The whole dynamic between the battle of cold exterior air and staying warm is displayed in the way Goretex is used to correctly stay warm and dry during heavy exersion in the cold and wet. Many people overdress under the goretex then open all of the vents and still sweat! The trick is to strip down under the goretex to avoid over-heating and seal all of the openings of the goretex. This will create the insulating air pocket and at the same time keep the sweat in a vapor state so that it can pass through the goretex. Not overheating because of excess insulation will eliminate the formation of sweat that is produced to just cool you off. See Mark Twight - Extreme Alpinism Pg 83. Sure, the air layer is not an effecient insulator but effecient enough to do the job and just enough so - that's what makes it dynamic.

Sleeping bags need to be monitored in the same way but are the opposite in that there is no exertion going on so this is where all that insulation comes in. The less the bag drapes the more it needs to be sealed as things get colder. I have been in situations where I had to dry off wet clothes inside light dryloft down bags ( Apache weight ) while frost is falling off the tent interior because the wind is shaking it off and then melting on the bag. The bag in this situation would be a wet mess if it was not dryloft or the equivalent. You only do it by keeping that bag sealed tightly and by burning as many calories as you can. It may be a good idea to vent the bag slightly too and let some of that moisture turning to vapor out the easy way but you risk losing heat. Some will ask why not vent the tent to keep moisture off the walls so it does not freeze. Venting the tent in these situations only lets more snow in. There is much bad advice about venting WP/B tents - just like WP/B parkas. You overdo it and you let in so much cold air it will cause condensation/sweating. The snow would probably pile up on the bag and stop the breathing process that is going on that is drying my clothing - the only clothing I have because it's a carry over. The dynamic here is interesting because even though it is frosting and basically snowing in the tent, the Bag surface is staying warm enough to stay free of ice and breath. All of this works because the tent is sealed to the max and so is the bag. The critical thing is to maintain the down insulation and dryness and this only happens if the whole dynamic 'motor' is kept in motion.

I was in a situation like this on top of Rainier once, trapped for 2 days. I had a light dryloft bag that stayed dry the whole time and someone else in our party in another goretex tent had a non protected down bag - heavier than mine. He ended up hypothermic and we had to almost drag him down the mountain when things cleared. My partner in my tent in this situation actually did quite well with a light synthetic bag.