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What are the advantages of a vapour barrier?

~~I was recently reading an article on <a href="http://www.ukclimbing.com/articles/page.php?id=6903">UKClimbing by Andy Kirkpatrick</a> about clothing in winter. It talks about vapour barriers:~~
~~<blockquote>~~
~~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.~~
~~</blockquote>~~
~~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?~~

I was recently reading an article on <a href="http://www.ukclimbing.com/articles/page.php?id=6903">UKClimbing by Andy Kirkpatrick</a> about clothing in winter. It talks about vapour barriers:
<blockquote>
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.
</blockquote>
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?

winter clothing

winter clothing vapor-barrier-liner