Bivy sacks are an alternative lightweight shelter for mountaineering in winter and under a tarp in 3-season conditions as protection against rainsplatter. I pulled together this list of the bivy sacks so I could compare them by weight, fabric, and price.
Please also note the Fabric Guide below, where I have attempted to explain, in English, the differences between the different bivy top fabrics.
|Model||Style||Weight||WTB Top Fabric||MSRP|
|Big Agnes Three Wire Bivy||Bivy Shelter||25||3 Layer Membrane||329.95|
|Black Diamond Spotlight Bivy||Bivy Shelter||18||Nanoshield||220|
|Black Diamond Twilight Bivy||Bivy Sack||10.7||Nanoshield||150|
|Black Diamond Big Wall Hooped Bivy||Bivy Sack||29||ToddTex||240|
|Black Diamond Bipod Bivy||Bivy Shelter||27||ToddTex||290|
|MSR E-Bivy||Bivy Sack||9||Silnylon||100|
|MSR AC Bivy||Bivy Sack||16||WTB||200|
|NEMO Gogo Elite||Bivy Shelter||19||OSMO||430|
|Outdoor Research Molecule Bivy||Bivy Sack||21.5||Proprietary||119|
|Outdoor Research Alpine Bivy||Bivy Shelter||32||Gore-Tex||245|
|Outdoor Research Advanced Bivy Sack||Bivy Shelter||39||Gore-Tex||320|
|Outdoor Research Aurora Bivy||Bivy Sack||24||Gore-Tex||200|
|Outdoor Research Helium Bivy||Bivy Shelter||18||Pertex Shield+||169|
|Rab Storm Bivy||Bivy sack||16.4||Hyperlite Storm||165|
|Rab Ridge Raider||Bivy Shelter||36.4||Event||375|
|Rab Ascent Bivi||Bivy Sack||22||Event||280|
|Rab Alpine Bivi||Bivy Sack||16.6||Event||265|
|Rab Survival Zone Lite Bivi||Bivy Sack||8.4||Pertex Endurance||150|
|Rab Survival Zone Bivi||Bivy Sack||15.5||Hyperlite Storm||120|
|Terra Nova Moonlite Bivi||Bivy Sack||6.7||Proprietary||149|
Bivy Fabric Guide
One of the most important elements of a bivy is the breathability of the fabric used on it’s top half. If you’ve ever had your sleeping bag drenched by touching the condensation that accumulates on the inside of a silnylon tent, then you know exactly what I’m talking about.
Here’s a quick definition of the fabric types listed in the table above:
Gore-Tex is a waterproof breathable membrane that has pores in it. Each pore is about 1/20,000 the size of a water droplet, making it impenetrable to liquid water, but still large enough for water molecules to pass through. The breathability of Gore-Tex and other proprietary laminates like it (Conduit SL, HyVent DT, REI Elements, MemBrane Strata, Watergate, and DriZone), depends a lot on the fabrics that it is sandwiched between. A lot of the proprietary laminates have 2 or 2.5 layers of fabric making them potentially more breathable than 3 layer Gore-Tex.
ToddTex is a proprietary membrane like Gore-Tex that was originally developed by Todd Bibler of Bibler tents. ToddTex is also treated with a fire retardent. Black Diamond acquired Bibler and uses ToddTex for its tents and shelters.
Sympatex is a waterproof, windproof, and breathable membrane, but it does not have pores in it like Gore-Tex that let water molecules pass from one side to another. Instead, water molecules are absorbed by the membrane and evaporate on the outside. Reflexion is a variant of Sympatex that is coated on one side with a reflective coating that traps body heat and reflects it back, like an emergency blanket.
Silnylon is silicone impregrated nylon. It is waterproof but it does not breathe. Epic is silicone impregnated polyester which is waterproof and breathable.
Momentum is a breathable, wind and water resistant fabric, but not waterproof. Pertex is also wind resistant and has better breathability than many waterproof laminates. Pertex is made with two fibers of different filament thicknesses. Moisture is absorbed by the larger filaments on the inside of the fabric and is transferred to smaller ones on the outside, moving moisture by capillary action.
eVent is the current top dog in breathable waterproof membranes. Like Gore-Tex, it vents moisture through millions of tiny pores. It is different from Gore-Tex in that it does not require an inner coating, providing a more direct path for moisture to evaporate.
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