There are a number of different materials that bivvy bags are commonly made from. These materials can be found on their own or together, depending on the type and brand of bivvy bag that you go for.
We’ve put together a brief explanation on each of these materials, how they work and whether or not they are any good, helping you to better make an informed decision on which bag is right for you and your needs.
Polyurethane (PU) and Silnylon
These materials are generally used for the bottom of a bivvy bag, but they may also be used for the entire bivvy. Both of these materials are waterproof, however, water vapour that develops as a result of the body sweating can’t be diffused by these materials, leaving the inside of the bivvy bag wet from condensation.
We urge caution when going for a polyurethane or silnylon bivvy bag as the current technology isn’t up to scratch. In fact, it’s our opinion that you may as well save your money and sleep directly in the rain, giving you a clear indication as to just poor their ability to keep you dry is.
There is currently a lot of research going into how to make these materials breathable, so things may change in the future, however the technology for bivvy bags made from these materials is simply not there yet which is why we recommend against investing in a bag made primarily of these materials.
Gore-Tex
Gore-text has been around since the 80s, and is made up of 3 layers of fabric – an outer layer, an inner layer of ultra-thin PU, and a middle layer of polytetrafluoroethylene (PTFE).
PTFE contains tiny pores, around 9 billion per square inch, that are approximately 1/20000 the size of a water droplet but 700 times larger than water vapour particles, preventing water from getting in whilst allowing water vapour to escape.
Gore-Tex can lose its ability to keep water out when coming in to contact with alcohol or excessing amounts of sweat, which is why almost all Gore-Tex bivvy bags complete with an outer PU layer to counter this issue. It’s also designed in such a way that the outer PU layer actually attracts water molecules to it, away from the Gore-Tex, making the combination of these materials more effective at keeping water out and diffusing water vapour together than on their own.
To find out which goretex bivvy bags we recommend visit our best goretex bivvy bags page.
E-Vent
E-Vent, like Gore-Tex, is made from polytetrafluoroethylene (ePTFE), however it differs from Gore-Tex as instead of having an inner PU layer, ePTFE is treated with chemicals that help to protect its breathable membrane from soiling, maintaining the materials integrity.
This breathability helps bivvy bags made from ePTFE effectively diffuse water vapour that is released from the body, helping to keep you dry and free from condensation on the inside of the bag after use.
Pertex
Pertex is made of two layers – the top layer being made of small filaments, and the bottom layer being made of larger filaments. The reason for this is that it allows water vapour to escape whilst preventing water droplets from getting in. Water will eventually get in to a Pertex bivvy bag in heavy rain, making it a water resistant material, not a water proof one.
This material is most commonly found as the outer layer of more expensive bivvy bags, adding an outer shell that helps to stop them soaking through.
Read more about Pertex
DWR
DWR, which stands for durable water repellent, is a coating that is added to fabrics to make them water-resistant. This material is typically used with other fabric, like Gore-Tex or E-Vent, to protect the outer layers of these materials from becoming saturated with water, helping to maintain breathability. Over time DWR becomes less effective, requiring the material to have another coat applied to it to keep it water resistant.
Although there are a number of bivvy bags on the market that are made of DWR, such as the excellent Aqua Quest Hoopla bivvy bag, however it’s important to note that like Pertex, these aren’t waterproof and would be best used with either an additional outer layer or tarp to prevent water from getting in during heaving downpours.
Read more about DWR materials
Polyethylene
Polythylene is used in a number of higher end bivvy bags, notable those branded by Cuben Fiber and Tyvek. It’s the lightest material used in campaign products that is currently available and is extremely durable, breathable and waterproof. It’s the best material available for bivvy bag design and construction, however it comes with a hefty price tag, which leads most campers to opt for lesser, more cost effective materials such as Gore-Tex and E-Vent, which are found in the majority of mid-range bivvy bags.
Polypropylene
Polypropylene is a material that has been on the market since 1996. It’s completely waterproof, as it doesn’t absorb any amount of water, and is both quick drying and breathable. Polypropylene also has low thermal conductivity, meaning that it is better at retaining head than other materials, and is 20% lighter than nylon, making it a perfect material for ultralight weight bivvy bag design.
Find out more about the best bivvy bags currently available and what we think of them on our bivvy bag reviews page.