A bivouac sack or bivy is an extremely small, lightweight, waterproof shelter, used by climbers, ultralight backpackers, mountaineers, soldiers and minimalist campers who need lightweight emergency weather protection.

Choosing a Bivy

Advantages

  • Lighter weight and more compact than tent
  • Excellent weather protection
  • Bug and insect proof
  • Works on tight places where tent won’t work
  • Closeness to nature
  • Fast and easy set up

Disadvantages

  • Condensation can occur
  • Can feel restricting in longer periods
  • Can’t change clothes easily
  • Can’t shelter boots and backpack

At the first glance a bivy is just a micro tent – lighter and smaller than normal tent, but with all the same protection and less bulk. They will also work where tent won’t – on high rock ledges, tight snow caves or tiny flat campsites, any flat surface long enough to stretch out on, and narrow enough to fit on. Bivy sack doesn’t have as much floor space as tent, neither room to sit up and your comfort can be limited. However, these are features not every backpacker needs.  Mostly backpackers biggest concern is weight they carry and reducing your backpacks weight can make a massive  difference on your outdoor experience. Generally, the more durable the tent is, the more it weights and bulkier it is, often taking a half of your backpacks space, whereas bivy will take so much less. Even most durable and heaviest bivies weights far less than a good 4 season tent and they perform just as well, sometimes even better.  It might get a little too uncomfortable sleeping in a limited space for extended period, therefor for longer trips is good to consider a good tent instead of bivy sack.

You might like bivy sack if you:

  • Often travel solo in the backcountry
  • Climb big walls that require more than a day to complete
  • Camp on long-distance biking trips
  • Want to go as light as possible
  • Camp in snow caves

Types of bivy sacks

Bug nets are designed for areas where weather is less of a concern than insects. Bug net bivies are also lighter and easier to pack than the more weather-proof models.

Minimalist bivies are constructed without a set of poles or extra features that increase weight and bulk. These lightweight models are ideal for most adventures, where weather can be expected and you plan to hike longer distance fast.

4 season models are made from very durable material, designed to cope with rough weather in all seasons. They will have a structure to keep the fabric off of the camper’s face, giving you more protection from elements than other bivy models.

Bivy design

  • The bottom tier normally is made of a durable grade of nylon coated with urethane to make it waterproof. It is the same material used for most tent floors.
  • The top tier is usually made of lighter ripstop nylon treated with a waterproof, breathable laminate such as Gore-Tex®.

Air circulation in bivy

A bivy is basically a single wall tent. Breathable, but waterproof laminates such as Gore-Tex® fabric is pushing vapour, produced by body heat through and out of the fabric. This works best in cool and dry weather, but in rainy and damp conditions, manually venting a zipper or flap helps maintain an acceptable interior humidity level. When vapour escapes from your body or lungs, it rises to meet colder air. At some point, the air can no longer carry all this moisture, so some may collect on the inner wall of the treated bivy fabric. In double walled tents this moisture passes through the canopy and settles on the rainfly.

Fabric Guide

Gore-Tex is a waterproof breathable membrane that has pores in it which are much smaller than water droplet. It makes it impenetrable to liquid water, but still large enough for water molecules to pass through. The breathability of Gore-Tex and other proprietary laminates depends a lot on the fabrics that it is sandwiched between. A lot of the proprietary laminates have 2 or 2.5 layers of fabric making them potentially more breathable than 3 layer Gore-Tex.

ToddTex is a proprietary membrane like Gore-Tex that was originally developed by Todd Bibler of Bibler tents. ToddTex is also treated with a fire retardent. Black Diamond uses ToddTex for its tents and shelters.

Sympatex is a waterproof, windproof, and breathable membrane, but it does not have pores in it like Gore-Tex ,that let water molecules pass from one side to other. Instead, water molecules are absorbed by the membrane and evaporate on the outside.

Reflexion is a variant of Sympatex that is coated on one side with a reflective coating that traps body heat and reflects it back, like an emergency blanket.

Silnylon is waterproof, but non-breathable silicone impregrated nylon.

Epic is silicone impregnated polyester which is waterproof and breathable.

Momentum is a non-waterproof breathable, wind and water resistant fabric.

Pertex is also wind resistant and has better breathability than many waterproof laminates. Pertex is made with two fibers of different filament thicknesses. Moisture is absorbed by the larger filaments on the inside of the fabric and is transferred to smaller ones on the outside, moving moisture by capillary action.

eVent is one of most popular fabric in breathable waterproof membranes. It also vents moisture through millions of tiny pores, just like Gore-Tex. But unlike Gore-Tex it does not require an inner coating, providing a more direct  path for moisture to evaporate.

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