A Beginner’s Guide to Bivy Sacks
Bivy sacks were first developed as solo shelters for mountaineering, climbing, ultralight backpacking and adventure racing where participants were interested in light weight and highly compressible gear. The original bivies were little more than waterproof sleeping bag covers that protected extreme athletes from rain or provided them with a temporary emergency shelter.
However, in recent years, a new class of bivy sacks has evolved that incorporates high tech breathable fabrics like Gore-Tex and eVent with elements of ultralight tents such as mesh screening and collapsible vestibules. Initially, these more advanced bivies were manufactured by a few specialist and cottage brands such as Integral Designs, Outdoor Research and Bibler (acquired by Black Diamond), but mass market manufacturers such as Marmot, Big Agnes, Mountain Hardware, REI, The North Face and MSR have introduced models in recent years, reflecting an increased audience for this class of solo shelter.
If you're not familiar with bivy sacks, I've collected a few videos for you to watch that show how they work and how to set them up. When you watch these, note the differences in the features provided by the models, especially around the wearer's heads.
Rab Storm Bivy ($90)
This model from Rab reflects the bivy sack's original purpose which was to protect a sleeping bag from the elements. Climbers camped out on a ledge would routinely cover their sleeping bag with a bivy sack like this because they didn't have enough floor space to pitch a tent.
Outdoor Research Alpine Bivy ($219)
This video illustrates the first stage of the bivy's evolution into a more comfortable solo shelter with the addition of a mesh screen and collapsible poles to improve ventilation and keep the screen off the user's face. This bivy has a Gore-Tex top to reduce moisture built-up in your sleeping bag from sweat. This is particularly important in winter when trapped sweat can significantly reduce the thermal efficiency of your bag.
Big Agnes Three Wire Bivy ($299)
This final video illustrates the most recent evolution of bivy bag architecture by a mainstream gear manufacturer, Big Agnes, with the incorporation of an external vestibule over the wearer's head. The top of this bivy is also made with highly breathable eVent fabric which is reputed to be even more breathable than Gore-Tex.
Sleeping in a bivy sack is an acquired taste and not for everyone. Given their space constraints, there's not much to do except sleep once you've gotten into one. Still, they provide an excellent way for you to eliminate the weight and bulk of a tent, particularly in winter, if you are mountaineering or climbing, and need to carry a lot of other gear and food.
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Good article! I’ve found that a good bivy bag + lightweight tarp is a great way to go: lots of “living space” (as long as it’s not crazy-windy) but your sleeping bag stays bone dry, even with the bivy bag half-open: also avoids the claustrophobia issue and provides for a much better sleep in a downpour (BTDT: rain can be amazingly loud when it is pounding 1/4 of an inch from your ear).
Depending on the bivy and pad you use, some pads won’t fit inside of some bivys … although having a $20 foamie between your $300 bivy and sharp rocks is maybe not a bad idea. In my totally unscientific experience, having the pad inside the bivy bag seems a little warmer: any idea if this is true, or just my imagination?
I often use a bivy, but I chose the alternative to waterproof materials, which is a DWR type made of Momentum. For me a bivy is to protect my down quilt from spindrift rain or snow, usually while under a minimal sized tarp. In this case it doesn’t have to be totally waterproof, just water resistant. The Momentum is windproof, but allows considerably more breathability than an eVent type material. When rain isn’t a threat, then it makes a quick setup shelter on it’s own. A big advantage of this type of bivy is the very low weight (around 7 or 8 ounces) and a very small packed volume.
I’m not advocating that this type of bivy is better than a waterproof one, but I’ve found that if significant rain is occurring I’d rather be under a minimal tarp with this type of bivy than exposed with a waterproof one.
Photo of my MLD Soul Sidezip bivy (open), Nunatak quilt, and NeoAir pad.
[IMG]http://i444.photobucket.com/albums/qq164/Quoddy1/bivy2.jpg[/IMG]
Quoddy – I’m working on a followup post to this that explains in english the difference between the different bivy tops fabrics. Momentum and Pertex are top choices in the non waterproof category.
Tim – having a pad inside you bad would probably be warmer. Think of the bivy as a vapor barrier and you’ll see how it would retain the extra heat.