Winter walking and condensation on sleeping bag issue

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Re: Winter walking and condensation on sleeping bag issue

Postby jdeks » Thu 12 Jul, 2018 12:38 am

jobell wrote:I had a damp night last night and trialled the VBL. My bag stayed dry...but I did end up a bit damp inside the VBL in the morning. Problem was too that I felt a bit cold for a good part of the night. I didn't have much of a say in my campsite location as I was in a caravan park. Ah well, it's a learning experience. More strategies to try and better campsites to find.

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Try wearing a light set of wicking thermals when you use the VBL. They can help to transport, or at least distribute, some of the moisture inside the liner.

That way, you can progress onto the next problem of having damp sweaty thermals and being too bloddi hot ! :lol:
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Re: Winter walking and condensation on sleeping bag issue

Postby spittle » Thu 21 Jul, 2022 3:10 pm

jdeks wrote:
Well, if the skin of your tent cools enough that its inner surface is below dew point for the (comparably) warmer, humid air pocket on the inside, you get some condensation on the inside of the skin, where the air touching it cools and condenses water droplets. If your tent skin was, say, goretex, some of that water vapor can escape first, reducing inner humidity, meaning the temperature would need to drop further to make those droplets form. Or, you could ventilate, allowing dry outside air to mix and reduce the humidity (and thus dew point) inside , even though it's still just as cold.

Of course, if the air outside already has a lot of water vapor (say, near the ocean....) then this doesn't work so effectively. And if it's humid and cold enough, maybe the outer skin of your tent, the grass, and indeed everything able to radiate its heat away to the sky, will cool below the dew point, and you get droplets of (wait for it..) dew! And if all the atmosphere outside drops below dew point, you'll get water droplets all throughout the air and on everything, aka fog.

Get the picture? It all comes down to how cold a given lump of air is, and how much water vapor is in it (aka relative humidity)

Now, sleeping bags:

The air contained in your sleeping gear has a given humidity (usually higher, being close to the body). The thing is, it starts to immediately cool as it moves away from your skin. Now if it's 38C inside your sleeping bag/doona/jacket/thermal cocoon, but say, 0 C outside, and the dew point for the relatively humid air in your insulation, is say, 5C...well, you have a problem. Because somewhere in your gear, between 30 and 0C, the air cools to the point that water droplets will form (typically near the outer shell).

Apologies for reviving such an old thread but I found this to be a very good explanation of the issue of condensation and dew. I found it interesting because a lot of the suggestions online are that you should vent your tent as much as possible to avoid condensation.

I sleep quite warm so try to to be on the lower end of my (down) sleeping bag temperature rating but I do sometimes experience some condensation in the bag. Nothing major, but it prompted me to look into this a bit further.

I recently bought a new tent that has great opportunities for venting on both ends. Would the takeaways be that if the relative humidity is predicted low then you’d vent the tent completely because there won’t be any dew, and if the air is close to saturation then ideally you want to open vents just enough to let some of the warm moist air out whilst maintaining the internal tent temperature so that it’s above the dew point (favouring condensation on the inner of the fly rather than somewhere inside your sleeping bag)? And I’m guessing with the latter if the temp inside the tent is say 1degree higher than dew point then that would be optimal balance for maintaining a dry sleeping bag, and the minimal tent condensation possible?
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