The bivvy sack is as simple as outdoor shelters get. If you can handle being in a confined space it's a good substitute for a heavy dome tent. You can lie in the warmth of your sleeping bag and watch the stars or get a full view of sunrise in the morning without have to get out of the bivvy.

With a bivy, you can arrive at camp, crawl in, and go to sleep.

The Stormproof Bivy Sack

If no water can come into the bivvy sack, it can be considered as stormproof. As a standalone shelter it can be used in the foulest of weather. The fabrics, seams and opening are channels where water can seep into the bivvy sack.

A stormproof bivvy has to use a fabric that is both waterproof and breathable e.g. Gore-Tex or eVent. Fabrics like these not only prevent water from seeping through but allows vapor to escape as well to prevent unwanted condensation.

The floor of these stormproof bivvies are made of waterproof fabrics as well but not necessarily breathable. Silicone-coated nylon is typically used for the floor. The main function is to keep water from seeping through from the ground. Additionally the floor has to be resistant to abrasions and punctures.

To be sure that water cannot leak in, a stormproof bivvy needs to have sealed seams and waterproof closures as well.

Water Resistant Bivvy Sack

When a bivvy sack is not made of waterproof fabrics, does not have sealed seams or does not have a stormproof closure, it is considered as water-resistant as opposed to waterproof. Depending on the weather, water-resistant bivvies can be better than stormproof ones.

Water-resistant sacks are much lighter because of the lack of waterproof fabrics, seam sealings and waterproof closures.

Another advantage of water-resistant bivvies is that they tend to be less prone to condensation because the fabric used is much more breathable. Vapors are expelled before it gets the chance to form a condensate.

Many people use this bivvy with a tarp to protect themselves from the rain. You can use this type of bivvy with a small tarp (e.g. 5’ x 8’) for adequate protection and warmth even in the worst weather.

However, no fabric can completely eliminate condensation inside a bivvy sack. When there is a huge difference in temperature between the outside of the bivvy and the inside, where the outside temperature is lower, condensation is likely to occur . Condensation is likely to occur inside the bivvy sack where there is a lack of ventilation or the lack of body heat. It is less likely to occur when the ventilation inside the sack is good or when the weather is dry.

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