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The Two Types of Bivvy Sacks
A bivvy sack is the simplest form of waterproof shelter. If you can handle being in a confined space it's a good substitute for a tent. They're very light, easy to set up and can be pitched anywhere there's enough room to lay down. Because they're so light, some wise day hikers carry them as emergency shelters.
Weatherproof Bivvy Sack
If water cannot get inside a bivvy sack, it is considered stormproof. It can be used in the worst weather as a standalone shelter. The fabrics, seams and opening determines whether a bivvy sack is stormproof or not.
The fabric used in stormproof bivvies are generally waterproof yet breathable fabrics such as Gore-Tex. Vapor from perspiration needs to escape the bivvy in order to prevent condensation so that the user can stay dry.
It is not necessary for the floor to be breathable but it needs to be waterproof. Polyurethane-coated nylon is the type of waterproof-nonbreathable fabric commonly used for the floor. The floor has to prevent water from seeping in from the ground. The floor also has to be resistant to abrasions and punctures on rough ground.
To be totally waterproof, sealed seams and waterproof closures are absolutely essential in stormproof bivvies.
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.
Because the fabric in water-resistant sacks are much more breathable, they're also less prone to condensation. Vapors are expelled quickly before condensation occurs.
This type of bivvy fails miserably as a standalone shelter but is excellent when used in combination with a tarp as a bivvy shelter. For added protection, only a small piece of tarp is good enough for use in heavy rain.
However, no fabric can completely eliminate condensation inside a bivvy sack. Condensation occurs when a warm body in a confined space is mixed with cold temperatures outside.
The head end and the foot end of the bivvy sack is succeptible to condensation but it is less likely to occur when the ventilation inside the sack is good or when the weather is dry.
Which ever one you choose, be sure that you are aware of its capabilities. Don't go out in the rain without additional shelter in a water-resistant bivvy. Be wary of condensation when you use a stormproof bivvy in cold weather.
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