sleeping bags and perspiration

Hi all,

I have a question on bags taking on perspiration in the winter. This may have been discussed before, by typing in "sleeping bags" into the search engine generated too many postings for me to see.

How many days would one have to be out in the Whites in winter before a down bag became impacted by perspiration? I know it's very vague. I have a MH -20 Wraith bag and am thinking about a rather long winter trip. I've heard rumors bags can get heavy, and the down loses its warmth, but I don't know if this is actually true or not, and if so, how much of an impact it makes.

Along with that, does anyone have any knowledge about drying a bag out on the trail? On Denali, we just left them in the tent for a day and they were bone dry. Could one, for example, strap a bag to outside of one's pack on a clear dry day?

I know these are tough questions. Any thoughts (or links to research) would be helpful.

Thanks!


Brian

Others will have to comment on a down bag as I have always stuck with synthetic in the Whites. Synthetic bags avoid this issue, although they are heavier and bulkier as well. I would consider a lightweight bag liner, I had a partner years ago who swore by his.

The most I have done in the winter is three nights in a row, but I haven't noticed any decrease in bag efficiency.

I use a a silk liner, wear a separate set of base layers for sleeping, and I don't use my bag to dry out clothes overnight. Also I base camp and day hike so the bag is aired out atop the tent if no snow is forecast.

Water collects every night unless you dry the bag out sufficiently.

A number of years ago, two friends spent a winter week or so on the Long Tr (VT). One carried a polyester bag, the other a heavier down bag. Before long, the down bag had less loft than the polyester bag. (I don't know how much effort they put into drying the down bag.)

A VBL is one method of keeping the down dry. (Using a VBL will probably also help to dry out a damp bag.)

Doug

After a long night in the cold, the air within the loft of a sleeping bag ranges from the ambient outside temperature, up to your body/skin temperature on the inside of the bag. The warm air next to the inside is loaded with moisture from your body. At some point on the thermocline within the bag, the humidity in the air reaches the dew point, maybe even the frost point. There's not much you can do about that, unless you sleep within a vapor barrier liner.

The very first thing you should do when as you crawl out of your sleeping bag in the morning is to squish it down tight to get as much of that warm moist air out as possible, before it too cools enough to reach the dew point. Squish it and expand it a couple of times. At least you can drive some of the moisture out and prevent it from accumulating on the loft.

Tents have a similar problem - put people inside and the interior gets warm and humid while the outside is cold; the result is condensation. This condensation hardly affects the tent's insulating abilities, but it's unpleasant. A good habit is to leave your tent wide open for a while every morning before you pack it up and/or in the evening before you get inside, and do the same with your sleeping bag. (Unless it's foggy, raining or snowing!) Even in below-freezing temperatures, fresh air will allow some moisture to evaporate.

nartreb said:

Tents have a similar problem - put people inside and the interior gets warm and humid while the outside is cold; the result is condensation. This condensation hardly affects the tent's insulating abilities, but it's unpleasant. A good habit is to leave your tent wide open for a while every morning before you pack it up and/or in the evening before you get inside, and do the same with your sleeping bag. (Unless it's foggy, raining or snowing!) Even in below-freezing temperatures, fresh air will allow some moisture to evaporate.

Click to expand...

I was taught to ventilate a tent as much as conditions allow to minimize the moisture build up. The lower the temp, the more important this becomes.

There are several important differences between a tent and a sleeping bag:
* The tent has a cold moisture barrier, the sleeping bag does not. (A VBL in a sleeping bag has to be at or almost at body temp to be effective.)
* There is free air flow between you (the warm, moist object...) and a tent wall, but not in a sleeping bag.
* You breathe (exhale a lot of moisture) inside a tent, but should not inside a sleeping bag.
* The down fill can absorb a significant amount of water (which reduces the insulation), the tent fabric only a small amount (which does not affect its function).

Doug

Good info here:
http://andrewskurka.com/2011/vapor-barrier-liners-theory-application/

I have been thinking of getting an overbag like the Integral Designs Bugaboo II Bivy. The idea would be to more the condensation point to the inside surface of the bivy rather than the inside the bag.

Before I bought my winter bag I used a down bag inside a wider syn bag. Worked great, weighed more, but the worst part was it took up tons of volumne in my pack or pulk.

I like to use VBs for my feet and carry gloves as an emergency VB for my hands but not to sleep in.

Tom_Murphy said:

I have been thinking of getting an overbag like the Integral Designs Bugaboo II Bivy. The idea would be to more the condensation point to the inside surface of the bivy rather than the inside the bag.

Click to expand...

This would only work if outside temp is very close to the condensation temp--a very narrow range of conditions. It would also decrease the overall moisture permeability of the system.

Since the bivy would go outside of the down, it would also tend to keep moisture in the down.

Before I bought my winter bag I used a down bag inside a wider syn bag. Worked great, weighed more,

Click to expand...

The key here is that the outer synthetic bag has a significant amount of insulation which tends to move the condensation point out into the outer bag. A bivy only has a small amount of insulation.

I have used the syn outer bag, down inner bag system myself. It is very effective, but as you noted, big and heavy.

Doug

I've used a synthetic bag with bivy sack for years, no tent (add tarp for rainy weather). Always woke up with wet feet from condensation until I learned not to breathe inside the bivy. In cold weather it's nice to pull it close around your face but make sure you're always exhaling outside the bivy.

When I began using a VBL inside my winter down bag I immediately eliminated all of my (big) problems with condensation, ice and loss of loft. I wore a thin layer of polypropylene underwear, top and bottom. I thought I would be damp and clammy but was surprised at how dry I was. Not only was the bag dry after multiple nights but it was a lot warmer too not only because of the maintained loft but perhaps also because of the air space between the VBL and the bag.

More recently on a two-nighter in lean-tos with only a 40 bag and temps in the lower teens I used the VBL and that time I slept fully dressed with thick fleece pants, a down sweater etc. under the VBL and managed to sleep pretty good for 5-6 hours and once again was very surprised at how dry I was in the morning. (No room inside for my boots and they were rock-solid but a fire solved that problem.)

I also use VBL socks but unless it's very cold I find that my skin tends to weaken and develop friction rubs that would lead to blisters so I remove them half-way through the hike. Not only do my feet stay a lot warmer but my socks and boots are a lot lighter at the end of the day.

Hey Neil, if you use a vbl in your sleeping bag, how do you dry your base layers and such on a multi-day trip? I've wanted to try a vbl in my sleeping bag as I use them to great effect inside my boots, but I rely on drying my clothes at night in my (synthetic) bag. How do you do it?

hikerbrian said:

Hey Neil, if you use a vbl in your sleeping bag, how do you dry your base layers and such on a multi-day trip? I've wanted to try a vbl in my sleeping bag as I use them to great effect inside my boots, but I rely on drying my clothes at night in my (synthetic) bag. How do you do it?

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Put wet base layer in zip-lock and keep it from freezing in sleeping bag. Last thing before heading out in morning is to put the wet base layer back on. If it was OK to wear it wet most of the previous day then it's OK to wear it wet again. A bit invigorating putting wet items on upon a frosty morning I will admit but it warms up quite quickly once you get moving.

Best though is to have a big winter fire and get everything dry while whiling away the dark hours although not applicable in all winter camping locations and circumstances.

hikerbrian said:

Hey Neil, if you use a vbl in your sleeping bag, how do you dry your base layers and such on a multi-day trip? I've wanted to try a vbl in my sleeping bag as I use them to great effect inside my boots, but I rely on drying my clothes at night in my (synthetic) bag. How do you do it?

Click to expand...

You can dry things by putting them outside the VBL (but inside the bag). This has the disadvantage that some of the moisture will end up in the bag insulation, but your body heat should eventually drive it off.

If the wet things are wet enough that moisture will conduct into the bag, I'd put them in something that will prevent them from directly wetting the bag but still allow water vapor out (eg an open plastic bag).

I haven't tried this myself but it should work as long as the amount of water isn't too great. (For comparison, you lose around 400ml/day (~130ml at night) of water from your skin (insensible perspiration: http://www.anaesthesiamcq.com/FluidBook/fl3_2.php). More, of course, if you sweat.)


BTW, there is good info on the use of VBLs in "The Complete Walker IV" by Colin Fletcher and Chip Rawlins and much more info in "The Complete Walker III" by Colin Fletcher.

BTW 2: you could bring the wet items (in a sealed plastic bag) into your bag to warm them up before putting them on. However, I'd avoid keeping them wet and warm overnight because it would encourage bacterial growth.

BTW 3: Some people have reported bacterial infections from heavy use of VBL. (They tend to keep the skin moist.)

BTW 4: if you wear double boots, you can get away with just bringing the liners into your bag at night.

Doug

Thanks Neil and DP. I'm a wimp in the morning - in my opinion, getting out of the tent on a chilly morning is THE most difficult aspect of winter backpacking. So I prefer to find a way to get everything dry before I step out. Hence, I dry everything in my bag in the evening. I find that I'm often very warm after dinner, so it doesn't feel like a hardship to have slightly damp base layers on during that time, and they dry quickly once I get into my sleeping bag. The vbl inside the sleeping bag would throw my whole system off; putting the clothes between the vbl and my bag might work, but I don't see an advantage over my own system.

FWIW, I find that the plastic bag vbl's PREVENT me from getting blisters by acting as a lubricant of sorts between my feet and my liners; my liners stay dry (one less thing to put in the sleeping bag), and my socks dry quickly in my sleeping bag, particularly with a hot Nalgene bottle or two in there.

So, my system in a nutshell:
Hike per usual, trying to sweat as little as possible.
Keep slightly damp base layers on while preparing dinner. Occasionally this period of time, before food is in me, is a little chilly.
Eat.
Climb in synthetic sleeping bag with a hot Nalgene bottle or two - usually my base layers are dry by now, but if not, they quickly dry in the sleeping bag. My socks also dry quickly in the sleeping bag.
In the morning, there is sometimes a layer of frost on the inside of my tent, but I vent my tent to a great degree to prevent that. I haven't noticed my sleeping bag getting heavier or losing insulation qualities, even after 4 nights with this system. And I wear one set of base layers for the entire trip (one emergency set in a zipper bag). The major disadvantage is the requirement of a synthetic sleeping bag.

Thank you to all those who commented. Yes, it's clear that vapor barriers are the way to go. If this trip comes off (and it most likely won't), I may have a some chances here and there to open the bag up and air it out while I have dropped my pack for the summit.


All the best,


Brian

So when I google VBL I found this: http://en.wikipedia.org/wiki/Véhicule_Blindé_Léger Am I supposed to tie the sleeping bag to the back and drive around to dry it out? Or am I suppose to just sleep in it?

As for my serious question, I carry a lightweight bivy year round. Would this be a sufficient barrier in winter, or are there some brands that offer more durable alternatives?

TJsName said:

As for my serious question, I carry a lightweight bivy year round. Would this be a sufficient barrier in winter, or are there some brands that offer more durable alternatives?

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I presume you mean for use as a VBL inside a sleeping bag. The key feature is that it must be fully waterproof--waterproof breathable will not do the job.

You can make a simple sleeping bag VBL from large plastic bags. Ideally you can get one that is long enough to pull up to your neck and just large enough diameter to fit around you (perhaps a water heater bag). Otherwise, you can make one from several garbage bags--use a normal bag around your feet and cut the bottom off additional bags and tape them on the open end to increase the length of the tube. Or perhaps you can take some ~1mil plastic sheet and tape it into a tube closed at one end.

Also, lightweight plastic bivies are made from thin plastic of an appropriate shape. You could start with one and tape it closed to form a tube.

Doug