No other waterproof/water-resistant shelter can be more simple than the bivvy sack. 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 sack, you can arrive at camp, crawl in, and go to sleep.
Weatherproof Bivvy
If water cannot get inside a bivvy sack, it is considered stormproof. As a standalone shelter it can be used in the foulest of weather. Water can get inside a bivvy either through the fabrics, the seams or the opening of the sack.
Gore-Tex is commonly used as a waterproof/breathable fabric in stormproof bivvies. These fabrics not only prevent water from seeping through the fabrics but also allows vapor from perspiration to pass through to the outside to minimize or prevent altogether condensation on the inside.
The floor of these stormproof bivies are made of waterproof fabrics as well but not necessarily breathable. Silicone-coated nylon is typically used for the floor. It functions to keep out water from seeping in through the floor fabric from the ground. Since the ground can be rough at times, depending on the terrain, the floor has to able to withstand punctures and abrasions.
A stormproof bivvy would also have sealed seams and waterproof closures to be sure that no water can leak in.
Water Resistant Bivvy
In the absense of waterproof fabrics, sealed seams and waterproof closures, a bivvy is considered a water-resistant instead of waterproof. Sometimes water-resistant bivvies are better than stormproof bivvies.
Many ultralight hikers prefer water-resistant sacks because they’re much lighter than stormproof versions.
Water-resistant bivvies are also less prone to condensation than its counterpart because the fabric used is more breathable. The type of fabric used removes vapors very quickly before it has the chance to condensate.
This type of bivvy is not suitable as a standalone shelter in bad weather but works great if there is additional shelter like when used with a tarp above or inside a cave. Even though carrying a tarp may mean extra weight but the weight saved from carrying a water-resistant bivvy more than makes up to it especially when it reduces the chances of condensation.
However, no fabric can completely eliminate condensation inside a bivvy sack. Condensation will occur when a warm body that emit perspiration, confined in a small space, is mixed with cold temperatures. The head end and the foot end of the bivvy sack is succeptible to condensation. Some people even with breathable fabrics open up their bivvy sack once in a while for ventilation to prevent condensation.
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