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 camping tent. They’re very light, easy to set up and can be pitched anywhere there’s enough room to lay down. With a bivy sack, you can arrive at camp, crawl in, and go to sleep.
If water cannot get inside a bivvy sack, it is considered stormproof. You can use a stormproof bivvy sack alone even in bad weather. The fabrics, seams and opening determines whether a bivvy sack is stormproof or not.
The fabric used in stormproof in general are either Gore-Tex or eVent fabrics which are waterproof as well as breathable. To prevent condensation, fabrics in stormproof bivvies need to allow vapor from perspiration to escape to prevent condensation.
The floor of these stormproof bivvies are made of waterproof fabrics as well but not necessarily breathable. The floor is usually made up of silicone-coated or polyurethane-coated nylon which is a waterproof-nonbreathable material. The main function is to keep water from seeping through from the ground. The floor also has to be resistant to abrasions and punctures on rough ground.
To be sure that water cannot leak in, a stormproof bivvy needs to have sealed seams and waterproof closures as well.
Water-Resistant Bivouac 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.
Waterproof fabrics, seam sealings and waterproof closures add weight to a bivvy sack. That’s why water-resistant sacks are much lighter.
One less obvious advantage is that water-resistant bivvies are less prone to condensation. The type of fabric used removes vapors very quickly before it has the chance to condensate.
Combined with a tarp, this type of bivvy works very well. This may mean you have to carry some extra weight but only a small amount of tarp is required and you may not have to worry about condensation so much.
Despite advancements in breathable fabric technology, there’s still not one that’s good enough to completely eliminate condensation altogether. Condensation occurs when a warm body in a confined space is mixed with cold temperatures outside. Condensation is likely to occur inside the bivvy sack where there is a lack of ventilation or the lack of body heat. The risk is increased when the weather is humid but if there is good ventilation it is less likely to occur.