Bivy sacs were originally engineered for ascent climbers to use as emergency protection with their sleep systems. They were particularly designed for making camp quickly and in any location. The early sacks were basically a nylon shell that would go over your sleeping bag to protect you from the rain or snow. Since conception the bivy has progressed in both design and usage. That being said the bivy only shines for very specific uses as compared to the flexiblity of using a tent shelter.
There are generally 2 different types of bivy’s. The regular bivy (some come with bug net) and bivy shelter that has a frame which lifts the bivvy off your face. The regular bivy is geared towards mountaineers or minamilist campers that can use them if the weather turns bad suddenly. The framed bivy is for the ultralight backpacker, cycle tourist or anyone who wants to keep their gear light but have more room around their head.
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Bivy Materials
There are different types of materials used in bivy construction. Some combine a good durable waterproof base with a breathable top. Here are a few examples of the materials used:
| Bivy Material | Part of sack best used | Cost | breathable? | durable? | weight | rain protect? |
| Silylon | bottom | low | none | moderate | light | proof |
| Gore-tex | top or all | high | high | high | mod-heavy | proof |
| eVent | top | high | high | moderate | mod-heavy | proof |
| Epic | bottom or all | med | moderate | moderate | light | resistant |
| ToddTex | top or all | high | high | high | mod-heavy | proof |
| Sympatex | top or all | high | mod-heavy | proof |
Are Bivy Sacks Waterproof?
As depicted in the chart under bivy materials some bivy’s are completely waterproof while others are only water resistant. While a shelter bivy with a pole support around the head can help keep you dry in a pinch sleeping under a tarp or natural shelter is best.
How Much Warmth Does A Bivy Add?
Sleeping in a bivy will add 10 degrees to the warmth of your sleeping bag. A bivy will also hinder air flow. This is a plus if you need the extra warmth but a negative if you are camping in warm temperatures and want to stay cool.
Do You Need A Tarp With A Bivy?
Unless you are caught in an emergency and the only shelter you have is a bivy than you should either use a tarp with a bivy or find a natural overhang or shelter that will help protect you from the rain and snow. While some bivys are completely waterproof, they will be difficult to climb out of while wet without letting water in. Bivy’s struggle with condensation issues just as do single walled tents. Being able to vent ones breath outside the shelter is an important factor in reducing condensation and increasing comfort in a bivy. This cannot be done effectively while completely exposed to the rain.
Are Bivy Sacks Worth It?
Bivy sacks are worth it if you are camping in an area with no room to pitch a tent, no where to stake a tent or otherwise secure a tent easily or effectively, or if one is into a minimalist camping experince. If weight is the only concern there are tents that can pack down and run just about as light as a bivy but with more benifits and less drawbacks.