Sleeping Bag Condensation Tricks?

[Chris W]

I went for a rainy, then snowy, then cold (15F) winter uphill and sleepover in an unheated ridgetop cabin. I used an older “expedition” down bag (EMS -40, circa 1970’s). It was dry when I got in… and woke up with it very damp. Lots from the top, and I assume plenty from the inside out. This reduced it’s insulation dramatically.

I was well-exercized getting in, but not sweating too much…

Any tips or tricks would be appreciated. Would a newer bag with a pertex shell and treated down or the like make a differece/

Thanks in advance

[James S]

There are lots of possibilities as to what was causing the dampness. Were you damp getting in to the bag or did you have dry sleep clothes? Were you overly warm throughout the night (i.e. sweating)? What sort of mat were you using? Was your bag in contact with the walls at all? Was the dampness evenly distributed? I use a sleeping bag liner at all times and for very cold nights will use a VBL (some discussion here: VBLs & synthetic overbags). A new bag might make a difference, but then again it might not. Without knowing a little more about the specific situation, it is hard to make a firm recommendation. I have a pertex bag with treated down and it will still get damp (from the outside usually) if I am not careful with things like the pitch of my mid. It seems there is often a rush to buy new gear when current gear will do the job within the right contexts.

[Chris W]

Thanks for the response!

I was warm getting in, not sweaty but perhaps a bit damp and I think I slept hot, with maybe too much clothes on from a bad weather trip up.

The dampness in the down seemed evenly distributed. I used a Therm-a-Rest NeoAir XTherm pad on a wooden platform, not touching any walls.

I wonder if I could treat the outside of the bag to limit condensation from the air? Seems VBL’s are well-liked…

Thank you again, for your response.

[Edward John M]

Just a thought here but how clean is the bag? Something that has been pointed out in the past is that a dirty bag may get damper/wetter than a clean one, so perhaps a good gentle wash and then a Nikwax treatment would help?

Was it cold enough to need a -40 bag? Were you venting? Was there condensation dripping from the cabin ceiling?

[James S]

I’ve slept in a lot of huts and usually look forward to it as condensation is much less of an issue compared to in a tent. Venting the hut somehow might have helped if there was running condensation from the ceiling/walls? I would almost always recommend to change into a dry set of baselayers for sleeping. I bring a baselayer for legs, warm sleep socks and usually a 100 weight fleece. It’s a small weight but ensures that you are not bringing external moisture into the bag. Some people argue that it is warmer to sleep wearing as little possible as you will heat your bag more efficiently, but I have not found this to be the case and it runs against the more intuitive view that more clothing equals more insulation.

I like to be on the cooler side when sleeping, and often am cold when I first get in the bag. If I dress to be ‘warm’ inside the bag in that sort of temperature, I will usually wake up sweating once my body has heated the bag. I prefer to be a little colder, and put a hat on if I wake up from cold. A liner will help keep the bag clean for longer and probably improves long-term performance. How much does your bag weigh? It’s likely that a modern bag will be significantly lighter for similar performance, or you could try a two bag system. Winter bags aren’t cheap though, so I suppose it would depend on how often you are using it.

[Peter Nash]

A vapor barrier liner will protect the down from condensation from insensible perspiration, and it will keep you warmer.  Of course, there will be condensation inside the vapor barrier liner, but that is manageable.

Vapor barriers are always the lightest option, the warmest option, and they work even in the middle of summer.  Waking up with condensation inside of a vapor barrier means it’s working, and that is good.

[Matt]

I have experienced the exact same problem.  I finally concluded that it was my body heat rising through the down, out through the porous shell, and then condensing upon contact with cold/damp air.  A good DWR on the bag usually kept the water droplets from soaking back in until I rolled around or tried to clumsily pat the droplets dry with a chamois towel.

One solution that works for me is to manage my sleeping clothes and avoid sleeping “hot”.  I also think a thicker/less porous shell material (such as the option EE offers) might keep some of the body vapor from so readily moving out to the exterior of the bag.  Failing that, I shake any visible droplets off as best I can as soon as I wake up.  Lastly, I gently blot the droplets with super-absorbent toilet paper.

[Jerry Adams]

did you breath into the inside of the bag?

I find that normal sweating and even clothes that are a little damp will be driven out of my bag by body heat.

[Matt Dirksen]

I’ve been using treated down for several years, and I’ve been pretty happy with it so far. The best thing I’ve done on cold weather trips is to use an outer bag which acts as a “sacrificial layer”. While many folks use synthetic for this, I’ve been fine using a 50d treated down bag. This is the layer where any condensation will take place. It is as permeable as possible so it will dry out as quickly as it can get wet. I also use a silk liner over top of all of this, which does a masterful job of wicking away moisture and rogue water drops from the fly overhead of me.

While I don’t personally use VPL’s, I understand they are quite proven in very cold conditions, but part of a good sleeping system is to become comfortable enough to actually fall asleep. I gather VPL’s take some getting used to, since our bodies “prefer” the right temperature and relative humidity to “feel comfortable”. And even then, one may still have moisture management issues to deal with in very cold-highly saturated air.

The general problem with sleeping in cold/wet conditions is the simple fact that the air itself is likely to be pretty saturated already. Leaving aside air transport, vapor moves from warm to cold, and from humid to dry. So if the air is already pretty humid and I jump into my sleeping bag pretty warm from a small workout, where would my body’s vapor want to end up?

So if the outside of my sleeping bag is pretty cold, and I’m hot, my vapor will migrate toward the cold along with my body’s heat. But vapor is also interested in seeking dryness as well. So, as the vapor migrates away from me, and the air outside is already mostly saturated and really cant “take in” any more moisture, then what?. Most likely it’ll settle down inside of the nooks and crannies inside of my bag’s outside shell, as well as any adjacent down plumes and fibers. And once enough vapor “settlement” has occurred, it will eventually condense into water droplets, some of which may freeze depending on the temperature. I also understand that this “capillary condensation” can occur at humidity levels as low as 80%, so I don’t need to be in completely saturated air for this to happen.

So even though a VBL can mitigate a bulk of the water woes, at some place between a warm body and the outside saturated air is a “sweet spot” where the vapor in the air itself will want to go. Not as much of a concern as body vapor, but can still be an issue that must be managed.

Bottom line: people got so focused on trying to “keep” water out of their system, they forget the fact that humans are mostly made of water, and we must deal with that, too. And in cold weather, this becomes a life-safety issue. So always have good ways to dry off, because you are already wet to begin with.

[James Marco]

Well, I really don’t agree, Matt@Zen. While some is surely a component of bag getting wet outside, on the surface, I believe that the most of the wetness was actual condensation INSIDE the down. But the solution you mention is spot on, in my opinion. Sleep cooler. Open the bag up and use it as a quilt. Ventilate your head/torso more. But trying to ventilate a -40F bag for 15F degree conditions, might be a bit much. I am comfortable doing about 20 degrees, but 55 is a LOT.

A -40 bag in 15F weather… Too much bag can do this easily. Anytime the dew point exists within the down, the down will condense water. This is a plus and a minus to using down. But it has to be balanced for the conditions. Under these conditions, a VB would have helped a lot. But this can occur under other conditions. For example at 40F and 100% RH you are using a 10F bag. The dew point, will occur inside the bag itself again, leaving a wetter bag. Of course, once down gets wet or damp, it insulates less, making it feel comfortable to the sleeper, but after getting out of the bag and letting it cool for a few minutes, you will notice it feels damp.

Anyway, other factors could be the cause, as has been said by others. Matt D. mentions using a sacrificial, low insulation top bag. This works also. It will maintain enough heat to raise the RH of the air till it hits the dew point in the outer bag. But this still can be over driven with a too warm bag for the actual temperature. You want to keep the temperature of the surface layers of the bag as high as possible to keep the RH as high as is possible. 100%RH at 20F is far different from 100%RH at 60F. (Relative Humidity is the amount of water vapor that can be held in the air. It goes up significantly with increasing temperature.) By the same token, keeping the surface of your bag warm, means you are loosing heat. This can be bad.

[Bruce Tolley]

Based on the description of events, I agree with James’ theory: “A -40 bag in 15F weather… Too much bag can do this easily. Anytime the dew point exists within the down, the down will condense water. ”

Plus all moisture that was in Chris’ clothes ended up evaporating off the clothes and then condensed before it passed through the down and into the ambient environment.

[corrected names].

[John Vance]

I would agree with the previous two posters. The dew point was inside your bag and a lighter bag would have performed better.

[Chris W]

ahhh… ok, so condensation, by definition is vapor in the air transforming to liquid because the air cooled and squeezed it out. So the only way condensation happens is when warm air – from inside the bag – gets to the outer layers where the outside temp cools it, squeezing water into the down and also on the outside of the bag, as the moisture moves from inside towards outside.

Hence the source is sweat and/or damp clothes (not the outside air itself)., and hence that is how/why VBL’s work.

And so a goretex or pertex shell is really only good for rain and such.

[Bruce Tolley]

There have been many good postings about the use VBLs on BPL.  Andrew Skurka has a good long article on his website https://andrewskurka.com/2011/vapor-barrier-liners-theory-application.

For very short trips of one or two or three nights, I think it is a question of whether you want the extra weight of the VBL (and the possible discomfort) or you think you can manage the moisture without using a VBL.

When snow camping in the Sierra Nevada below treeline where the coldest nightime temperature might reach 5 or 10 degree F, I sleep under a DuoMid inside a breathable water resistant bivy sack. I often start the night using 0 degree down bag as a quilt and only zip it up once I start to get cold. I have also tossed in one or two liter bottles filled with hot water to warm up my my feet which usually get very cold when I am cooking dinner.

For longer trips, there are trip reports on BPL and elsewhere on how much moisture starts to accumulate in the sleeping bag and corresponding adverse effects on the bags ability to insulate without the use of the VBL.

 

[James Marco]

Yeah, pretty much, Chris. There IS such a thing as being too warm in cold temps.

Normal perspiration is used to cool your body. I’m sure you know about that.

Insensible perspiration – you perspire a certain amount of water off your body even if you don’t need to perspire for cooling. It keeps the outer layers of your skin fairly flexible, because those cells are usually dead cells. In addition you secrete oils and urine and a variety of other things in much lesser amounts through your skin. It increases the waterproofness of the skin, flexibility, callus formation and maintenance, hair maintenance, some immunity, etc. You normally don’t see much evidence of this. But it is normal to loose about a half liter of water or so, even when you are mostly cold. This is like saying you dumped a cup of water in your sleeping bag.

In colder conditions, this will leave the warm environment of your body and condense in the down.

Normally, say you are using a 32F bag at a 32F temperature, enough will eventually evaporate off your bag to cause no problems. You wake up and your bag is dry in 5minutes or less due to remaining heat.

If you use a 50F bag at 32F, you might be too cold to sleep but your bag will be pretty dry. (This is what most would say is in the extreme or survival rating of an ISO(EN) rated bag.)

Above freezing, there isn’t much of a problem, because it allows the condensation to collapse the down, removing some of the insulating value. It will stabilize at some cooler level, causing less perspiration from you as you get a bit cooler. As an example, a 20F bag actually now insulates you at 32F, balancing heat loss and condensation at fairly comfortable levels. (Down will do this naturally where synthetics will not. Synth fills don’t collapse when damp. Anyway…) Your bag will feel slightly damp in the morning, but leaving it spread out, it will dry pretty quickly…usually in 10-20min. Even at 32F, the residual heat will cause it dry or mostly dry in that time.

The final case is when you are at 20F or lower and you have a -40F bag. The water still condenses in the down (and of course it migrates,) but instead of remaining a liquid, it freezes. Sometimes you can feel this. After a midnight run you can feel that your bag feels a bit stiff. Not too bad the first night. The second night it gets a bit worse as you add more condensation, and so on…it isn’t leaving your bag, it is frozen in. You have to take extra effort and precautions to encourage it to dry out.

Yes. A breathable jacket/shell/fabric will NOT work as a VBL (Vapor Barrier Liner.) Equinox, Western Mountaineering makes a liner for bags. I would suggest a silk liner or something inside though, I never cared for the damp, clammy feeling. You can make a bag liner out of an emergency blanket and some duct tape. It works OK provided you can keep it pulled up enough…cheap enough to try one out.

[James S]

I was going to ask whether a silk liner inside a VBL will work. Seems like you could avoid the clamminess from the VBL but might get too cold as the silk liner absorbs the condensation from the VBL? Silk liners dry quickly so it shouldn’t be an issue over multiple days. I will test it out.

[James Marco]

Actually, silk will absorb some water directly into the cloth fibers. In many ways, a lot like cotton, but to a lesser extent. (Merino wool is about the same.) It is a good compromise between the plasticky/wet feeling of any VBL liner and cloth of a sleeping bag at colder temps. I only mention this as a typical use of a bag liner that is actually worth while. They don’t add really add much warmth for their weight, it is more about separating your skin from the plastic.

In actual, practice, you can just use your long johns.  Again, VBL’s are mostly useful on longer trips (5 or more days) at constant below freezing temps when you cannot get your bag to dry (arctic/tundra/antarctic like conditions.) Getting up and wearing the long johns for a minute will dry them out fairly well.

Example: Back when, I used to get out of my bag, pull the VBL out, turn it inside-out, and shake it out…placing it on top of my bag to dry out (the VBL was usually pretty dry after shaking it out.) After donning my day cloths (I wear the long johns as a base layer,) I fold and roll the VBL. I spend a minute or two breaking down my pillow and preparing my pack/pad, so by then my bag is normally dry enough to pack up. It looks a bit haphazard, but everything gets a minute to dry out with a bit of body heat and/or residual heat, and, I get useful stuff done. This is just one system that works, there are undoubtedly other systems…

If you are headed out for a few days (say less than 4-5 days,) I would not worry too much about the VBL. They are really only necessary for sustained cold (<20F) long duration trips (>week,) are optional for 4-7 days and don’t make much difference at 1-3 days for most hikers. I believe there was a note somewhere about someone flying in new bags every week in the antarctic… Here in the NE corner, we don’t use them much, we just get a morning fire going.

[Eric Blumensaadt]

Andrew Skurka has been converted to the VBL (Vapor Barrier Layer) “persuasion” to control this problem. After his first winter trip where he experienced excessive wetting of sleeping bag and traveling clothes from sweat he “saw the light”. GOOGLE him & VBL to see his recommendations.

SOLUTION: Order some light silnylon material from a place like Seattle Fabrics or Ripstop by the Roll and get a men’s pajama pattern from a fabric shop.The sit down at yer sewing machine and make a VBL suit with the top a pullover with a zippered “turtle neck” opening. Use elastic for cuff and neck openings. Seal all seams.

Too inept to make yer own? Sweet talk yer wife of Significant Other into doing it for you. Or buy VBL suits from Warmlite.

I wear light polyester long johns inside to avoid the clammy feeling. They dry fairly rapidly when removed. If they freeze you can shake most of the frost off.

[AlpineIce]

Yep, VBL 100%

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