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Bivy Sack Buyer’s Guide

Mountaineering Bivy Sack

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.

ModelStyleWeightWTB Top FabricMSRP
Big Agnes Three Wire BivyBivy Shelter253 Layer Membrane329.95
Black Diamond Spotlight BivyBivy Shelter18Nanoshield220
Black Diamond Twilight BivyBivy Sack10.7Nanoshield150
Black Diamond Big Wall Hooped BivyBivy Sack29ToddTex240
Black Diamond Bipod BivyBivy Shelter27ToddTex290
MSR E-BivyBivy Sack9Silnylon100
MSR AC BivyBivy Sack16WTB200
NEMO Gogo EliteBivy Shelter19OSMO430
Outdoor Research Molecule BivyBivy Sack21.5Proprietary119
Outdoor Research Alpine BivyBivy Shelter32Gore-Tex245
Outdoor Research Advanced Bivy SackBivy Shelter39Gore-Tex320
Outdoor Research Aurora BivyBivy Sack24Gore-Tex200
Outdoor Research Helium BivyBivy Shelter18Pertex Shield+169
Rab Storm BivyBivy sack16.4Hyperlite Storm165
Rab Ridge RaiderBivy Shelter36.4Event375
Rab Ascent BiviBivy Sack22Event280
Rab Alpine BiviBivy Sack16.6Event265
Rab Survival Zone Lite BiviBivy Sack8.4Pertex Endurance150
Rab Survival Zone BiviBivy Sack15.5Hyperlite Storm120
Terra Nova Moonlite BiviBivy Sack6.7Proprietary149

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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One comment

  1. I was searching for a the perfect bivy and stumbled on your list which is one of the most complete lists on the net so much thanks! After trying lots of different brands I ended up with one not on your list but well deserving. Little to no condensation, waterproof and lots of ventilation. The Über bivy by Miles Gear, there’s one hanging in the Whitney Portal store. Best I found and I tried them all!

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