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?