A very interesting thread, and since I have only very limited experience of spending time outdoors at those kinds of temperatures I am learning a lot of new stuff here.
I think that the lowest I've experienced asleep is probably not less than around -5°C to -7°C, and in waking hours probably not less than -20°C on a very few occasions (though -10°C to -15°C quite a few times in both the USA and in Russia).
Now it seems to me that if you have a vapour barrier layer over a thin base insulating layer, then the thin base layer will get wet. Grey Owl has explained that even at rest this will happen as the body tries to maintain a humid environment around the skin. Next you have some layers of insulation around that which stay dry because the vapour barrier layer prevents any body moisture from getting into it. Now at -20°C the air is going to be bone dry, and so that outer layer will stay dry and not lose any of its efficiency through damp.
I can understand all that, and I think it sounds very reasonable.
Now to the bit about waking up and wanting to take off the vapour barrier layer... I think that I would want to get out of the clammy base layer, and into dry underclothes. I think that at -20°C, if I was out of the wind, I would be able to do that in less than a minute, with a good insulating mat to sit or stand on.
This would leave me comfortable, back in daytime clothing that I can open and vent to allow me to dissipate "metabolic heat" generated through exertion. Leaving my night-time base layer exposed to the -20°C temperatures which would instantly (or near enough) crystallize all the water as ice. Now I can beat the frozen stiff clothing with a stick and shake out all the ice, leaving it dry. This would be the change of underclothing for the next morning.
I think that the lowest I've experienced asleep is probably not less than around -5°C to -7°C, and in waking hours probably not less than -20°C on a very few occasions (though -10°C to -15°C quite a few times in both the USA and in Russia).
Now it seems to me that if you have a vapour barrier layer over a thin base insulating layer, then the thin base layer will get wet. Grey Owl has explained that even at rest this will happen as the body tries to maintain a humid environment around the skin. Next you have some layers of insulation around that which stay dry because the vapour barrier layer prevents any body moisture from getting into it. Now at -20°C the air is going to be bone dry, and so that outer layer will stay dry and not lose any of its efficiency through damp.
I can understand all that, and I think it sounds very reasonable.
Now to the bit about waking up and wanting to take off the vapour barrier layer... I think that I would want to get out of the clammy base layer, and into dry underclothes. I think that at -20°C, if I was out of the wind, I would be able to do that in less than a minute, with a good insulating mat to sit or stand on.
This would leave me comfortable, back in daytime clothing that I can open and vent to allow me to dissipate "metabolic heat" generated through exertion. Leaving my night-time base layer exposed to the -20°C temperatures which would instantly (or near enough) crystallize all the water as ice. Now I can beat the frozen stiff clothing with a stick and shake out all the ice, leaving it dry. This would be the change of underclothing for the next morning.