Condensation in bivy?

If my sleeping bag is seriously overrated for the ambient temperature, I just lay down on my sleeping pad and lay the bag on top of me.

It sounds like your bivy sack is doing a better job of a vapor barrier than as a rain protector.

–B.G.–

Andrew, you are working in the worst possible conditions for condensation in any shelter or bivy. Partly it is just the temps and the humidity, partly too warm a bag so the dew point is at least between the bag and the top of the bivy.

Here is my in depth piece on bivy condensation

https://40yearsofwalking.wordpress.com/2011/07/04/the-bivy-condensation-conundrum/

Try sleeping in the bivy, but only half in the bag, stick body parts out to thermoregulate. In the end, if you stayed warm, you did okay. Sometimes we try to replicate living in a greenhouse when we really can't control everything as we can inside.

Wool works best for me in those conditions, though it doesn't dry super well.

I like to up my energy output in that kind of weather and hike very fast or run, wear 200g/m2 wool top and bottom, and a wind shell on top or bottom if I get cold or slow down. You'll get damp, but you'll be warm, and your body heat and breathable clothes will be constantly pushing the water out. Then throw on a puffy and/or wpb shell when stopped. The less clothes you can wear, the better, because they stay dry in your pack, which means moving faster is the best option.

At night, you can sleep in your damp clothes in a bag built for colder temps, though maybe not a 5 deg bag, that is a lot of bag, and a bivy doesn't work well for this, because the vapor that the bag moves out will get trapped inside the bivy. Or you can dry them with a fire or hot tent set-up, though starting and maintaining a fire can be tricky in those conditions.

Perhaps a tarp or a similar set-up with good air-flow, no bivy, and the wet wool inside a conservatively warm bag for the temps, like a 20deg bag maybe, would do well, or you could try a 40deg down bag with a summer synthetic overbag, and the vapor coming off you would land in the synthetic.

Alex, great article.

I've been thinking about using something like Argon 67 with no DWR or a lighter DWR, because it would be very breathable, moreso than M90 I think, and then just have good coverage on a tarp if needed for moisture from outside.

Also I've been wondering if you could get away with just using an argon 67 top, perhaps attached to a pad or ground cloth at a few points, and no waterproof bottom, so there is extra moisture moving out, but you still get wind coverage and can use a lighter bag. The hot water bottle is a great idea too.

Thanks for the replies. I think part of my problem staying warm was a calorie deficit, I packed way too little food for the amount of postholing I ended up having to do and by the end of the week I felt really drained, and noticed it when I got wet. I was wearing 200wt wool top long sleeve and nylon pants, didn't bother with rain pants. My wpb shell was just gone, completely wetted out and useless so I might as well have switched to my houdini just to block the wind. I think I'll need to add another light baselayer to sleep in, maybe something capilene, that will stay dry in my pack, and just up the calories and hike in wool top and bottom with wind layers and just stay wet but move fast. Problem is I have to stop and clear blowdowns, etc, as a ridgerunner.

Also, tarping is not an option in the smokies as I have to stay in the shelters per park regulations.

Next week I think I'm going to try my 20 degree quilt instead of my 5 degree bag, lows are probably going to be about the same, 30ish, so I should be better off. Dropped a bunch of cash on the WM antelope and wanted to use it, but the conditions just don't warrant it. Although tonight the lows are probably in the single digits and I'm off the trail, so it's good to have just in case.

Alex,

That article is excellent and basically answers all my questions. Thanks!

Alex,

As was mentioned before, comfortably sleeping cold/moist air is difficult. What has worked for me (as weird as it may sound) is to use a silk liner as my "bivy", and bring my sleeping bag inside if it. I also ALWAYS use a set of sleeping clothes, something thin, long sleeved and designed for wicking. Even in hot/humid weather, I sleep this way. I just don't sleep under insulation.

From my observation, the silk liner acts as a "film" which can offset the temp delta between the ambient air I'm sleeping in, and the surface of my sleeping bag. Seems that just that few degrees can help a lot. And if it gets hit by a drop or two of water on the outside of it, can easily be dried out. In very cold weather camping, I'll use a 2nd "outer" bag for a similar purpose. (There are some good discussions on this list about it.)

Now I have only used this method when it's cool/humid, and always under shelter (usually a Gatewood cape), but it works well toward "protecting" my bag, just not from bulk water, but from debris.

Hope this helps,

Matt

“I’ve been thinking about using something like Argon 67 with no DWR or a lighter DWR, because it would be very breathable, moreso than M90 I think, and then just have good coverage on a tarp if needed for moisture from outside.”

This is a pretty good thread on the breathability of the various fabrics, looks like Argon is one of the best, most breathable.

Matt —

I like that idea about the silk liner on the *outside.* I had been thinking along similar lines with some sort of micro-fleece MYOG bivy/quilt-topper project, but the micro fleece weight seem prohibitive.

How much does a roomy-enough silk liner weight?

Would it be large enough for an inflatable pad too? (otherwise it might interfere w/attachments straps, which I find useful)

In my experience, no Goretex bivy condensed like shown in the pictures. So I gave little attention to the matter. As an long-time bivy user, I can say back in the day, water did not work like that…

But more seriously, the question has to be "What changed?" Back in the day, bivies were not black and I had little condensation. Now most of the very light bivies are black and black radiates well to a clear sky. Perhaps the shell infrared-induced temperatures are significantly lower on the outside surfaxe than the inside surface.

At -5F, under a clear sky, no shelter, on snow, my yellow Goretex bivy did not show visible signs of condensation — although the condensation could have been within the down. I've had frost on the outside of that bivy on other trips with no sign of condensation on the inside.

White bivy experiments anyone?