Winter walking and condensation on sleeping bag issue

[jdeks]

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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Jo,

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 !

[spittle]

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?

[jobell]

I’m going to revive this thread once more with some new thoughts. Those thoughts involve discussion of menopausal symptoms so if that makes you uncomfortable look away now! To be honest I don’t think it’s talked about enough and that doesn’t help women so I am happy to bare all... so to speak. Plus, I figure it might help someone else work out WT *$&# is going on for them lol.

So... new theory on what went wrong for me on the Heysen. It was 2018 when I had these issues with the damp sleeping bag... and it turns out, in hindsight, that that was when my monthly cycle started changing aka I first had symptoms of perimenopause.

It’s taken me ages to put this theory together for myself.. as in, until today. But basically what I am now theorising is that I started having night sweats in that time. So I was still a cold sleeper, all rugged up to stay nice and cosy as I had previously done with no issues, but then, sometime in the night, I got hot and sweated. That sweat wetted my sleeping bag and all layers especially around my torso. When it got wet it got cold and I woke up. Voila! It wasn’t the conditions or my gear that had changed... it was me...

Unfortunately with the wet weather on the Heysen I had no real chance of drying my gear properly during the day so with an increasingly damp sleeping bag and no idea on why it was happening I pulled the pin on that walk - hoping to finish it next year btw.

So. Back to my previous line of action - it’s a VBL for me all the way, with wicking thermals on my body inside the VBL, and if I need extra layers I shall put them in my sleeping bag but outside the VBL. Watch this space... I shall report back!

I did some walking last year but it seemed that my night sweats subside when I am less stressed and out bushwalking so I didn’t have a conclusive result. I’m walking the GPT this May... and then the Larapinta in June. I reckon after those I will have a pretty firm idea.


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[andrewa]

Wicking thermals inside a VB seem a reasonable option, and should dry out pretty quickly if you continue to wear them once you’re out of bed.

Interesting comment about hot sweats and stress - I’ve not come across that in discussion with my patients, but, who knows? Maybe that’s why the antidepressant SNRI venlafaxine works well to reduce menopausal hot sweats??

What about a VB that opens from neck to waist, for times when you really need to allow some evaporation off? Haven’t used a VB in Oz, though I’ve thought a lot about them - more specifically about making a cuben fibre quilt, with cuben on both the inner and outer, and a strip of breathable fabric somewhere (? the sides) to allow it to be compressed into a stuff sack.

I sleep in all my clothes, inc down pants and jacket in the snow, with a lightish quilt (700g total weight) on top. Allows me to vent when too warm, and I don’t have to get dressed to go to the loo in the middle of the night.

The simplistic reason why a good quality sleeping bag gets damp on the outside is that, if it’s insulating you properly, water vapor is travelling at snails pace by the time it gets to the outside of the sleeping bag/quilt, and there is no temperature differential to assist the vapor getting through the outer fabric,which is why goretex sleeping bag covers don’t work. Then add in meteorological reasons beyond that!

Andrew A