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?