I can say that no shelter feels better on your back than a 1 pound 1 oz shelter. With small poles to hold the sack off of your face, and all the bells and whistles of OR sacks, the Helium packs a huge punch for 1 pound.
It is, of course, rather delicate, and I found myself being particularly delicate as I wormed around on volcanic debris while in the bag.
Outdoor Research Bug Bivy (Best Warm Weather)
Personally, if I’m planning on heading out on a warm-weather trip, especially one with no precipitation in the forecast, I’ll either totally forgo rain protection or bring an emergency rain shelter, should I wake up with snow or rain on my nose. That whole plan backfires, however, when the bugs come out. One of the big hurdles of bivy sacks is that they get sweaty, as the ventilation is poor.
Several companies make bug net bivy sacks, for the overnight trip where you want to watch the stars and not end up with 100 mosquito bites. Both OR and REI make bug net bivy sacks, but I prefer OR’s rounded designs. It should also be noted that a bug sack inside a nylon bivy bag would not go well. Don’t do that.
Outdoor Research Alpine Bivy (Most Bomber Sack)
This one is easy – Outdoor Research’s Alpine Bivy. With two big poles, removable bug netting for the entrance and burly fabric, this thing is the tank of bivy sacks. I’ve been really impressed with the nights I’ve spent inside. It felt way more like a shelter than other bivy sacks I’ve used.
The Interstellar, OR’s newest bag, gets an honorable (lighter) mention as well. It’s the newer, slightly lighter version of the Alpine, and I’m excited to try it.
SOL Sacks (Best Emergency Sack)
For work and play, I carry SOL sacks. Even though they cost a bit more than fabricated a reflective sleeve myself, they pack really well and surprisingly durable. They’ve seen me through a number of hypo-wraps and storms with impressive aplomb. I would definitely spring for a modern one if you’re planning on carrying it a lot, as they have more than halved the weight of those things in the last 10 years.
Recap and Myths
Bivy sacks have a reputation as being used by crazy people or in bizarre situations, or both. While this isn’t untrue, you don’t have to be a lunatic to use one. And no, you won’t suffocate in a bivy sack; they’re not garbage bags. Remember, it is important that you really understand what you’ll be using the shelter for before you pull the trigger. These things are flexible, but they really shine in their niche.
Can you suffocate in a bivy sack? While it may seem a claustrophobic nightmare, actual asphixiation in a bivy sack is impossible. The nylon and zippers both let in enough air to rest easy for a night. If the sack were to be totally entombed in snow, that story might be a bit different, but that isn’t a bivy-specific problem.
Are bivy sacks warmer than tents? Of course, it depends on the bivy sack and the tent. Bivy sacks are easier to keep warm, given how much less trapped air there is inside them, and it is also easier to pitch them in more protected locations.
As to whether the fabric of one versus the other emits more trapped heat, I think it’s 6 of one, half a dozen of the other, but in terms of how warm you can stay circumstantially, it is far easier to tuck your bivy sack beside a fallen log to block the wind than a tent.
Are bivy sacks waterproof? They are just as waterproof as tents, but that, again, depends on what you buy. Burly mountaineering bivy sacks will hold out water, especially when laid on snow, a lot better than skimpier summer bags. but with taped seams and waterproof, shielded zippers, bivy sacks will prevent outside moisture from getting in.
Do you need a sleeping bag with a bivy sack? Yes. While a bivy sack is warmer than sleeping with nothing, it is just a nylon shell, and has almost no insulating properties.
How do you stop condensation in a bivy sack? Ventilation! This is a big (minor) drawback for bivy sacks. Once they’re sealed up, they get steamy quickly, which is why many sacks come with mosquito netting for the head opening, so that there is extra airflow.
The more you can open the vents, the more the sack will release condensation, but if you’ve got an old-school nylon tube, you’re going to have to sleep with the thing open and hope you wake up when it starts raining!
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