Originally Posted by
UnCruel
The condensation occurs on the tent walls, where it is cold. It releases its heat into the tent wall (because it's cold). Most of that moisture came from the occupant's exhalations, which means that the heat being transferred is heat that the occupant has already lost. Sure, taken in isolation, that heat contributes a positive for the occupant, but really it is just part of the overall transfer of heat being lost outside through the tent walls.
The heat released isn’t directional nor are the tent wall particularly good at transferring heat. Not much better than the air surrounding the tent. The tent, being water proof, also doesn’t lose heat to the outside due to evaporation which would reduce the heat in the tent.
Condensation really does make us cold, and it fits fine with thermodynamics. Primarily, liquid water defeats the effectiveness of the insulation around us, because heat conducts through water faster than it transfers through the air gaps in the loft of the insulation. (The effectiveness of insulation while wet varies depending on the material.) Secondly, water on skin conducts heat away from us faster because of the relatively high density and specific heat of water.
Liquid water defeats the effectiveness of insulation but inside the tent the water isn’t in the form of liquid. It is vapor and, as such, can carry more heat than dry air. Think of what humidity does to summer weather in much of the eastern part of the US. Much the same happens in a tent and the smaller the space, the greater the humidity.
As to when the water condenses, A well designed tent will direct the water droplets away from the inside of the tent. The condensation won’t wet the insulation of the sleeping bag. The amount of water condensed is also quite small. Even here in Colorado where the temperature differential between the tent outside and inside is greater…thus increasing the efficiency of the condensation…the amount of water condensed is probably less than 100 mL (about 1/2 cup). Even on a multi-day trip, the amount of water picked up by the sleeping bag is minimal.