Mar 29, 2009 at 1:56 pm #1235165
I realize that the issues of bivy use and moisture management are addressed in many articles and threads on this site. For such an apparently simple concept- sleeping in a bag- effective bivy use gets pretty darn complicated, taking into account factors such as bivy material, exposure to night sky radiation, wind conditions, humidity, temperature, insulation amount and type, variations in individual physiology, etc. A post elsewhere on BPL describes moisture management in a bivy as a bit of a "black art," and this is not because when you are in a bivy things are generally pitch black :). Of all the variables beyond one's control in the wilderness, I would like to get other's opinions on something we can control, specifically the optimal amount of insulation for staying warm and dry overnight.
I own an MLD Soul Bivy,
and despite the fact that I use it predominantly in dry desert conditions with low humidity and little precipitation, I still experience some condensation in the bag from time to time. For example, on a recent trip in southern Utah, overnight temperatures were just below freezing, and I was sleeping under the stars in the bivy with a MH Phantom bag rated to 32 F. Sleeping on the cool side, and pushing the comfort range of the bag a bit, I woke up in the middle of the night with some condensation on the inside surface of the bivy and a chill. I had thoroughly dried the bag out the previous morning, so moisture collecting in the bag over multiple days was not an issue. I threw on my MH Phantom down jacket, and went back to sleep. By morning, I was warm again, and the condensation was gone.
Perhaps this is just a story with a happy ending- ultimately I added some insulation, went back to sleep, and awoke warm and dry- but I am still not quite sure how best to regulate insulation in a bivy. One theory goes that if you sleep too cool, you will (of course) be cold, and there will also be insufficient heat inside the bag to drive moisture out, causing condensation. The other theory is that if you sleep too hot, you will be adding too much moisture to the system through sweating and overwhelm the breathability of the system, causing condensation.
So, is the trick simply to find a happy medium between these two extremes? Is it possible that I would have more success with a different brand of down sleeping bag? Any tricks out there for staying in the bivy sweet spot without waking up in some state of discomfort? Is some of this finickyness just the price of business with any bivy, even a very breathable one like the Soul Bivy?Mar 29, 2009 at 4:17 pm #1489626
@owareLocale: Steptoe Butte
Sleep in a quilt or sleeping bag unzipped so you can easily
vent in the night and prevent overheating.
In cold, damp conditions, use a hot water bottle to
help drive moisture out.
Finally, just deal with it. Condensation is like
mosquitoes. The more you let it bother you the more it
will. Unless you are Scott on the way back from the
South pole, a little condensation is only a nuisance.
If you are not in a bivy, then the condensation is
forming inside your sleeping bag, you just don't see it.
It is easier to dry a bivy than a whole sleeping bag.Mar 29, 2009 at 10:05 pm #1489701
@ngatelLocale: Southern California
Not so much a dark science, you just have to learn from your mistakes :)
Good advice from Dave.
Another thing, when it gets cold many people tend to stick their head into the bag and breath into it. That does help warm you up at first, and then dumps a lot of moisture into the bag. For me the best way is to get the correct mix of clothes in conjuction with conditions. Too few clothes and it is cold, too many clothes I get hot and sweat. The unmeasureable is the difference in physiology between individuals.Mar 30, 2009 at 12:05 pm #1489801
Nick and David,
Thank you for the advice. I will continue to tweak my system and seek that perfect mean.
JamesMar 30, 2009 at 1:03 pm #1489818
@rezniemLocale: San Francisco
I had a problem with a DWR nylon top bivy where the dew accumulated before I went to bed and soaked through the bivy into my bag. I'm not sure if a tarp would have helped but it seems the nylon is not good at resisting small amounts of water.Mar 30, 2009 at 2:06 pm #1489832
@robleeLocale: Southern High Plains
Under the conditions you described you might consider the use of a vapor barrier (liner or clothing). There are some good threads.Mar 30, 2009 at 2:08 pm #1489833
@owareLocale: Steptoe Butte
Water vapor can move in or out depending on temperature
gradients.Mar 30, 2009 at 4:44 pm #1489882
Nathan- is it possible that the DWR on the bivy was compromised? In my experience, most wp/b bivy fabrics with DWR should be able to shed dew and keep it from getting into the bivy- in the situation I described above, I'm fairly certain the condensation was from inside the bivy. However, I do think a tarp may have helped in a few ways. First, by blocking infared radiation from the night sky that can lower the temperature of fabrics well below the ambient air temperature, thus creating a cool surface for condensation. Second, by potentially creating more of a layer of protected, warmer air around the bivy surface. There are, however, conditions where a tarp could also inhibit much needed airflow.
Rob- I've had good luck with vapor barrier liners at lower temperatures, but don't yet know the upper limits of my comfort range- might be something to explore further.
David's point is a good one- temperature gradients determine where and if moisture condenses in your sleep system. If the presence of a bivy fabric moves this point from inside your sleeping bag to the inside surface of the bivy, then it has done its job, and most SB fabrics will keep this out of the down. I think this is where the challenge of balancing your sleep temperature comes into play. If you intentionally sleep cold to avoid perspiration, you may be moving the point of condensation deeper inside your sleep system. Too hot, however, and you may be producing more moisture than it can expel.
The merit of a hot water bottle is that it produces dry heat, unlike the human body.
JamesApr 3, 2009 at 1:02 pm #1491024
@trebiskyLocale: Southern Arizona
On a cold clear desert night, IR coupling to the night sky could definitely bring the temperature of the bivy material down to below freezing, even if the air temperature was not this cold, and then inside condensation could most definitely take place. As you point out, being under a tarp would break this link and this is probably the best solution, even on a clear and windless night. To may way of thinking though, I would just put up with a bit of frost inside and give the bivy and bag a few minutes in the morning sun to dry in lieu of fussing with a tarp.Apr 4, 2009 at 12:22 pm #1491234
Great point- in desert conditions, rare is the time when you cannot quickly dry out in the morning, and so the tarp still may be an unnecessary hassle- especially when in conditions where easy staking is not available- slickrock, sand washes, etc. A bivy is perfect for desert conditions when precipitation is not a big worry, as it lets you tuck in to sleep almost anywhere with a minimum of fuss and impact.
Funny, I camped a few nights ago under a tarp at about 9700' at Lost Lake near Nederland, CO. Temps were in the mid 20s F, deep snow, and conditions shifting from partly clear skies to falling snow by morning. In the mld bivy w/ an REI sub kilo 20 F down bag, had very little if any condensation or frost in the bivy.
I hiked up by moonlight and headlamp, and boy did it take a while to not get spooked every time the lake ice creaked and moaned!
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