Pertex Quantum, condensation and a sleeping bag

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mattheus
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Re: Pertex Quantum, condensation and a sleeping bag

Post by mattheus »

(useful info, thanks pj)
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horizon
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Re: Pertex Quantum, condensation and a sleeping bag

Post by horizon »

Tigerbiten wrote: 23 Jul 2021, 1:51pm
As you move around in your sleep, this air passes through the outer shell of the bag from the down and hits the colder air, this extra water vapour condenses out on the hydrophobic shell.
Hence the water on the outside of the bag.
In reply to this and I think something thirdcrank and Vorpal have also noted, I'm not too worried about the exact source of the moisture. I'm presuming there is some residual moisture in the air from the environment but that most of it is from me either directy from my breath or from my body (as in perspiration though the bag). I don't know if this moisture condenses directly onto the bag or on the inner tent and then finds its way (by contact or by drip) onto the bag. However, I think that the wetness on the bag was concentrated mainly along contact points on the side and at the feet. It could be that the bag was doing a grand job in soaking it all up.

The question would still remain though: is the bag collecting more than other items due to some factor such as the nature of the material? Remember, this is a new problem for me: I've used a mixture of tents and sleeping bags over the decades but this is the first time I have experienced a condensation problem serious enough to consider ending the tour early.
When the pestilence strikes from the East, go far and breathe the cold air deeply. Ignore the sage, stay not indoors. Ho Ri Zon 12th Century Chinese philosopher
nsew
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Re: Pertex Quantum, condensation and a sleeping bag

Post by nsew »

I have this in my camping notes. Found on the web a few years back and appears to be a reply to someone having similar issues.


- 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 38C 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 use a fine bug mesh inner to divert drops from your bag)
ossie
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Re: Pertex Quantum, condensation and a sleeping bag

Post by ossie »

horizon wrote: 23 Jul 2021, 6:25pm
The question would still remain though: is the bag collecting more than other items due to some factor such as the nature of the material? Remember, this is a new problem for me: I've used a mixture of tents and sleeping bags over the decades but this is the first time I have experienced a condensation problem serious enough to consider ending the tour early.
Could it simply be the Pertex coating as you suggest . I have a Pertex coated Montane jacket and despite its excellent water repellency it refuses to let go of water droplets after rain, even after shaking it off. So much so I have to hang it in the garage overnight on a makeshift washing line for wet gear. I use it for touring and as you can imagine its a bit of a pain if wet, it seems to be a magnet for water droplets so never enters the tent.
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pjclinch
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Re: Pertex Quantum, condensation and a sleeping bag

Post by pjclinch »

horizon wrote: 23 Jul 2021, 6:25pm ...but this is the first time I have experienced a condensation problem serious enough to consider ending the tour early.
There's a first time for everything, and it's not necessarily the bag or the tent.

On one occasion we camped two nights on the Black Isle, same site, similar winds and temperatures, no rain, summer. First night nothing unusual, second night we woke to the heaviest dew I've ever camped in and the fly was literally dripping inside and out, inner damp to the touch and condensation on the floor and IIRC some on the bags. If you've just experienced a night like that, that might explain everything, and that was with tried and trusted, well respected gear 24 hours after a totally unexceptional night. Camp often enough and a monster condensation event is increasingly guaranteed, but I'd already been going camping for decades before that event.

Down bags don't work well if they're wet, but they need to be pretty much soaked for that to be an issue, not just a bit damp, mainly in the shell. In the UK that'll generally mean the sort of wall to wall rain where you can never dry anything and you'd quite possibly be thinking to hell with it anyway, quite apart from your sleeping bag.

You need to take care of down, but that doesn't mean having to treat it like paper lace.

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pjclinch
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Re: Pertex Quantum, condensation and a sleeping bag

Post by pjclinch »

An addendum, I think nsew's description is on the money (including the probable source of the extra moisture being you, and condensing on the bag shell just after having left in to a colder atmosphere), but...
A problem, as illustrated by my Black Isle anecdote, is you can't necessarily tell before you go to bed whether you want to force up interior temperature by shutting everything or keeping the potential for air moving around greater. The whole thing can swing on a degree or two and/or a small change in relative humidity.

In a humid place like the UK it'll happen from time to time, but good gear will cut it unless everything is getting wetter and wetter day after day. I'd start thinking about an abandon because it's getting miserable, not because you're worried about your sleeping bag.

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nsew
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Re: Pertex Quantum, condensation and a sleeping bag

Post by nsew »

The bag is certainly too warm for a high summer journey. Rather than abandon I’d just post it home and pick up a cheap circa +10deg bag. I used to leave in early March with a warm bag that would have to be fully unzipped and used as a half blanket for the warmer months. Even then it would accumulate so much perspiration / moisture the down clogged up. To remedy I purchased a +4 deg bag at half the weight and bulk and layered up when temps dropped. Small tent isn’t optimal either in humid conditions. Plenty of variable weather conditions while travelling so the ability to layer up or down is essential.
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horizon
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Re: Pertex Quantum, condensation and a sleeping bag

Post by horizon »

nsew: thanks for that and your previous posts. The bag is indeed overkill for the weather we have been having. It was intended for the "shoulder" months but even for July we have had exceptionally warm weather. If you think that this condensation (i.e. on the bag) was caused by the bag being too warm then I am happy to look at that. I consider myself a "cold sleeper" so giving up a warm bag isn't easy. Moreover I don't really have a good comparison: this is a new, good quality bag and my older bags are just that - old. However, I've got some short trips coming up so I am going to experiment and hopefully report back.
When the pestilence strikes from the East, go far and breathe the cold air deeply. Ignore the sage, stay not indoors. Ho Ri Zon 12th Century Chinese philosopher
nsew
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Re: Pertex Quantum, condensation and a sleeping bag

Post by nsew »

That’s a fine bag that will serve you well. After that trip it should be hung in a warm dry area of the house for a few days, the warmer the better. Rustle it up as you pass to open up and loft the down. If you have older synthetic bags they loose their insulating properties after ~ 5 years. Down when cared for will keep insulating forever.

https://www.phdesigns.co.uk/down-and-sy ... n-compared

https://www.phdesigns.co.uk/getting-the ... eeping-bag

https://www.phdesigns.co.uk/the-truth-a ... eping-bags
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horizon
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Re: Pertex Quantum, condensation and a sleeping bag

Post by horizon »

thirdcrank wrote: 23 Jul 2021, 1:36pm On the subject of moisture in down, weigh your sleeping bag after a period of use then give it a good tumble dry; you may be surprised how much lighter it will then be.
Fortuitously, I weighed the bag I took away for the "experiment" (a different bag) before I went. I weighed it on my return after the two-night trip. Unsurprisingly after such a short time it wasn't much different, if at all, but I am now drying it out properly to see if the weight changes, even slightly.
When the pestilence strikes from the East, go far and breathe the cold air deeply. Ignore the sage, stay not indoors. Ho Ri Zon 12th Century Chinese philosopher
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horizon
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Re: Pertex Quantum, condensation and a sleeping bag

Post by horizon »

horizon wrote: 25 Jul 2021, 10:02pm I've got some short trips coming up so I am going to experiment and hopefully report back.
So here is the first report:

Sleeping bag: down with cotton shell
Flysheet material: polyester
Inner tent material: polycotton
Outside nighttime temp: 9 deg C (clear night, no cloud or rain, both nights, moderate dew)
Ventilation: none
Innerside of flysheet: totally wet from condensation
Layering: full and feeling cold(ish)
Condition of bag overnight/in morning: no discernible condensation (i.e. dry)
Inner tent: no discernible condensation (i.e. dry)

I'm not drawing any conclusions from this as the weather conditions may have been slightly less extreme (as in temp differential day/night) and the assessment of the bag was very subjective (i.e. it could have been damper than I realised). But the fact that I was cold may be indicative. I am going to see what happens on the next trip.
When the pestilence strikes from the East, go far and breathe the cold air deeply. Ignore the sage, stay not indoors. Ho Ri Zon 12th Century Chinese philosopher
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Re: Pertex Quantum, condensation and a sleeping bag

Post by nsew »

horizon wrote: 5 Aug 2021, 4:03pm
horizon wrote: 25 Jul 2021, 10:02pm I've got some short trips coming up so I am going to experiment and hopefully report back.
So here is the first report:

Sleeping bag: down with cotton shell
Flysheet material: polyester
Inner tent material: polycotton
Outside nighttime temp: 9 deg C (clear night, no cloud or rain, both nights, moderate dew)
Ventilation: none
Innerside of flysheet: totally wet from condensation
Layering: full and feeling cold(ish)
Condition of bag overnight/in morning: no discernible condensation (i.e. dry)
Inner tent: no discernible condensation (i.e. dry)

I'm not drawing any conclusions from this as the weather conditions may have been slightly less extreme (as in temp differential day/night) and the assessment of the bag was very subjective (i.e. it could have been damper than I realised). But the fact that I was cold may be indicative. I am going to see what happens on the next trip.
Site location? Cold or warm before climbing in the bag? When did you last eat? R rating of sleeping pad used? How much fill in the bag and does it loft? Cotton shell or liner sleeping bag? There’s a lot of cotton, a poor material for the outdoors.
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Re: Pertex Quantum, condensation and a sleeping bag

Post by nsew »

Tricks for being warm and comfortable on colder nights:

Get into your bag quickly while still warm from exercise. Don’t let yourself get cold.
Always carry a pair of merino wool socks. Put them on.
Always carry a merino wool beanie. Put it on before you feel cold and leave it on.
Eat fatty foods while in your bag. The digestive system has to work harder, which radically heats you.
If you remain cold, do a bunch of sit ups or press ups while in your bag and trap the resultant body heat in.

A good sleeping bag doesn’t warm you, you warm it - the insulation (if it’s working) keeps that warmth in. If a good sleeping bag is immediately toasty and you nod off quickly from tiredness it will be much too warm later in the night.
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horizon
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Re: Pertex Quantum, condensation and a sleeping bag

Post by horizon »

Thanks nsew. I only said I was "cold" because it implies I wasn't too warm and generating too much heat/perspiration. I was well layered (as advised) and the sleeping bag itself is old and probably rather poor. The bit I am interested in is the lack of condensation.

The hypothesis I am working on is that the dew point of generated moist air wasn't reached until it had escaped through the polycotton inner-tent and onto the inside of the flysheet, where the moisture condensed.

So:

1. Cooler sleeping bag (as advised)
2. Layering to appropriate level of warmth (i.e. not too warm) (as advised)
3. No ventilation (keeping internal temp higher than dewpoint) (as suggested)
4. Highly breathable inner tent (also recommended)

The weather conditions were slightly more favourable but still high humidity days followed by cold, clear nights. I'm trying to arrange another test but still deciding which tent and sleeping bag. Unfortunately it is likely that both will be different rather than just the one which will make comparison more difficult.

Having said that, I'm very pleased with progress so far and I'm continuing to base what I do on what you and others have suggested, that the source of the condensation on my Rab sleeping bag was body heat/perspiration rather than general condensation in the tent.

Although the cotton bag is still "drying out", I'm not expecting much change in weight; but the fact that it appeared dry points to the difference between the cotton shell and the Pertex (the former absorbing the condensation, the latter beading it). It also might mean though that the moisture got as far as the flysheet before condensing and I'm hoping that can be replicated with the Pertex Rab.

By the way, I worked out that the body moisture had possibly five potential condensing points:
1. Inside the bag (unlikely in summer conditions)
2. On the outer shell (which is what happened with the Rab)
3. In the air above the bag (I'm not sure about this)
4. On the inside of the inner tent (perhaps less so with polycotton?)
5. On the inside of the flysheet.

Getting it exactly right might not be easy!
When the pestilence strikes from the East, go far and breathe the cold air deeply. Ignore the sage, stay not indoors. Ho Ri Zon 12th Century Chinese philosopher
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Re: Pertex Quantum, condensation and a sleeping bag

Post by pjclinch »

horizon wrote: 9 Aug 2021, 12:01am <big snip>
Getting it exactly right might not be easy!
I suspect you're over-thinking it. I'm all in favour of experiments but unless you have full control over the environment (and you don't) and can be bothered to do so many experiments that you might as well abandon your bedroom I don't think you'll get much hard data.

I also wouldn't get too hung up on specific materials. The difference between polyester and nylon is very probably less than the difference between the particular fibre and weave of the resulting cloth, and will be further changed by any coatings. Things with a DWR coating will encourage water to bead up on their surface which is good in some respects, bad in others, spreading coatings do the opposite of DWR (help moisture soak in for easier evaporation, rather than forming drops) and have pretty much the opposite sets of pros and cons, but either might be better than the other in some situations.

I think what you want is gear that will generally cope with most of what will be thrown at it (I suspect you're already there) and stop worrying so much about condensation: it happens, it sometimes can't be avoided. With time you'll be able to choose between keeping everything hot or keeping everything vented based on experience, but at the end of the day condensation is what it is and your bigger goal is to worry about comfort. If it's all a bit damp in the morning is less important than did you get some decent kip. Condensation is a pain, but it's not that much of a pain as to be a deal breaker almost all of the time in the UK. Your sleeping bag will not be destroyed by it or rendered useless.

Much is made of the problems of wet down, but if you wash a sleeping bag in your bath at home then that's what we're talking about as "wet", not a wee bit damp to the touch and a bit clammy inside.

Pete.
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