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A pretty common trick when winter camping is to dry anything that gets damp/wet (e.g., base layers and socks) in your sleeping bag (cf. How do you dry wet socks when camping/backpacking if it's raining and the socks cannot be hung to dry outside). Similarly, people will also sleep with their boots to keep them from freezing and try and dry the liners out.

Where does the water go when sleeping with wet/damp clothes? Aren't you just transferring the moisture into your sleeping bag?

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    I can't believe this would work, for exactly the reason you've identified. This would be an especially bad idea if you're using a down bag, because down loses its insulating ability when it's wet. – Ben Crowell Feb 8 '16 at 15:12
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    This method also mentioned in this answer. Note that they mention using a synthetic sleeping bag. I think it turns into humidity and some escapes and some probably condenses in the sleeping bag. – Chris Mendez Feb 8 '16 at 15:25
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    Having slept in a synthetic sleeping bag inside a biivy bag on a cold night (I was reminded of this by a question here the other day) it's quite obvious that a lot of moisture can leave the sleeping bag. The inside of my plastic bivvy bag was properly wet from condensed sweat and any damp that got in on whatever I was wearing. So if you're in a tent or hut that moisture will have soemwhere to go before it condenses. – Chris H Feb 8 '16 at 16:25
  • Evaporation (which is where the water would go) consumes large amount of heat. Any moisture added to the sleeping bag will consume some of heat generated by your body. Personally I would rather be warm with damp cloths the next day than hypothermic with dry cloths. – user5330 Feb 8 '16 at 20:51
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    @mattnz the pros and cons seem hard to evaluate since dry socks could be the difference between frostbite/trench foot. Do you have numbers about how much "heat" is required. Presumably it can be offset by more food or a warmer bag. – StrongBad Feb 8 '16 at 21:00
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The water will either absorb into the sleeping bag materials, and or evaporate, or absorb then evaporate. The rate of evaporation is determined by the humidity inside the sleeping bag, and this is determined by the rate water vapor escapes into the outside air.

The latent heat of evaporation of water is around 2264 kilo Joule/Kilogram (629 WH/Kilogram.) - a sleeping person generates about 60-80W, so if you add damp cloths containing 100ml of water (1/2 a cup) to your sleeping bag, it will take all the heat you produce in 1 hour, while asleep, to evaporate it. It won't normally evaporate at this rate, as the humidity will quickly raise to 100%, however, in the first tens of minutes, the moisture will be sucking the heat out of you body at the same rate you produce it - you will become colder. In many cases, you will spend the rest of the night shivering, struggling to recover the lost heat.

So effectively the water is lost by turning the water + body heat into water vapor which then escapes though the sleeping bag into the atmosphere, and some will still absorbed in the sleep bag materials.

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Yes, the moisture is transferred to your sleeping bag. But unless you are sleeping in a plastic bag, it keeps going and makes its way out of the sleeping bag into the air because sleeping bag shells are porous.

By the way, there is no reason not to do this inside a down bag. Even if you sleep naked, your breath and body produce plenty of moisture. Your socks and gloves aren't going to make a huge difference.

Whether you use a down or synthetic bag, the moisture accumulation makes your bag less efficient every night you use it. This is probably more pronounced with a down bag, but probably not by much. My solution is to let my bag air out during the day (or for as long as possible between the time I wake up and break camp). It won't make your bag 100% dry, but it will do a pretty good job.

Winter air is really dry, after all, so it sucks moisture out of your bag pretty quickly.

Edit: There seems to be some misconception about how warm you will be with synthetic materials versus down. First, I know you can do this with down because I dry my gloves and socks inside my down bag every night when I am winter camping. I've been doing it for years in temperatures as low as -30ºF.

Second, while synthetic materials do retain some insulation value when wet, a 0F bag doesn't stay a 0F bag when it's damp. Try soaking a Thinsulate jacket, wringing it out, and then walking outside with it. Not warm, is it?

The point is, you want to avoid getting your sleeping bag soaked no matter what it is made of. But a little extra moisture from your gloves and socks isn't going to make any significant difference, especially since airing out your bag during the day is a pretty effective way to dry it out.

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    I am pretty sure it does not magically just keep going. Whether it makes it out must depend to an extent on the permeability of the bad, and the relative temperatures inside and outside the bad, and maybe other factors. Would love to see some numbers explaining how much stays in the bag and how much gets out and how effective airing out the bag is. – StrongBad Feb 8 '16 at 20:10
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    The moisture doesn't "magically" keep going. It keeps going because of evaporation. To clarify, it doesn't happen instantly. The shell of the bag definitely slows it down. My winter bag, for example, has a Gore-Tex shell, which is less permeable than a typical nylon shell. So moisture is slower to evaporate from my bag. At least until I open it up to air it out. But either way, it gets out. – samglover Feb 8 '16 at 20:29
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    If it's not below freezing, why would you worry about keeping things inside your bag so they don't freeze? Just hang them up to dry overnight. – samglover Feb 8 '16 at 20:48
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    Moisture ≠ wet. Look, I go camping in a down bag every year in the Boundary Waters. It's been as cold as -30ºF. I put my socks and gloves on my chest to dry them out every single night. It's never been a problem. – samglover Feb 9 '16 at 18:50
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    Stuff gets amazingly dry if taken with you into the bag. I do this all the time, but don't overdo it. A pair of trekking socks and a wool shirt is fine, or a pair of boots. Maybe your pants. If you take it all with you, you'll have a miserable night and stuff will still be damp in the morning. I have a down bag BTW, don't worry as long as you don't bring anything dripping wet or too much. – Stian Yttervik Nov 19 '17 at 21:40

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