What is a Bivy Sack? | Ultralight, Compact and Warm Outdoor Sleeping Option

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If you’re going on a hike for several days and sleeping along the way, a bivouac sack (also known as the “bivy sack”) may be the better alternative. But what is a bivy sack, and why is it preferable to old standbys like a camping tent and sleeping bag?

A bivy sack or bag is a kind of shelter that you can carry around on hikes. It’s waterproof, and it’s personal-sized as well so it’s not as bulky and as heavy as lugging around a camping tent and a sleeping bag. You might say that the bivy bag is actually a combination of camping tent and sleeping bag instead.

Sunset on the lake

This was first used by hikers who were going on excursions that lasted several days. Nowadays it’s still used for that purpose, but others also prefer it simply because an ultralight bivy sack doesn’t pack as much weight.

You’re able to sleep literally underneath the stars with this, and this act appeals to so many outdoorsy types. 

Basically, the design of the bivy shelter consists of your sleeping with the notable addition of a thin waterproof fabric to provide additional protection against the elements.

The bivy cover may offer an opening for your eyes, and also you to breathe through. But more modern bivy sacks can use fabrics that are both waterproof and breathable, so some of these openings won’t be necessary anymore.

Do You Need a Bivy Tent?

You may want to get a bivouac tent if you’re going on a climb, a hike, or a biking trip that will need more than a day to complete. You’ll need to cut down on the weight of your load, and a bivy tent will be much lighter than a combo of camping tent and sleeping bag.

A sleeping bag bivy will also be ideal if you’re going winter camping and staying in a cave along the way. An ordinary sleeping bag just won’t do.

Some soldiers also go for military bivy sacks with proper camouflage so that enemy patrols won’t find them easily.

If you frequently travel alone in the backcountry, a bivy sack will also be much more suitable for your needs.

Bivy Sleeping Bag Design

The earliest bivy sacks were basically waterproof nylon slipcovers for sleeping bags. They were meant to protect the sleeping bags from getting wetting from the rain and snow. However, these slipcovers weren’t ideal for ventilating the vapor as a result of body heat.

Blue and yellow sleeping bags

Today, modern bivy design involves the use of 2 levels of fabric. The bottom level of the bivy sack generally uses the same material that you’ll find in a tent floor. This is usually a durable type of nylon, with a coating of urethane for waterproofing.

The top level fabric is lighter, as it is typically made from ripstop nylon. It is then treated with a breathable waterproof laminate. Some of the more famous laminates of this type include Gore-Tex.

Bivy Sack vs. Bivouac Shelter

While some tend to use the terms interchangeably, these are actually 2 different items.

Traditionally, the bivy sack is designed to meet the needs of mountaineers. However, plenty of hiking enthusiasts have liked it too, especially those who are determined to minimize the load they’ll carry on their incursions.

The sleeping bag bivy has 2 main functions. One is to make sure that the sleeping bag if the camp stays dry. The other function is to enhance its warming capacity, usually by 10 degrees F.

The traditional bivy sack has a head opening, which allows you to see out and to breathe. The problem is that this is also potentially an opening for moisture to get into the sleeping bag.

You can reduce the risk of this by pulling on the drawstring of the hole as snugly as you can. That works, although the drawback is that it turns the head hole as nothing more than an opening for the nose. Some have no problem with this, to others may find it a bit too restrictive.

A bivouac shelter is also known as a bivy tent. This is a low-rise tent, with the head opening featuring mesh panels to keep the bugs out. This comes with hoops or poles to keep the fabric away from your face.

Sky above the mountains

This shelter can also keep the rain put. But if you want to remain dry, you’ll need to think about careful venting during the rain.

If you’re a cyclist out on a multi-day excursion or a backpacker insisting on keeping it minimal, then these shelters work out well. You get a nice fortified wedge of space for your head, so that it doesn’t feel as restrictive as with a bivy sack.

On the other hand, if it’s raining for an extended period of time this type of shelter may not be all that appealing. You can’t sit up, so lying there while it rains forever will be rather trying. 

Features of an Ultralight Bivy Sack to Look For

There are obviously many types of bivy sacks, and you can pick and choose the ones which offer the features that you want. Here are some features that a backpacking bivy sack user might want:

  • Securing straps. These straps will make sure that your sleeping pad stays in place. A sleeping pad that slides around can make you feel cold and wet in the end. 
  • Well-made sealed seams. Waterproofing is certainly a crucial issue with a bevy sack. The seams can be a likely cause of water seeping in if they’re not sealed properly. With premium bivy sacks, these seams are sealed tightly.
  • Full-length zippers. These can provide you with some ventilation options.
  • Multiple zipper sliders. You don’t have to just lie there when you’re in the bivy sack. These zipper sliders can let you have armholes which you can then use to perform certain tasks even while you’re protected by the bivy sack. With these armholes, maybe you can cook or make coffee, or you can sort out your gear.
  • Proper shape. Do you have big shoulders? You should then check out the shoulder girth to see if it fits you or if it provides the wider space you prefer. If you have big feet, then you should also check out the shape of the foot box.
  • Size and weight. Obviously you will need to confirm that the bivy sack you’re buying is lightweight and compressed. These are the main reasons why you’re getting a bivy sack in the first place. Some of these bivy sacks can weigh only 200 grams, while others with more features can weigh heavier than a kilogram. The extra weight may not mean much, but you may feel it over long hikes.
  • Waterproofing. Some of these bivy sacks come with hydrostatic head (HH) ratings. If you have am HH rating of 10,000 or more, then you can stay dry in most weather conditions. The lower HH rating you have, the greater chances there are that the water will seep through when you have puddles and heavy downpours. A 1,000 rating is fine for brief showers and spills, but will then let in water in more extreme conditions.
  • Breathability. If you have a bivy bag that uses brands like Pertex, eVent, or Gore-Tex, you’re going to be okay when it comes to breathability. The measure of a bag’s breathability is set by the moisture vapor transmission rate (MVTR), and a rate of 10,000 is good. If you have a lower MVTR, things can get sweaty and damp inside the bivy sack. When you have Gore-Tex or other special fabrics, there’s no problem. The MVTR for such fabrics can go up to 25,000 or more. 
  • Price. A nice Tact Bivy Compact Ultra-lightweight sleeping bag can weigh just 4.8 ounces and will be windproof, water-proof, and even tear-resistant. These can go for less than $25. But a Rab Ridge Raider with even more features like a mosquito net door and zip pulls that glow in the dark can cost $375.

Dealing with Condensation Issues when Bivy Camping

The moisture buildup inside the bivy is often the main problem when you’re using a bivy bag or tent. One solution to this is to use a bivy tent with branded fabrics like Gore-Tex, but that’s somewhat expensive for some people.

View from the tent

So what’s the alternative? Here are some tips that can help.

  • Don’t camp close to water sources or on valley floors. Instead, find an elevated position.
  • Don’t sleep in wet clothes.
  • Don’t breathe into your sleeping quilt.
  • Sleep under low-hanging branches.
  • If possible, leave the bivy somewhat open.
  • Avoid anything that makes you sweat inside the bivy. Don’t use an overwarm quilt or overdress when you sleep.

All in all, a bivy sack is great if you can stand the space restrictions and you can deal with the moisture buildup. It’s easy to carry, keeps you dry and no trouble to set up unlike an actual tent. Just pick a top-rated bivy sack for your needs and you’re good to go!

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