Weird Condensation Issue

Poser

Well Known Rokslider
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Dec 27, 2013
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4,511
Location
Durango CO
I’ve been using this MLD bivy sack since this summer. No issues during the summer months, but since it’s gotten cold, I’ve had a consistent problem on the left side which is the zipper side. The bottom of the sack is silnylon, the top is E-vent material.

The setup when I’m sleeping:
11f601a3829605f43a79302fe2f898bb.jpg



The right side is enclosed, the left side (the problem side) has a U zipper. I’ve been sleeping in a hot tent with a stove on my right, but if I’m asleep for 7 hours, a stove might be burning for 1 of those hours.

The left side, where I’m getting condensation, is next to the tent door (which has a good amount of frozen condensation in the morning, but no moisture on the top of my bivy sack)

44b03f5ea74d3f2591873783914d9584.jpg


I first experienced condensation on this left side when I was leaving the zipper More unzipped than pictured here. I just spent 3 nights out and zipped it up as shown here and still had condensation on this left side, but nowhere else.

Right side for reference (again, next to the stove but that doesn’t burn most of the night).

402d4cc22f5c556e4538f2ab2bfbee56.jpg



The only difference is the presence of a zipper.
Any theories or diagnosis? It’s a minor amount of condensation, an amount that will dry in the sun in under 5 minutes, but I want to understand what is happening to cause condensation in this one spot. I toss and turn constantly all night sleeping on both sides, stomach and back. I’ve been using a bag liner since it gotten cold this season as my quilt is only 20 degrees, but the liner is just a fleece sack so I don’t see how that would contribute to one specific area which is the left side about the length of the torso. Not lower and not at the very top of the quilt.
 

Centralcoastca

Well Known Rokslider
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May 3, 2020
Messages
542
Seems to me that the easiest way for the moisture from your body to escape with help from a little heat is through the zipper so that’s why it ends up there. I’ve been tempted to try a bivy sack for a quick light set up but it seems like their downfall is the build up of moisture from your body, maybe there’s a few people who have good luck with them, but I have heard too many stories from people who wake up with soggy sleeping bags when using one. If you’re already in a tent anyway, the best solution would probably be a warmer sleeping bag without the bivy
 
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Poser

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Joined
Dec 27, 2013
Messages
4,511
Location
Durango CO
Seems to me that the easiest way for the moisture from your body to escape with help from a little heat is through the zipper so that’s why it ends up there. I’ve been tempted to try a bivy sack for a quick light set up but it seems like their downfall is the build up of moisture from your body, maybe there’s a few people who have good luck with them, but I have heard too many stories from people who wake up with soggy sleeping bags when using one. If you’re already in a tent anyway, the best solution would probably be a warmer sleeping bag without the bivy

I use a bivy for a couple of reasons:

I like the versatility to forego a shelter and do so regularly.

I toss and turn relentlessly so it helps keep everything contained: I store extra clothes in the headspace, plus the cold sensitive gear im sleeping with in my foot box and also just helps keep me on my pad as I’ve been known to Roll and slide off pads during the night, even had my sleeping bag sticking outside of the shelter before.

Floorless shelters are often dirty, dusty or muddy so a bivy sack helps keep my sleep system clean.

That is an interesting point about the zipper line being a prime place for heat to escape. It can’t escape the right side so goes to the left and condenses once it hit the cold air. I suppose I could try sleeping with the flap completely open and see if that spreads out the condensation enough that it’s not noticeable. The last 3 nights got pretty cold, but not until very late. My experience with bivy sacks is that if you get enough temperature differential between inside and outside of the sack, you’ll be able to push moisture through the breathable membrane (Goretex or whatever you’re using). However, if the difference in temps is not substantial, you’ll experience condensation. In this case it was mild and relegated to one specific area, but still something to think about.
 
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