On 22-Dec-2003, Gary S. Idontwantspam@net wrote:

Good technical description at www.warmlite.com, specifically:
http://www.warmlite.com/vb.htm (Note to the prudish: close your eyes
when viewing this site)

Most of which is of secondary importance. The real reason for a vapour
barrier in a sleeping bag or winter clothing is to keep the insulation
dry (just like the vapour barrier in your house walls).

Polar adventurers have suffered from getting their down sleeping bags
soaked with water vapour that has condensed in the insulation (at the
point where the temperature in the insulation is the dew point).
Eventually, they end up with a bag that is frozen solid. This can't keep
you warm. One of the documentaries in the Banff Mountain Film Festival
showed this a few years ago.

Heat loss is irrelevant compared to this. However, it seems that the
manufacturers have picked up on it as a major selling point.

Mike