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I was recently reading an article on UKClimbing by Andy Kirkpatrick about clothing in winter. It talks about vapour barriers:

I’ve played around with vapour barrier socks over many years, and think just having a cheap thin shopping bag next to your skin, then a good sock over the top, is a no brainer on any super cold, or extended routes. Having it next to your skin means you don’t end up with stinking liner socks, and you also have a little more friction between the layers when front pointing. You really get the advantages from a vapour barrier system when your day becomes extended, say that 12 hour winter route on the Tacul draws on to a 24 hour epic, where saturated socks suck the heat out of your feet with the combination of nighttime temperatures and fatigue.

I don't really get this, I would of thought that keeping moisture next to your skin would be a bad thing.

What's the thinking behind this? When should they be used?

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When to use it: An vapour barrier is used when the temperature is very cold and when a wet down-suit/sleeping bag, clothes or shoes can cause serious hypothermia. It's also used if you can't dry your stuff because of no sun, cold temperature or bad weather, so everything stays dry at least from the inside.

Thinking behind this: When the isolation layers get wet they are not functioning well or not at all. So it's better to have the sweat directly on your skin and dry it off in the evening/morning with a small towel than have all layers wet and cold. And wet feet in a insulated shoe are not the same as wet feet exposed to cold wind. And the theory goes that you will stop sweating once your skin is completely wet. So your feet will be damp, but not soaking

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    Also, the theory goes that you will stop sweating once your skin is completely wet. So your feet will be damp, but not soaking.
    – Fred the Magic Wonder Dog
    Commented Jan 20, 2015 at 15:53
  • Thanks @FredtheMagicWonderDog ! I've added your point to my answer
    – ibex
    Commented Jan 21, 2015 at 8:35
  • @FredtheMagicWonderDog Where is evidence for the theory that you stop sweating when your skin is completely wet? You still sweat significantly even when swimming, it's just harder to observe: swimmingscience.net/2010/05/sweat-loss-in-swimmers.html
    – shimizu
    Commented Jan 21, 2015 at 14:39
  • I have no evidence, but I have read that statement in books such as Extreme Alpinism by Mark Twight. I guess I wasn't clear, it's a hypothesis that exponents of vapour barrier put forth. I haven't been able to find an experimental evidence for it.
    – Fred the Magic Wonder Dog
    Commented Jan 21, 2015 at 15:32
  • Lot's more info on VB's home.comcast.net/~pinnah/DirtbagPinner/vb.txt
    – Fred the Magic Wonder Dog
    Commented Jan 21, 2015 at 15:43

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