How to Choose a Bivvy Bag

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Most wild campers begin their camping careers in a tent. A tent is a good bit of kit. As long as it’s not rubbish, it will keep the wind and wetness out and you can organise your kit around you. If it’s a gopping day, you can cook inside. However, a tent is heavy and campers can sometimes feel a certain ‘separation’ from nature. At this point most campers take a leap of faith and opt for a bivvy, which is why, we suspect, you are here. Now let’s take you through the motions so you get the right one because there are a lot of good ones and some pretty rubbish bivvies out there, too!

Bivvy Nomenclature

First things first. No one really knows for certain how to spell the word “bivvy.” It comes in the following singular and plural names:

Singular: bivy, bivvy, bivvi, bivi

Plural:  bivies, bivvies, bivys

If you do an internet search all of them will reveal a variety of bivouac bags which is the proper term. This has been shortened to the above names and the reason why we are telling you this is so that you can do a good internet search should you feel the need. However, on a pedantic note, we feel that the appropriate English (British English, mind you) should be bivvy (singular) or bivvies (plural) as it would allow the “i” to be pronounced similarly to the “i” in bivouac, rather than the “i” in the word “like.” Now that the English lesson is over let’s get down to the real business of advice on choosing a bivvy.

Bivvy Styles

In terms of styles of bivvies, there are two styles:

 1. Bivvy bags: These are the lightest weight option and are literally sacks that you crawl into with your sleeping bag and roll mat. They may have a draw string hood or fully enclose your head with a zipper.

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2. Hooped bivvies or bivvy shelters: These contain a little more material around the head area and a lightweight pole that when bent, lifts the material off the head. This allows the user to read and a less claustrophobic experience in a storm. Some companies might use a small loop that attaches to some paracord that will pull the material off the face. However this does require something to hang the cord from.

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Materials

There are several materials to choose from. First up are are two constituents that I shall group together; polyurethane (PU) and Silnylon which is a silicone liquid applied to nylon. These are generally used for the bottom of bivvy bags but they may be used for the whole bivvy. These are waterproof and will keep the rain out but there is a problem, which is best exemplified with one physiological fact. The body releases 1/4 litre of water vapour from sweat glands every hour. If this water does not get out of your bivvy, it condenses. I saw the end result of that process when my mate came camping with a silnylon bivvy. The next day, he and his sleeping bag were absolutely sodden.

​It therefore becomes apparent that a bivvy should be:
1. Waterproof- water droplets must be kept from getting in
2. Breathable- water vapour must be able to diffuse out

We would unsurprisingly urge caution with getting a polyurethane or silnylon bivvy because with current technology you may as well sleep in the rain instead. However,   there is a bucket load of research going on with polyurethane and the quest is on to create a breathable, yet waterproof, PU coating. Two examples of PU coated products come in the form of the REI Elements and Marmot Membrain Strata product ranges. However, a variety of reviews suggest that bivvies with these PU coatings are subject to condensation. The technology is not yet there but it soon will be.

Next up is Gore-Tex. Gore- was invented by Wilbert Gore, Rowena Taylor and Wilbert’s son, Robert, in 1980. It consists of three layers of fabric an outer layer, an inner layer made of ultra-thin PU, and a middle layer that is a porous fluoropolymer membrane (aka polytetrafluoroethylene (PTFE) aka teflon aka goretex) that contains tiny pores that are about 1/20000 the size of a water droplet but about 700 times larger than water vapour. This means that liquid water can’t get in but water vapour secreted from your breath and sweat glands every hour can get out. Gore-Tex has about 9 billion pores per square inch.

Now some of the more discerning amongst you will be asking the question “but I thought you said PU was not breathable? Why do I need it with Gore-Tex?” The answer is that if Gore-Tex is soiled with alcohol or sweat containing substances it loses its waterproof properties. Hence you need a PU layer to protect the Gore-Tex. This inner layer is made ultra-thin and is engineered in such a way that it is hydrophilic- water molecules are attracted to the PU. They then diffuse to through it and reach the side that faces the Gore-Tex. Then the vapour simply diffuses out of the bag. This water vapour transfer process is slower when compared to using Gore-Tex alone and some Gore-Tex products do suffer with condensation as a result of moisture collecting on the inside of a PU film.

​E-Vent contains the same ePTFE material as Gore-Tex but instead of having the PU inner layer, the ePTFE is smothered with a chemical (that is currently a trade secret) that protects breathable membrane from soiling and maintains ePTFE integrity. This means that water vapour from sweat can diffuse directly through the ePTFE layer, which in theory, should increase breathability.

Pertex is worthy of mention to the beginner. Pertex comprises two layers. The top layer has small filaments whereas the next layer down as larger filaments. The upshot of this is that water vapour can get out but water droplets struggle to get it. The key word here is STRUGGLE; water will eventually get in and a pertex bivvy will end up wet in rain and is therefore water resistant, not waterproof. It will withstand a mild dew, however. Many higher spec down sleeping bags come with a pertex outer shell to avoid them soaking through.

DWR (durable water repellent) is a coating added to fabrics at the factory to make them water-resistant (or hydrophobic). They may be used in  conjunction with fabrics like Gore-Tex to prevent the outer layer of fabric from becoming saturated with water, which will reduce breathability as water vapour transport from the goretex layer to the atmosphere will be impaired. Nickwax is a good example of DWR coating. DWR degrades over time and becomes less effective which means that the fabric will need another DWR coat. Although some of the lighter bivvies are made with DWR, it must be noted that this is not strictly speaking waterproof. Like pertex, it has water resistance properties only.

Cuben Fiber and Tyvek are branded products made from ultra high molecular weight polyethylene (UHMWPE). Tyvek is manufactured by DuPont whereas Cuben Fiber is made from the Cuben Fiber and Cuben Tech Corporations. It is perhaps the lightest weight material on the camping market and is extremely durable. It is breathable and waterproof. Top end bivvy manufacturers such as Mountain Laurel Designs use this product. However, this material comes with a BIG price tag attached.

Another material that hit the market in 1996 is the little known but exceptional product called polypropylene (aka polyolefin). It is completely waterproof and as the textile does not absorb water, it is quick drying. It is also ridiculously breathable; my bivvy is made of polypropylene and I can confirm that I have never suffered with condensation issues. There are other advantages too. It has a low thermal conductivity so heat is retained and is really lightweight being 20% lighter than nylon.

Weight

The weight of the bivvy will be governed by three factors:
1. The design. An enclosed bivvy will add a hundred grams to the bivvy weight. A hooped bivvy will add another two hundred grams.
2. The dimensions. The larger the bivvy the greater the room but the penalty is an increased weight
3. The material. Better material is lighter and more durable but generally costs more

So which bivvy should I choose?

Your choice will be determined by your camping style and we suggest visiting the following page called choosing your camping style. Have a look at our diagrams and decide what you will be doing and where you sit in the Zone of Feistiness and Triangle of Feistiness. If you are climbing big walls on multiple day ascents then clearly you will need to focus on lightweight and durable with some comfort sacrifices. If you are doing standard winter camping in the UK then why not get something a little more palatial?

One salient point soon becomes apparent. We can’t tell you which bivvy is “the best” because this is determined by your use. Be mindful of companies that are trying to punt “super duper ultralight” bivvies? Do you really need a 350 gram bivvy that costs close to £350 and is the size of a hamster cage? For the weight of three more apples in your backpack you could get something cheaper, equally bombproof, and comfortable.

We shall conclude with a few last pointers. If you want to have a quick look at the bivvies on the market today with some vital statistics, feel free to check out our bivvy comparison page. If you would rather read some more in depth reviews then please peruse our bivvy reviews at leisure with a fine single malt

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2 thoughts on “How to Choose a Bivvy Bag”

  1. Great article, full of useful advice. I think the most important feature of a bivy should be the breathability of the fabric. That way your sweat can leave the bivy.

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