Bivy Bags: Sleep with Protection under the Stars

Bivy bags are a simple element, designed to insulate from the wind and humidity. Nothing else to say about them? Quite the opposite. The uses, limits, materials, construction and design are all important. Here''s how to choose the right bivy bag for you.

Sleeping out with no worries about getting wet. Photo Alberto Hernández

For Don Quixote, when sleeping under the open heaven, "he performed an act of ownership that helped to prove his chivalry" and Gaston Rébuffat said "Some mountaineers are proud of having done all their climbs without bivouac. How much they have missed!".

Whether used for multi-day trekking or in an emergency, over time, the bivy bag has evolved into designs that are much more comfortable, resistant, lightweight and practical than the original sheepskin covers used by Count Henry Russell or the more recent plastic bags used when there was no other way of protecting your sleeping bag. If the conditions are right, it’s not an essential item, but it certainly prevents major problems and is a back up in the event of a threatening cloud hanging over your camp.

Want to know more about bivouacs? Check out this link on tips for bivouacking in all conditions consejos para realizar vivacs en cualquier circunstancia and then continue to read this article to learn how to choose your bivy bag.

What is a Bivy Bag?

The bivy bag is a basic element for lightweight camping, designed to cover your sleeping bag and add waterproof protection. An ultra-lightweight substitute for tents, it has traditionally been used for outdoor activities that were not compatible with a classic tent.

Carlos López-Torres Photo Carlos López-Torres

Since the early days of mountaineering, bivy bags have accompanied those who prioritise lightweight gear, be it for big wall climbing routes or multi-day treks in predictably stable weather or as a back up, in the event of not being able to make it to the hut in time.

The reasons for dispensing with a tent and opting for a bivy bag are varied, but they are mainly used during activities where pitching a tent is not possible. There may be no suitable place for pitching, such as a bare rocky summit or vertical wall or pitching a tent may be prohibited in that area, or maybe a compact bivy bag simply saves time and space in your backpack. If fast & light is your thing, we invite you to read our article on Lightweight mountain gear and material de acampada ligero.

Mountain Equipment Photo Mountain Equipment

Rain is not the only enemy when sleeping out. Snow is, of course, also a problem, but let’s not forget the fog, the wind or even the morning dew when sleeping on grass. The probability of waking up in a wet sleeping bag due to humid outdoor conditions is high when not using a tent.

What are the Benefits of Using a Bivy Bag?

We have already mentioned that using a bivy bag instead of a tent gives you extra space in your backpack, as well as making it lighter and therefore easier to carry. This allows you to carry a smaller and lighter backpack, so that you can move faster and complete a route with more time to spare and less fatigue. This, in turn, means you are more likely to finish your activity without unforeseen mishaps.

The main use of a bivy bag is to keep you dry while sleeping. Not only is sleeping in a wet bag uncomfortable and an obvious health risk, but wet gear becomes much heavier to carry and can become damaged, especially in the case of down sleeping bags.

A good bivy bag should not only insulate against humidity, but have good windproof capacity. Wind can quickly turn a bivouac into an unpleasant experience, so in addition to preparing the sleeping area properly, the windproof capacity of your bivy bag will also help get through the night in comfort.

Carlos López-Torres Photo Carlos López-Torres

Carrying a suitable bivy bag will save you from having to move your sleeping place in the middle of the night. Before the invention of high quality bivy bags, it was not uncommon for a night shower or heavy fog to ruin your bivouac long before dawn, leaving no choice but to move to another spot, if that was even possible. Remember that when opening a climbing route or climbing a big wall, it is not always an option to leave when it starts to rain, and sometimes you just have to hang on and wait for the rain to pass and hope that the effects are not too severe.

A bivy bag is also designed to protect your sleeping bag from damage and make it longer-lasting. From the most trivial problems such as a stain while eating, to the most disastrous such as losing the down or synthetic fill due to a tear in the fabric.

Another benefit is that a bivy bag increases the warmth capacity of your sleeping bag by several degrees. However, while using more layers clearly provides greater warmth, this is only practical when the quality of the bivy bag is able to provide sufficient breathability in case of excessive heat. More on that later.

Restrictions of Use

So far, a bivy bag seems the perfect solution for saving weight, space and effort, but there are also some disadvantages. If you want a piece of equipment that can withstand the harshest weather conditions, you will have to either resort to a tent or be prepared to turn back when the conditions are not suitable. You can read here about How to choose a tent.

Alberto Hernández Photo Alberto Hernández

A tent also provides living and storage space for two or more people, which is practical when sharing activities such as cooking and eating, consulting a map, planning the activity or socialising.

Equally, it would be imprudent to rely on a bivy bag as the only means of protection against the elements on a multi-day trek. Weather forecasts are not always reliable, especially when given several days ahead. Finding yourself in the middle of a storm, at night, with no protection other than a bivy bag, can turn your dream trek into a nightmare.

As with most mountaineering equipment, designs, materials and finishes vary, and this is reflected in technical performance. Check the features of the bivy bag you have chosen carefully, to avoid unpleasant surprises.

Mikel IraolaPreparing a bivouac. Photo: Mikel Iraola Gracia

What Materials Should I Look for in a Bivy Bag?

The outer layer of a bivy bag is usually made of two main materials: polyester, which is lightweight and water-repellent or polyamide, which is a resistant and durable material.

The inner layer is similar and is normally made of the following materials to ensure waterproofing. These are:

  • Microporous Teflon: the technical name for this material is expanded polytetrafluoroethylene (ePTFE). This is very resistant and microporous and guarantees considerable waterproofing capacity with more than satisfactory breathability. The only downside to using this material is the price, as it is usually used for the most expensive bivy bags available. The best known examples are eVent and Gore-Tex.
  • Microporous polyurethane: Induced polyurethane inner lining with a microporous structure that allows water vapour out but prevents water droplets from entering. Pertex Shield is one of the best known names in this system.
  • Polyester: pore-free membranes made of hydrophilic molecular chains that absorb water vapour and wick it away to the outer layer. Models made of Sympatex are the best known in this range.
BarrabesMillet Trilogy Bivy with Pertex Shield

An essential feature to check when looking for a bivy bag, is thermosealed seams. However good the quality of the bivy bag fabric, if the seams are not thermosealed, water will get in. This is worth remembering because sometimes discounts are very attractive until you discover that not all the seams are thermosealed.

What Features are Needed for a Bivy Bag?

Among the features that should be checked, the most obvious is to ensure your sleeping bag is able to fit comfortably inside the bivy bag. Most bivy bags are usually large enough, but if your sleeping bag is on the bulky side, it is useful to check before making an investment and, above all, before going into the mountains to try it out.

Not only is the size important for fitting over your sleeping bag, but also to ensure your sleeping bag is able to fully expand, especially if it is down. If the down is not able to fully expand, it will not perform correctly as it will not create enough warm air to keep the down warm. Normally the minimum size required is 215 centimetres long and about 80 centimetres wide.

The next reason is basic: the waterproofing capacity has to be sufficient to withstand moderate rainfall over a short period of time. The waterproofing capacity is expressed as the hydrostatic head (HH). A bivy bag with a 5000 millimetre HH will provide adequate protection in most outdoor conditions, while those with a HH over 8000 millimetres are the best for withstanding even a short downpour.

BarrabesRab Trailhead. 10000 mm HH and 10000 g/m2/24 h MVTR

Most bivy bags are treated with DWR, or Durable Water Repellency and nowadays this is an essential treatment in the manufacturing process of all bivy bags. We recommend that this treatment is free of fluorocarbons so that their degradation does not harm the environment.

It is not only waterproofing that is important. Although sweating is not usually a problem when sleeping out in the mountains, it is possible that a bad choice of bivy bag can lead to getting overheated, even in wet weather. That is why you should not overlook breathability and your bivy bag should have a minimum MVTR of 5000 g/m2/24 hours. Those with 10000 g/m2/24 hours are the most recommendable.

Another factor that can be decisive when choosing a bivy bag is its ease of transport. This is determined by the weight and dimensions of the bag. The usual weight range would be between 300 and 700 grams (there are also much lighter and heavier models) and the dimensions should be no larger than 13 x 20 centimetres.

BarrabesUltralite Bivy by Mountain Equipment. At just 108 grams.

It should be noted, that packability is determined by the thickness of the bivy bag fabric, which provides strength and waterproofing capacity, so a heavier model is not necessarily less practical. However, the specifications of fabric thickness are not usually sufficiently detailed on most models; generally the base is thicker than the top, but there are differences. A 40 or 50 denier polyamide bivy bag is considered quite robust. All the variables of the main fabric, from thread thickness to the number of threads per surface area have to be taken into account, but this data is not always easy to find.

It is also useful to consider the accessories included. A zip for easy access is useful, but is also a weak point, so check it is well made and designed. The quality of any other closures should also be checked and the size of the hood should provide sufficient coverage when closed.

We hope this article has helped you get an idea of the criteria needed when choosing a bivy bag. If you have any further queries, please visit our website or any of our shops and ask our staff for advice. We are here to help you.

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