Originally Posted by
cyccommute
It's not an either or situation. A tent is made to protect you...period. Part of that protection is to provide shelter from rain and wind and cold. It won't do a whole lot for the cold by itself but it keeps the wind from sucking the heat away and keeps the inside warmer. Condensation isn't much of a problem here where the relative humidity is extremely low...thin air doesn't hold much moisture.
To use a winter time bicycle analogy, a tent is like a wind shell over inner layers. The inner layers will keep you warm but the wind shell will keep the wind from sucking the heat away. By themselves, the inner layers don't do enough to trap the heat and keep it near your core. Sure you have to deal with some condensation but it's better to keep the condensation near you than to have it stripped away, along with the heat it carries.
If you use your logic, the best way to sleep would be with out a tent at all. That way you'd get maximum ventilation, minimum condensation and you's have an infinite space to heat. It just doesn't work that way.
No need to take it to an extreme. I simply said a larger air space is better for keeping condensation at bay, and a tent air space is not there to add warmth from body heat as you stated. A tent is there to deaden the air space in windy conditions, but you want it to breathe, just like a waterproof shell if you a riding in it. The more air that gets pumped through, the better in terms of controlling moisture. A larger air space is better for that. It is still a dead air space. As much as a shelter protects from wind though, it's more to protect against precipitation. Again, if you are not finding your insulation to be enough, it's because you don't have enough bag, and likely really don't have enough pad.
If you look at a bivy set-up, like the current military modular system. It is measured for warmth using the CE standard for bags. The bivy adds 5F to the system as a tight outer for the Lower Limit rating. The temp rating for all bags meeting the standard is measured using a 5.0r value pad. The associated temp rating of any pieces will be slightly less with a lower r-value pad. That's the only place where a bivy or an outer has been measured for warmth value to the system. Basically, even that close to the body, the shell isn't accounting for much in terms of warmth and it is the least desirable form of shelter in terms of comfort and space.