Make no mistake – it is more comfortable to sleep inside a tent than a bivy sack. A tent experiences less condensation, offers more elbow room, and keeps you drier on a rainy night. Still, for those looking to pack small and light, reducing the weight you carry by two pounds or more, than a bivy sack might be the perfect solution.
What is a Bivy Sack?
Bivouac sacks, more commonly referred to as bivy sacks, are a smaller, more transportable shelter option than a traditional tent. Originally designed to provide emergency weather protection for climbers and mountaineers making particularly lengthy or perilous ascents, these versatile shelters have since evolved into a handy alternative for all varieties of adventurer – assuming they're minimalists.
Modern day bivy sacks are made with an array of lightweight fabrics, each with their own impressive properties, such as detachable hoods and fully taped seams. But at their core, every shelter is striving to deliver the same two comforts: dryness and warmth.
When determining which type of bivy sack you'd like to purchase, an important consideration should be the shelter's fabric. Because the shelter is so confined, breathability will be extremely important. The last thing a person wants to do when camping is wake up to a sleeping bag and tent soaked from condensation. Certain materials will help ward of moisture better than others.
Gore-Tex: One of the most popular fabrics on the market, Gore-Tex is a breathable, waterproof membrane, which uses multiple layers of laminates and thousands of tiny water droplet-sized pores to keep liquid water out. At the same time, the porous shell allows water vapor to pass through the tiny holes to provide superior dryness.
Silnylon: As its name suggest, Silnylon is a combination of silicone and nylon. While the material offers impressive waterproofing, it doesn't allow for much breathability, and so condensation can become a problem
Epic: Epic is similar to Silnylon in that it's made with silicon, but instead of nylon the fabric is primarily polyester, which results in both water resistance and breathability.
ToddTex: A proprietary product developed by Bibler tents, ToddTex is very similar to Gore-Tex apart from one stark difference: It's fireproof. Prior to leaving the factory, each Bibler bivy sack is treated with a special fire retardant - something that could come in handy should you require a fire on a windy night.
Momentum: This particular fabric succeeds in protecting campers against both wind and water. However, the material only resists moisture. Momentum shouldn't be your first choice if you're expecting rain - or any precipitation.
Pertex: Pertex is innovatively designed using two fibers varying in thickness. The larger filaments on the inside of the bivy sack work to absorb moisture, pushing water molecules to the shelter's exterior where smaller fibers where they can evaporate. This is called capillary action, and it helps improve breathability and wind resistance.
eVent: In terms of breathability and waterproofing, eVent is the optimum choice. It mimics Gore-Tex with an expansive network of tiny pores for improved moisture protection, but doesn't require any internal coating, so inside water vapor can more easily move to the membranes surface where it can evaporate.
Sympatex: Sympatex is a high quality fabric on par with eVent and Gore-Tex. It's waterproof, windproof and impressively breathable, but not as a result of pores. Instead, Sympatex uses more of a wicking technique, in which moisture is drawn from the bivy sack's interior and through the interior where it can collect on the exterior surface and eventually evaporate.
How Do You Use One?
The beauty of bivy sacks rests in their simplicity. Of course, complicated science is behind their abilities to protect you from the elements, but properly using one of the shelters requires little more than a desire to spend some time outdoors.
The first step is to slip the entire tube-like sack over your sleeping bag. Some campers prefer to wait until they've reached their campsite to get things setup, but because of their lightweight material and their independence from long, heavy poles, if you'd like to roll your bivy sack up with your sleeping bag already inside, that's definitely a realistic option.
Once at the campsite, shelter fully prepared, sleeping is fairly easy. If your sack has a detachable hood, consider the weather before putting it on, but remember that if you choose to use, leave a large opening near your mouth. Allowing a quick escape for breath will help reduce additional condensation.
Depending on the size of your bivy sack, a sleeping pad or roll might have to stay on the outside, which shouldn't be a problem if the weather is reasonable.
Finding the Best Fit
On a clear night, when the wind is still and each star can be traced from one to the next, sleeping in a bivy sack can make you feel like a pioneer, alone in the frontier with nothing but your gear and the world. But that's not for everyone. Some people prefer the secure feeling of a big, domed roof over their head, like they're protected from the elements. While bivy sacks may offer much of that same protection, the extremely confined quarters tend to break down the sensory separation between yourself and the surrounding landscape. On a windy night or during a tough storm, it's entirely reasonable that some people might feel uncomfortable in such a small shelter. Consider what you're able to handle before purchasing a bivy sack and setting off into the great unknown.
Despite the protection many bivy sacks offer from wind and rain, their lightweight and thin materials still make them susceptible to severe weather. When making your purchasing, first decide what climates you'll most likely be traveling through. Be sure to consider the bivy sack's fabric during your selection process. Remember that while some are waterproof, others are only water resistance - there is a big difference.
The Right Design For You
There are two primary designs for bivy sacks. The first are more the luxury models - if you can say that about such a small, outdoor shelter. They generally include a thin, flexible pole or built-in hoop that creates a bit of a tent effect over your head, like Outdoor Research's Alpine bivy, allowing the camper to sit up and even store a few smaller items. The second option is more in line with the traditional emergency bivy sacks, which provide virtually no additional head room. While the two serve the same purpose, personal preference will certainly help you determine which the right fit is for you.
Additionally, the weight of your sack should also play a role in your purchasing decision. All are built for portability, but some go quite a bit farther than others. Some bivy sacks can weigh less than 19 ounces and be packed to fit the volume of a water bottle, while others are built for harsher climates and might weigh a few pounds.
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