Bivy Bags - Waterproof and Water-Resistant Sacks
Wednesday, July 22nd, 2009A 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 lightweight and extremely easy to set up. You can even pitch it just about anywhere you want. With a bivvy bag, you can arrive at camp, crawl in, and go to sleep.
Weatherproof Bivvy
If no water can come into the bivvy sack, it can be considered as 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. Waterproof-nonbreathable fabric such as 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.
A bivvy sack can only be considered as stormproof if it can totally prevent water from seeping in by having sealed seams and waterproof closures.
Water Resistant Sack
A water resistant bivvy sack does not have sealed seams, waterproof fabrics and waterproof closures. Depending on the weather, water-resistant bivvies can be better than stormproof ones.
Many ultralight hikers prefer water-resistant sacks because they’re much lighter than stormproof versions.
One less obvious advantage is that water-resistant bivvies are less prone to condensation. Vapors due to perspiration are expelled quickly, reducing the chances of a condensate forming.
This type of bivvy fails miserably as a standalone shelter but is excellent when used in combination with a tarp as a bivvy shelter. I use a 5 x 8 foot tarp for use with a bivvy sack which works well the foulest of weather.
However, no fabric can completely eliminate condensation inside a bivvy sack. As a warm body perspires, giving out vapor and in a confined space, condensation occurs when the temperature outside is lower by a big margin. The head end and the foot end of the bivvy sack is succeptible to condensation. The risk is increased when the weather is humid but if there is good ventilation it is less likely to occur.
