Winter walking and condensation on sleeping bag issue

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Winter walking and condensation on sleeping bag issue

Postby jobell » Fri 22 Jun, 2018 9:12 pm

Hey all,

I'm just a few weeks back from the better part of a five week walk in South Australia on Kangaroo Island and on the Heysen Trail. My intention was to end to end the Heysen Trail in one hit, but the conditions got the better of me and I pulled the pin after about 400kms of walking. I have encountered minor condensation in my previous walks but never anything like I experienced on this walk and I have a few questions/issues I am trying to sort out before I head back out there for another go. I'm hoping the brains trust that is the bushwalkers of Bushwalk Australia might have some suggestions for me.

I currently use a Tarptent Double Rainbow. Condensation wise it's never been a drama and still isn't. I had my first experience of misting on this walk and it took me a moment in the middle of the night to think of Franco's recommendation and start wiping down the interior. That did the trick and apart from not noticing I was squeezing the excess water out of the chux cloth straight into my hiking shoes all was good. But what was a drama was my very damp aka wet sleeping bag, night after night. None of this wetness on my sleeping bag was transferred from the tent as far as I know - I have the Double Rainbow to myself so there's plenty of room to stay away from the walls and apart from that one incidence of misting there was no water falling from the tent roof. I will add I do pitch my tent for maximum ventilation and use the features of the tent (ie rain porch) as much as I can to enhance ventilation as well. If I'm not using the rain porch I will sleep with the fly doors open most nights if weather permits. I'm also usually quite picky about location of campsites but there wasn't the freedom of choice on the Heysen that I'm used to to. There also wasn't much wind or breeze most nights which didn't help.

Anyway on more nights than not I would wake in the morning with moisture on and soaking into my sleeping bag. I've encountered this a little in the past but never to this degree where my bag was actually losing loft and thus warmth as well in the night due to the dampness. Most of the wetness was on my upper chest (I'm assuming it came from the moisture from my breath) but the rest of it was on my leg area with a little moisture on my middle body area as well. At the time I assumed it was the contact of the cold moist night air with the warm air rising through my sleeping bag. I also assumed this is what is referred to as the "dew point". Feel free to set me straight if I"m wrong in these assumptions. I have researched it but am still struggling to get my head around the concepts.

I don't think my wet sleeping bag is anything to do with sweating; on the whole I have to layer like mad to stay warm in bags that would make most people cook. I'm a hopelessly cold sleeper. To illustrate - on the AAWT in Nov/Dec a few years back I used a Mont Franklin with a minimum COMFORT temperature of -12'C (850gram fill of 700+ loft goose down), and that was just right with thermals top and bottom, merino socks, beanie, down jacket and if it approached anything close to/below 0'C I just added my rain jacket and pants which act as like a VPL and was nicely cosy. This walk I took a hoodless sleeping bag that Undercling Mike customed for me which has 600g of 950 fill HyperDRY goose down and taller baffles to match and providing I layered to a similar degree as in the past and used the separate hood combined with a beanie I was fine warmth wise down to as low as 0'C.

But I was starting to feel the cold in the early morning hours as my bag got more damp/wet and lost loft and it was getting more and more difficult to find the time in the short winter days as well as suitable weather (sun and wind) to dry my bag properly every day. Add to that an ear infection that dogged me for the WHOLE five weeks and wasn't getting better and I decided I had had enough of winter walking in South Australia.... for the moment anyway.

So after that long background story (sorry!) what I'm wondering is, in similar damp air/humid conditions:

- Would using something like a very light bivy such as the Montbell Breeze Dry Tec Ul sleeping bag cover help to move the dew point (if that's what it is) out from the surface of my sleeping bag? Ie - if that moisture is coming out of the air and not from my body would it settle on the outside of the bivy bag? Has anyone used a bivy or something similar for this purpose with any success?

- Or... given my Double Rainbow has seen a lot of use and is starting to show some wear and tear what if I replaced it with something like the Tarptent Notch with the semi solid interior - assuming if I used the semi solid interior that might help to raise the overall temperature inside the tent and thus hopefully help to move the dew point further out again perhaps as far as the tent's fly? I don't care about the tent being wet. I do care about keeping my sleeping bag as dry as possible.

I'm keen to get out and do more winter walking but I have to work out how to make it work for me first.

TIA...
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Re: Winter walking and condensation on sleeping bag issue

Postby andrewa » Fri 22 Jun, 2018 11:08 pm

I've found that condensation is a real PITA to deal with...

You need reasonable airflow through tent for a start.

Beyong that it's a case of applying a water resistant finish to you sleeping bag/quilt, and beyond that it's a case of just accept and move on!

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Re: Winter walking and condensation on sleeping bag issue

Postby Franco » Sat 23 Jun, 2018 10:35 am

"None of this wetness on my sleeping bag was transferred from the tent as far as I know" -
One way to have a better idea about that is for you to test this at home.
It's winter so if you do have a non heated room or better still a covered area outside (say a car port) sleep out one night with that mat/sleeping bag and clothing and see if you do get the SB damp or not.
My bet is that you are getting into the bag cold* , so you need a lot of clothing to warm up, then start sweating in your sleep and that wakes you up when the by now damp bag feels cold.
I don't know for sure , but I have seen that happening .
*Hanging around after dinner outside or simply not moving inside the tent will do that. A brisk walk around camp will help.
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Re: Winter walking and condensation on sleeping bag issue

Postby Moondog55 » Sat 23 Jun, 2018 10:58 am

I haven't had a problem with condensation on my sleeping bag forever
BUT
Long W/E at Mt Franklin we had condensation like you were living under water and I had to get my Goretex bivvy bag and use it inside the big tent.
That W/E nothing I did seemed to ameliorate the problem and I do not not exactly what caused it but exacerbated by a non-breathable tent, constant drizzle and saturated wet ground
Vapour resistance of the groundsheet is the culprit I think, a material can be water resistant to a high degree but still allow water vapour to move through it, especially when pressurised by the weight of feet and furniture
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Re: Winter walking and condensation on sleeping bag issue

Postby jdeks » Sat 23 Jun, 2018 11:18 am

Hmm, lots of potshot guesses going on here...often seems to be the case on this topic.

First off, lets lay out some of the physics here. Bear with me - knowing the mechanisms at play is important in understanding whats going down with your bag (see what I did there?)

Dewpoint isn't a physical location. It's a temperature.

All natural air has water vapor in it. This isn't visible steam/mist, but rather lone H20 molecules sneaking invisibly between the rest of the air. The warmer the air, the more molecules can squeeze in. This is that 'humidity %' thing the weather man talks about. 80% humidity means the air has 80% of the max amount of water vapor it can invisibly hold, for that given temperature.

So what happens when it gets to 100%? Well, notionally, H20 molecules bump into each other so much they collect into suspended droplets of liquid, aka mist/steam. For boring physics reasons, this actually heats the air up again, and the droplets immediately evaporate. This is why mist doesn't just randomly appear on its own whenever water is around. Instead, it appears when temperature drops. Cooling the air robs away this heat energy as well as reducing its capacity to hold water vapor, effectively increasing that "relative" % humidity despite no H20 being added. Do it enough and water droplets condense out and stay there. Critically, the higher the starting humidity, the less you need to cool it for droplets to condense out.

This action presents in heaps of different ways, prime example being when hot water vapor shoots out a boiling kettle (or even your breath) and hits cold morning air, you get a plume of mixed, still humid, but much cooler steam. All are cases of comparatively 'warm' air, being cooled to the point that the air can't hold the vapor it has (saturation), and it forms droplets.

The temperature at which this occurs? That is the Dew point. But what does this have to do with your bag and tent?

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).

This is what I think is happening to you.

This phenomenon used to slowly freeze early antarctic explorers' sleeping bags solid over the weeks. It happens to me a lot, when I hike in the snow. I get it because the temp drops low enough outside, for the bag's own temperature to drop below dew point even in 'dryer' air. You're getting it because the humidity/dew point where you hiked is already quite high. Same thing though. Looking at BoM observations for Kangaroo Island, I'm seeing a lot of high relative humidity conditions with ambient overnight temperatures approaching or dipping below the dew point. Meaning the air you're sleeping in, thats filling your sleeping bag, is already close to saturation, and the heat from your body isn't enough to counteract overnight temperature drop with the extra humidity you're producing. You've essentially got fog forming inside your down.

Fixing this isn't simple. But some of these might help:

-Dont use a bivvy bag. It might keep the odd tent wall drip off, but the big issue here likely isn't water from the outside getting on your bag, its from the inside. Even breathable shells will just impede vapor escape even more. The skin of the bivvy will reach dew point just the same, and you'll get water droplets all over the inside, leaving you with wet bivvy and bag.

-Similar for water repellent treatment.

-Use a THINNER bag. Similarly, wear less layers inside the bag, it sounds like you're not layering effectively anyway. Yes, I know, it sounds counter intuitive, using less to stay warmer. But the thicker the bag, the more likely the dew point is reached inside the insulation. A thinner bag hopefully means all the insulation stays warm enough that the water vapor escaping doesn't cool mid-down, and the bag stays dry. It's balancing act but you should be able to tune it, especially if your'e in fairly warm coast conditions.

-Close the tent doors and vents. Again, I know, counter intuitive. Idea here is if the ambient humidity is already high, you can't reduce it with venting anyway, so instead, trap heat in. If you're lucky and you can boost the ambient temperature above dew point, the problem goes away. You will get EPIC wall condensation where the tent skin is cold though, so just gonna have to wake up now and then to mop it down (or us a fine bug mes inner to divert drops from your bag)

-Vapor Barrier liner would solve the problem entirely, at the cost of being probably sweaty and sticky and uncomfortable the whole night due to the high humidity in the bag.

- Go synthetic - down struggles in humidity, synthetic stuff really shines in these conditions. This is the simplest solution.

Hopefully everyone's still awake?
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Re: Winter walking and condensation on sleeping bag issue

Postby jdeks » Sat 23 Jun, 2018 11:30 am

Moondog55 wrote:I haven't had a problem with condensation on my sleeping bag forever
BUT
Long W/E at Mt Franklin we had condensation like you were living under water and I had to get my Goretex bivvy bag and use it inside the big tent.
That W/E nothing I did seemed to ameliorate the problem and I do not not exactly what caused it but exacerbated by a non-breathable tent, constant drizzle and saturated wet ground
Vapour resistance of the groundsheet is the culprit I think, a material can be water resistant to a high degree but still allow water vapour to move through it, especially when pressurised by the weight of feet and furniture


Water under the groundsheet didn't play into it. Unless you're using goretex as a floor, your typical groundsheet is a vapor barrier. Atmospheric conditions dominate.

Constant drizzle often means you had a very high ambient humidity, especially at altitude. Air was likely close to dew point before you even pitched the tent. Once you go in it, you warmed the air inside up a bit, but equally pumped even more water vapor into it as well. The moment it comes in contact with the tent skin and cools back to the colder outside air temp (likely already around dew point to begin with), it's already supersaturated. A goretex tent wouldn't have even saved you.

Dehumidifier or woodfire campstove might have.
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Re: Winter walking and condensation on sleeping bag issue

Postby Moondog55 » Sat 23 Jun, 2018 11:42 am

jdeks wrote:Dewpoint isn't a physical location. It's a temperature.

True but I prefer to think about it as both, because when it gets really cold it helps you to know when adding an extra layer helps or when to use a VB liner.
Personal experience says to use a bivvy bag to protect the down and vent, that is I personally sleep with the bivvy bag as open as conditions will allow.
But even if I was using one of my synthetic sleeping systems it would include my bivvy bag if I thought things could get gnarly. YMMYV naturally
Water vapour can and does move through most groundsheet materials. Polyurethane isn't a good VB material and neither is silnylon. PVC is but it is heavy
Ever used a CCF pad on a tent floor and woken up with water under the mat even tho it was dry when put down? That is vapour cming though the tent floor and why the very best tents in the days before UL came with PVC floors, we thought the extra weight had distinct advantages; it's one of the reasons I continue to use by old Fairydown stuff
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Re: Winter walking and condensation on sleeping bag issue

Postby crollsurf » Sat 23 Jun, 2018 1:05 pm

Just an observation from ski-touring back in the day. I had, still have, an old Dacron Hollofil bag and I would sleep in, with wet gear and skiboot inners inside the bag. The gear would be pretty much dry in the morning and the bag would have droplets on the outside which I would brush off. The bag otherwise felt dry. I sleep warm.

Maybe synthetic bags work better at wicking the moisture to the outside of the bag in these conditions. Although I believe a lot has been done over the years, to make Down less water absorbent. The down side (no pun intended) is that synthetic is heavier.

Just as a side note, I used to always get a laugh watching mates trying to get into their frozen ski boots in the morning, because they didn't stick their inners in their sleeping bag at night.
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Re: Winter walking and condensation on sleeping bag issue

Postby Moondog55 » Sat 23 Jun, 2018 2:29 pm

crollsurf wrote:Just as a side note, I used to always get a laugh watching mates trying to get into their frozen ski boots in the morning, because they didn't stick their inners in their sleeping bag at night.


Well a lot of modern sleeping bags simply do not have sufficient room inside to do that [ in fact most don't have enough room for feet bigger than average which is size 9 from memory] more than a few times I have had to thaw my old leather Scarp boots over the Primus or SVEA in the AM
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Re: Winter walking and condensation on sleeping bag issue

Postby Lamont » Sat 23 Jun, 2018 2:55 pm

Perhaps the solid inner would help and I might conjecture a smaller tent. Especially in the conditions you describe. If the temp is lower than what you describe this might not be relevant. This is what I can relate from my use of my one person tent in the conditions you describe from about -2C and upwards- non snow.
The mesh (I also have a mesh inner on a two person tent which I have used down to about 4C) can still be a bugger because it will still provide resistance to air movement, so on a night with very little air movement outside, will likely be nil inside the tent "capsule" thus the moist air falls on you even more easily than normal. Even if there was some wind outside but it was slight, it wouldn't/mightn't reach you. I have a solid inner in my Revolution Big Sky 1 person (pics on their website) and yes the temp is higher inside. I really don't know how much (2-4C?) but it is clearly noticeable. In recent rainstorms and other cold weather the fly was wet but only under the porch area, above the ground, where the inner "capsule" did not reach and my quilt remained dry. Using a Goondie 2 person in NZ a couple of years ago near Aoraki Mt Cook it was so cold both outside and inside (bigger tent -I could not feel a difference in inner and outer temp.-i.e one person in a 2 person tent-nothing wrong with the Goondie!) my sleeping bag got wet and it did it several times in coldish non windy conditions, but it was negligible. Last year in the same conditions same place, frost on the outside of the tent but in the One person double wall -inside dry as a bone. The little capsule was more easily "warmed" and was warmer. I feel certain this has a bearing on condensation given my repeated experience. I always have both top vents open on the fly and they are directly opposite each other and not far apart and the door windows open, but as someone said, close them up, and I do (but never completely) the colder it gets. It has worked so I am happy. All conditions down to the -2 (not had anything less than that yet) rain, hail or shine it has been brilliant. I would love to try (and own -ha ha) the Notch as well and I would like to know if the solid inner which does not come up as high as My Big Sky is better/worse or the same. I am looking at the Notch now myself for a meshy milder weather tent myself. i.e 3-5 degrees and up. I did use the Rev 1 atop Baw Baw one warmish night around 12C down to about 10C and I slept ON my quilt until about 3 am I was that warm in there.Good luck with your choices.
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Re: Winter walking and condensation on sleeping bag issue

Postby wildwanderer » Sat 23 Jun, 2018 7:20 pm

As others have said. I would investigate the following.

- your cold when you hop into your sleeping bag and then at some point in the night (maybe your food digestion starts later or something) your body warms up. and due to the warm bag/clothing layers and dew point you start to sweat. This used to happen to me. Id go to bed and feel cold, put warm layers on and then wake up at about 4am and the bag was soaked in sweat especially the footbox. Not sure what solved it, it may have been I was younger and my metabolism was higher so I sweated more at night. I also now use a bag that has a water resistant/vapor barrier exterior. Its an old bag made for winter above the tree line with the exterior fabric made of gore dry loft. Which is a very thin goretex.

Currently I can sleep in a moisture laden area and wake up in the morning with tent walls dripping but im sweat free and sleeping bag dry. So either I stopped sweating at night due to changed metabolism/other body factors or the goredry loft fabric did the trick.
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Re: Winter walking and condensation on sleeping bag issue

Postby jobell » Sat 23 Jun, 2018 10:02 pm

Wow lots to work with! Thanks for all ypur thoughts. I am going to try to reply to each.

Franco wrote:My bet is that you are getting into the bag cold* , so you need a lot of clothing to warm up, then start sweating in your sleep and that wakes you up when the by now damp bag feels cold.


Franco - I actually make myself delayer when I first go to bed so I definitely do get in a bit cold but without my extra layers. I warm up enough in my bag usually before I go to sleep (except my feet but that's another story...). But often I wake up in the night with this whole body shiver thing happening and then add layers to counter it. On odd occasions I have overcompensated and been aware of myself sweating, but it's rare. More often I am still aware of being only just warm enough....

I am confident the moisture isn't a result of a direct transfer from my tent. I have slept in it very wet in and out on multiple occasions yet previously remained nicely dry! Happy to vouch for my Double Rainbow.



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Re: Winter walking and condensation on sleeping bag issue

Postby jobell » Sat 23 Jun, 2018 10:10 pm

Moondog55 wrote:Long W/E at Mt Franklin we had condensation like you were living under water


Lol moondog. I had visions of you in scuba gear in yout tent reading this. Interesting though about the moisture from the ground. I did read somewhere a recommendation that a groundsheet should extend out under the fly through the vestibule to help prevent moisture rising up. I use tyvek under the inner body only and have noticed a lot more moisture under the fly where it isnt present. I think ground moisture is a factor in all this.



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Re: Winter walking and condensation on sleeping bag issue

Postby jobell » Sat 23 Jun, 2018 10:31 pm

Jdeks - yes still awake! But I had to read your reply twice. Took me a while to take it in. What helps it make sense to me is that these nights were more often foggy than not so that water vapour was well and truly visible! Your suggestion to try closing up the tent might tie in with Lamont's thoughts on using a smaller tent that might hold a bit more heat. I might even get brave and try a lighter layer in similar damp conditions. To be fair most of the foggy nights weren't the coldest nights - likely between 5 and 10'C. I have, like many here, a bit of a collection of sleeping bags....in fact, in winter I often sleep in one at home much to my other halfs amusement. There's something about being able to roll over in bed at night and never have a cover lift to allow cold air down my back that appeals. Not to mention sleeping bags are my happy place.

A smaller closed up tent might take a little adjusting to after my Double Rainbow palace though. On the upside it has occurred to me that downsizing my tent might allow for downsizing my pack in which case I might not need quite so much room in my tent anyway.

Synthetic sleeping bags are beyond my carrying capabilities these days I think. The older I get the less my body is happy to carry.

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Re: Winter walking and condensation on sleeping bag issue

Postby jobell » Sat 23 Jun, 2018 10:49 pm

I am currently staying on the NSW South Coast and am eyeing off my old bushwalking stomping grounds in the Budawangs so will put some of these ideas to test when I get off my bum and get up there. I'll think of you lot as I do laps around my tent before bed and let you know if things improve. I think just not being in the saturated winter night air of tge Mount Lofty Ranges in SA will help the situation immensely....

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Re: Winter walking and condensation on sleeping bag issue

Postby Neo » Sun 24 Jun, 2018 9:32 am

Some say to wear less and fill the voids around you with the extra layers. This means less air space inside the sleeping bag to be warmed up / kept warm.

A full bladder can make it both hot and clammy or, the body gets cold elsewhere keeping the bladder warm...


Jumping on your thread, in Nov/Dec I used a synthetic half bag inside a bivy bag and got water in the bivy around my feet. It is a Haglofs and has a PU base with a polycotton top fabric. Was thinking of adding a zip at the foot end for venting. Already added one at the top to make getting in it easier!

On nights around zero degrees, if I had my down STS bag inside this bivy bag, would it just sweat and make the sleeping bag wet? Am thinking either the bivy as a cover will block any draft, or the polycotton won't breath very well and just make things worse..?
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Re: Winter walking and condensation on sleeping bag issue

Postby legend » Sun 24 Jun, 2018 10:47 am

Your body is constantly expiring water vapour during the night. When you have a temperature difference between inside and outside your bag, the vapour will condense on the colder surface. This happens whether you are under a tarp or in a tent. I have noticed this when walking in the Central Australia, when the temps are below zero and no visible signs of frost, but the bag is still damp on the outside.
In calm conditions there is no way of avoiding this.
If possible, air the bag out at lunch time or when camp is reached.
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Re: Winter walking and condensation on sleeping bag issue

Postby Graham17 » Sun 24 Jun, 2018 7:17 pm

Yeah well, nothing beats a camp fire!
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Re: Winter walking and condensation on sleeping bag issue

Postby Zapruda » Sun 24 Jun, 2018 8:26 pm

jdeks wrote:Hmm, lots of potshot guesses going on here...often seems to be the case on this topic.


Thanks for the correct answer to this issue jdeks. This is one of the best explanations of the phenomenon that I have read.
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Re: Winter walking and condensation on sleeping bag issue

Postby jdeks » Sun 24 Jun, 2018 10:30 pm

jobell wrote:Jdeks - yes still awake! But I had to read your reply twice. Took me a while to take it in. What helps it make sense to me is that these nights were more often foggy than not so that water vapour was well and truly visible! Your suggestion to try closing up the tent might tie in with Lamont's thoughts on using a smaller tent that might hold a bit more heat. ...

A smaller closed up tent might take a little adjusting to after my Double Rainbow palace though. On the upside it has occurred to me that downsizing my tent might allow for downsizing my pack in which case I might not need quite so much room in my tent anyway.

Synthetic sleeping bags are beyond my carrying capabilities these days I think. The older I get the less my body is happy to carry.


Glad it managed to make some sense. :)

If the nights were visibly foggy, you're REALLY working from a hard place (Although forgive me for being being a pedant, but on a foggy night the water vapor is still invisible, what you're seeing are is water condensate as airborne droplets). The air would already be fully saturated, adding moisture to the down from both without and within. Short of getting inside, say, a house, and establishing a warmer pocket of dryer air, honestly there's very little to be done there - you are going to get damp insulation.

The smaller tent suggestion is good for two reasons - one, yes, easier to establish your own warm air pocket, but two, it's lighter! The double-rainbow is good, but big! And heavy. Additionally, you may be surprised to know you can get synthetic sleeping bags to rival down in terms of weight for the 5C + range these days. If the conditions will be a regular thing for you, I'd swap to say a TarpTent Moment or MLD Cricket or Solomid or something in the 1-pers tent range, and go for a synthetic bag, with a set of dedicated, wicking sleeping thermals. I think you'll find that much less prone to the creeping soggyness, and likely lighter too on the whole anyway.


Moondog55 wrote:Water vapour can and does move through most groundsheet materials. Polyurethane isn't a good VB material and neither is silnylon. PVC is but it is heavy
Ever used a CCF pad on a tent floor and woken up with water under the mat even tho it was dry when put down? That is vapour cming though the tent floor ...


Hmmm, sorry but you're mistaken I'm afraid. Think about the physics here...

Yes, PU is technically breathable but the rates are negligibly low compared to, say, the natural gaps in any tent around seams and zips.

Moreover, vapor does not 'rise' spontaneously, especially from a cold ground towards a 'hot' body. Osmosis is driven by thermal gradient and concentration, direction is irrelevant. In this case, water vapor will not transfer from a cold ground towards a warm body, especially when that warm body is producing its own water vapor, and the internal air is already as saturated as outside from transfer around zips/seams etc anyway.

If the moisture was indeed 'rising' through the PU floor as you say, considering the CCF is far more permeable that then PU, would not the moisture would continue to 'rise' until absorbed by your bag, and there would be no damp patch between the mats?

The water you're seeing under the mat is in fact condensation of ambient and body moisture, against the cold inner surface of the floor - the same as water droplets on the inside of the cold tent roof. It just collects under the CCF mat because it's very porous (yes, even CCF) and readily transfers and hold body moisture to the colder floor, where it condenses and and cannot re-evaporate (and possibly also some direct liquid osmosis from the ground but thats another matter).

Leave a slightly damp towel or sponge on the floor overnight, you'll see the same thing. Stick a sheet of PU between you and the CCF, the damp spot will typically go away (handy tip to prevent you mat freezing to snowcave floors). Try it next time!


crollsurf wrote:Just an observation from ski-touring back in the day. I had, still have, an old Dacron Hollofil bag and I would sleep in, with wet gear and skiboot inners inside the bag. The gear would be pretty much dry in the morning and the bag would have droplets on the outside which I would brush off. The bag otherwise felt dry. I sleep warm.

Maybe synthetic bags work better at wicking the moisture to the outside of the bag in these conditions. ....



This works great, until you get a really cold snap.

The bag will feel dry, but it's not - all the moisture is under the shell, where you can't feel it, often not for 3-4 sleep cycles. That moisture has to escape somehow, and even if your body heat evaporates it initially, if you hit dew point mid-insulation, it will condense out and if cold enough, freeze. Read some of the logs of the early antarctic explorers - many mentions of bags literally frozen solid with body moisture.

Synthetic will alleviate this and help wick it out (providing it doesn't freeze first) but you can't fight physics. You might get away with it a few nights but long term, better to keep as much moisture as possible out of the bags.

Zapruda wrote:[Thanks for the correct answer to this issue jdeks. This is one of the best explanations of the phenomenon that I have read.


No problem, it's important stuff. Too often people just give up with the technical stuff and wind up following 'easier' random hunches and pseudoscience, then wonder why it never quite works....
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Re: Winter walking and condensation on sleeping bag issue

Postby north-north-west » Mon 25 Jun, 2018 9:12 am

jdeks, thanks for the science. Also for phrasing it in easily understood terms. But I still have one question.

Why is the foot of my sleeping bag always the first (often only) place to get damp, and always the dampest? The Scarp breathes well but on still nights I always end up with slightly wet material at the foot of the bag. It isn't from contacting the walls because a) I make sure I'm set up so that there is no contact (helps being a shortarse) and b) the walls inside are dry. It isn't sweat because I have less than ideal peripheral circulation and the feet are the first bodily part to get cold (without movement).
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Re: Winter walking and condensation on sleeping bag issue

Postby jdeks » Mon 25 Jun, 2018 11:27 am

north-north-west wrote:jdeks, thanks for the science. Also for phrasing it in easily understood terms. But I still have one question.

Why is the foot of my sleeping bag always the first (often only) place to get damp, and always the dampest? The Scarp breathes well but on still nights I always end up with slightly wet material at the foot of the bag. It isn't from contacting the walls because a) I make sure I'm set up so that there is no contact (helps being a shortarse) and b) the walls inside are dry. It isn't sweat because I have less than ideal peripheral circulation and the feet are the first bodily part to get cold (without movement).



Bunch of theories, but this is less sciencey and more just my own testing and speculation. I think it's a combination of the following, I get damp footbox too.

-Feet do emit moisture, even if not as palpable 'sweat' , even if cold. Quite a bit actually. Varies person to person, for some it's very pronounced (hyperhidrosis). Not sure on the physiology of why they do so, but I'm sure every hiker's experienced damp socks after even an easy, cool day in full boots. They're deffo a vapor source.
-Less overall down volume in footbox. Torso being larger in diameter than feet, even with same (or greater) wall thickness, the total amount of down is waay less. Meaning less to sponge up condensed water vapor. Meaning the down becomes palpably damp to the touch faster
- Feet move more. Constant foot wriggling (say, due cold tootsies?) massages and compresses the down. Typically condensation forms on the inside of the outer bag shell (much like the inside of a tent wall, that's the coldest surface). The more you fuss the down about into the wall, the more condensate is absorbed before it has the chance to wick out of the bag.

Again - just theories. What I DO know is since I started I sleeping in fresh socks with a garbage bag over my feet (my usual snow sleeping arrangement) I don't get damp footbox any more (or cold feet. Ever.).
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Re: Winter walking and condensation on sleeping bag issue

Postby north-north-west » Mon 25 Jun, 2018 12:37 pm

jdeks wrote:
north-north-west wrote:jdeks, thanks for the science. Also for phrasing it in easily understood terms. But I still have one question.

Why is the foot of my sleeping bag always the first (often only) place to get damp, and always the dampest? The Scarp breathes well but on still nights I always end up with slightly wet material at the foot of the bag. It isn't from contacting the walls because a) I make sure I'm set up so that there is no contact (helps being a shortarse) and b) the walls inside are dry. It isn't sweat because I have less than ideal peripheral circulation and the feet are the first bodily part to get cold (without movement).


Again - just theories. What I DO know is since I started I sleeping in fresh socks with a garbage bag over my feet (my usual snow sleeping arrangement) I don't get damp footbox any more (or cold feet. Ever.).


OK, I might try the breadbag thing on the feet and see if that works. I always keep my bedsocks dry anyway as there is nothing that feels quite as good at the end of a day as putting on clean dry (warm) socks.
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Re: Winter walking and condensation on sleeping bag issue

Postby CasualNerd » Mon 25 Jun, 2018 12:42 pm

I've noticed the foot end of the sleeping bag being damp too, when I can find no trace of tent leakage. I always assumed that because your torso is nice and warm the sweat moves to the outer faster, but at your feet which are colder there's less warmth to move that condensation or sweat away. Just another theory !
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Re: Winter walking and condensation on sleeping bag issue

Postby nq111 » Tue 26 Jun, 2018 8:29 pm

Interesting discussion.

I started using a vapour barrier with my sleeping bag as standard in cooler climates (towards zero and under) a number of years ago. As we generally cook in the vestibule, don't have a ground sheet and my usual walking buddy does not use a vapour barrier we still often get lots of condensation. For example some recent nights on the plains after the Western Arthurs it was like rain inside :)

Since using the vapour barrier my sleeping bag stays really dry over a week of camping. This is despite it getting drenched on the outside (particularly the foot end) on many nights. Sometimes I notice puddles on the tent floor. My sleeping bag is not a waterproof shell which I believe are counterproductive by reducing the vapour transmission from within the sleeping bag. I can only conclude that the majority of the dampness and loss of loft previously experienced with the sleeping bag over a weeks cold weather use was condensation from my perspiration. Reading up on the issue over the years and my experience seems consistent with the evidence.

No need for synthetic fill - down is so superior. The couple of occasions i have had properly wet down sleeping bags (a dry-down and a non-dry down bag - both due to 'busted fly' events - don't ask :) ) I have been pleasantly surprised how well they do dry out in the field and the amount of warm still available in a very damp down bag. Amazing feat of evolution those feathers.

So I am firmly in agreement with those saying to work out to manage the moisture coming out of you rather than try to prevent it coming in. Whether that is sleeping colder (amazing how much the core body temp. goes up after the hot meal at the end of the day) or a vapour barrier or both.
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Re: Winter walking and condensation on sleeping bag issue

Postby philm » Fri 29 Jun, 2018 1:55 pm

Interested to know what vapour barrier you use?
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Re: Winter walking and condensation on sleeping bag issue

Postby nq111 » Fri 29 Jun, 2018 5:55 pm

Western Mountaineering Hotsac
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Re: Winter walking and condensation on sleeping bag issue

Postby Neo » Thu 05 Jul, 2018 3:28 pm

[quote="Neo"]It is a Haglofs and has a PU base with a polycotton top fabric...

On nights around zero degrees, if I had my down STS bag inside this bivy bag, would it just sweat and make the sleeping bag wet? Am thinking either the bivy as a cover will block any draft, or the polycotton won't breath very well and just make things worse..?[/quote]


To answer my own query, just did three nights of sub zero under a tarp and no problems with this combo, synthetic halfbag was inside the down. Just icey dew and condensation outside, so the outer bag helped with that. Used a Mont rain jacket over the foot end the third night.
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Re: Winter walking and condensation on sleeping bag issue

Postby jobell » Wed 11 Jul, 2018 11:04 am

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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Re: Winter walking and condensation on sleeping bag issue

Postby Lamont » Wed 11 Jul, 2018 11:47 am

Thanks for the feedback jobell, Keep trialling and please let us know the results.
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