The Two Types of Bivvy Sacks
If you're looking for a simplest waterproof shelter then you can go no further than the bivvy sack. If you can handle being in a confined space it's a good substitute for a tent. There's a reason why some people prefer to use a bivvy sack than to share a tent. They probably don't want to share a bulky, heavy 2-person tent with a smelly, grubby mate. Because they're so light, some very smart day hikers carry them as emergency shelters.
Stormproof Bivvy Sack
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 are channels where water can seep into the bivvy sack.
Waterproof/breathable fabrics are the main components of a stormproof bivvy. To prevent condensation, fabrics in stormproof bivvies need to allow vapor from perspiration to escape to prevent condensation.
While the floor of a stormproof bivvy needs to waterproof, there is no need for it to be breathable. The floor is usually made up of silicone-coated or polyurethane-coated nylon which is a waterproof-nonbreathable material. The floor prevents water from seeping through the fabric 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.
Waterproof/breathable fabrics are the main components of a stormproof bivvy. To prevent condensation, fabrics in stormproof bivvies need to allow vapor from perspiration to escape to prevent condensation.
While the floor of a stormproof bivvy needs to waterproof, there is no need for it to be breathable. The floor is usually made up of silicone-coated or polyurethane-coated nylon which is a waterproof-nonbreathable material. The floor prevents water from seeping through the fabric 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.
Top Stormproof Bivvies
Integral Designs Unishelter eVent Bivy Bag
Suitable as a standalone shelter in the worst weather. Highly breathable fabrics, a durable, bathtub floor, waterproof zippers, ventilation that can be opened in stormy weather, and livable space provided by an overhead pole. Packs a tremendous performance into its sub-2-pounds package. Also has great ratings as a summer bivvy.
Nemo Gogo Adventure Bivy
The Nemo Gogo is one of the best bivvy sacks you can find. It has a unique front entry and retractable vestibule option, making it easy to ventilate and stow wet gear outside the main bivy sack. Now added with Swallowtail, an innovative system to tension the bivvy above the your sleeping bag which maximizes interior space and increases ventilation. The reengineered airbeam and integrated pump keeps the weight down to 1.9 pounds.
Integral Designs South Col eVent Bivy
Based on the proven design of the South Col bivy, using the lightest materials available, the South Col, eVENT version is designed for climbers looking for high performance and light weight. Made from lightweight 3oz. eVENT three layer fabric which uses the Dry System Technology TM and a lightweight coated nylon floor, staying dry won't be a problem. A wire-stiffened hood keeps the fabric off the face (no bugnet) and a 22" low profile waterproof zipper (left or right) allows for ease of entry and increased venting. This bivy is the best option for the unexpected night out or the planned big wall sleepover.
Water-Resistant Sack
If a bivvy sack does not have a waterproof fabric, sealed seams or stormproof closure it is only considered as water-resistant. 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.
The main advantage of a water-resistant bivvy is the breathable fabric used which reduces the tendency for condensation. 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. For added protection, only a small piece of tarp (e.g. 5' x 8') is good enough for use in heavy rain.
Advanced fabric technology still cannot eliminate the risk of condensation inside a bivvy sack but one day it probably will. Condensation is likely to occur when cold temperatures are mixed with a warm body that emits perspiration and that body is confined in a small space. Condensation is likely to occur inside the bivvy sack where there is a lack of ventilation or the lack of body heat. Some people even with breathable fabrics open up their bivvy sack once in a while for ventilation to prevent condensation.
Many ultralight hikers prefer water-resistant sacks because they're much lighter than stormproof versions.
The main advantage of a water-resistant bivvy is the breathable fabric used which reduces the tendency for condensation. 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. For added protection, only a small piece of tarp (e.g. 5' x 8') is good enough for use in heavy rain.
Advanced fabric technology still cannot eliminate the risk of condensation inside a bivvy sack but one day it probably will. Condensation is likely to occur when cold temperatures are mixed with a warm body that emits perspiration and that body is confined in a small space. Condensation is likely to occur inside the bivvy sack where there is a lack of ventilation or the lack of body heat. Some people even with breathable fabrics open up their bivvy sack once in a while for ventilation to prevent condensation.
Montbell Breeze Dry-Tec UL Bivy
Great for use with a bivvy shelter. The MontBell Breeze Dry-Tec UL bivy has a water resistant fabric and extremely lightweight at just 6.3 ounces.
