Epic Shelled vs. Breathable Bivy

[Anonymous]

I know there’s a lot on this question on this site, but I can’t quite put it all together in my head:

What’s the advantage to a separate bivy over a shelled bag in the following scenario:

Feathered Friends Osprey w/ Quantum shell is 24 oz, $300. Add an MLD epic bivy at 7.75 oz, $145, and you’ve got a 31.75 oz system for $445.

Or, get the Osprey shelled in Epic for $300, at 27 oz. Seems like you’re saving 5 oz and $145.

I guess you’d have to factor in a groundcloth if sleeping in a tarp, w/o bivy… And a quilt would obviously be lighter, but I thrash alot and don’t think it will work for me.

Any thoughts on this?

[Anonymous]

I’m still not 100% clear on that one either, but I was ashamed to admit it… aside from adding warmth, what’s the value of using an Epic bivy instead of an Epic sleeping bag? Seems like more layers for moisture to escape through, more money, more weight, and more complication.

No?

[Jason Shaffer]

Adding a breathable bivy will not ~reliably~ add warmth. That’s not a benefit. You should never automatically plan on being able to use, say, a lighter bag just b/c you’re using a bivy.

However, one major benefit of the bivy approach is the ability to shed wind, and errant precipitation/sprindrift/external condensation, as well as manage internal moisture, in a layer which can be separated from the insulation for drying and storage.

Take those two general (if imprecise) distinctions:
1. internal moisture, which can be dampness in clothing, humidity in the microclimate of the bag, or the vapor which rises constantly out of your body. You want this moisture to pass through the system as easily as possible.
2. external moisture (precipitation, blowing snow, dripping condensation under a tarp or in a tent, or just humidity in the ambient air). Obviously you want to keep this out to begin with. At least, if it does soak into the outermost layer (which is managable, you don’t want it soak into the insulation).

Regarding internal vapor and condensation: An epic sleeping bag shell, under the right conditions, will trap water vapor inside the down, where it can condense. This can visibly flatten a down bag over 4-8 days easy, tho less or more time is possible depending.

With a bivy and a breathable, dwr-coated bag shell, the same thing can occur, but that vapor is more likely to condense just inside the bivy, but outside the bag’s shell. In this case, the shell of the bag can usually shed that level of external moisture reliably, until morning, when the bivy is removed, shaken off and briefly air dried if possible (a bivy can dry just as quickly as a good windshirt.) Then the bivy is stored AWAY from the insulation. With an epic shell, when you stuff the bag, you force moisture in or under the shell back into the insulation.

Either approach can work. I personally have used both, depending on conditions, but for 3-season use, I prefer a Pertex Quantum bivy with a Pertex Quantum shelled bag most of the time. In conditions where external moisture clearly overwhelms the concern for internal condensation, an epic bivy would be even better. I also prefer open tarps, which in terms of external moisture makes a big difference in my decision too.

A damp bag is never the lighter bag, esp since it takes so much longer to dry (IF you can get it to dry) than a wet bivy. Also the number of layers that moisture has to escape through is not as much of an issue as the breathability of each layer, and the temperature gradients which the consecutive layers create.

Sorry, this is a quick post, necessarily simplified, but hopefully concise enough to be understandable. If this is confusing, you’d be better off considering the climate and conditions you most often hike in, as well as the length of trip, rather than considering the abstract merits of two similar approaches.

[Anonymous]

Jason, thanks for the detailed explanation. Seems like principally what you’re saying is that a) if it’s the surface that’s getting wet, it’s easier to dry the bivy than the whole sleeping bag, and that b) Epic being somewhat less breathable than Quantum has more potential to hold moisture in. Did I get it right?

I usually do 5-9 day trips, spring, summer, fall, splitting time between desert, rockies, sierras, and some pacific northwest, camping under a tarp, in the open, and occasionally in a conventional tent. Seems like maybe the PNW is the outlier there from a moisture perspective.

For the other areas, do you think shelled bag or bivy combo would be better? Of course the catch here is that I’m fairly broke and trying to buy one bag that will cover a range of conditions…

Regards.

[Jason Shaffer]

As for bivies being restrictive, yes it takes some getting used to. But I’ve come to the conclusion that its a versatile system, like a tarp compared to a tent, and should be used as such. You wouldn’t pitch a tarp the same in a storm as you would on a still, humid night. A breathable bivy is the same way. When conditions favor brathability over external protection anyway, roll the bivy down past your elbows like an elephant’s foot, and enjoy the open-ness. When wind or precip starts, roll it up and enjoy the coverage.

[Jason Shaffer]

Oops, posted right on top of you.
>Did I get it right?
That’s my experience anyhow.

I think the shelled bag is more specialized. If you’re climbing denali or fastpacking someplace wild, you could spend time hunkered down in a snowcave or out in the open in contantly fierce weather. Screwing w/ a bivy may not be an option. at altitude it might be tough to figure a bivy out! Short term, the simplicity of an epic or event shell would be good. In fact, any short term situation, 2-3 days max, the epic shel might be better, b/c the effects of poorer breathability don’t have a chance to accumulate. but for 5-9 day trips, I think a bivy is the more versatile route. Best of luck.

[Anonymous]

Is this the conventional wisdom? Be curious what a few of the other longtime posters here think…

[Jason Shaffer]

I’d like to hear others’ experiences too. I’m surprised nobody else has chimed in yet.

>I’m fairly broke and trying to buy one bag that will cover a range of conditions…

If that’s the case: I’d still go for the more breathable shell fabric (Pertex Quantum is my choice), but with an enclosed tarp instead of an open one, no bivy. It is interesting that while Ryan J. uses the open tarp + bivy combo, Glen Van Peski of Gossamer Gear skips the bivy and uses his Spinnshelter in many of the same conditions. The former is heavier (unless poncho-tarping), less prone to condensation, but more exposed; the latter is more prone to internal condensation (on the tarp walls), but less exposed. Both do work w/ some skill, and the latter is also $100-200 CHEAPER!

[jeff woods]

you guys are scaring me. i got this arc alpine with an epic shell (thinking i was so smart.) now i’m worried that it’s going to become damp all the time from my condensation. i’ve used it only on a few overnighters so far, and it’s been fine (except for the time i got into it with fairly wet clothes in january and then it got damp pretty fast.) please tell me that i have nothing to worry about.

[Jason Shaffer]

Hi Jeff,
Well, as per your January experience, cold temps will always be the worst in terms of condensation, with any fabric. Epic is noted for being tricky in cold, and very damp conditions. Thats the norm for me, so I’m just hyper-aware of this. I’m not saying epic is bad. Every top bag shop offers it for a reason. I’d only suggest being aware as to the strengths and limitations of any choice. Extra sun-dry time might be in order on occasion. (And it IS only 2.5″ of loft, not the 4+” winter bags I’ve seen w/ epic.) Other times, you might see benefits that others will miss out on. Splitting hairs is for those who have yet to drop their money. Then the real trick becomes getting out there, making it work, and thinking about something other than gear.

Besides, if your talking about the Nunatak Arc Alpinist, you’ve already got yourself one of the finest bags ever made — so get out there and enjoy your trip!

(then post back and tell us what YOU think! ; )

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