Originally Posted by
License2Ill
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.
The whole point of a dead air space is so that wind doesn't strip the moisture from the tent. Air doesn't carry that much heat but water carries a lot. The water you breath out carries your body heat with it. When it condenses in the top of the tent, it releases heat which goes to heat the dead air of the tent space. A double walled tent then allows the condensed water to flow out under the rain fly and drip onto the ground. A single wall tent condenses water just the same but the water doesn't have any place to go so it drips back on the tent occupants. Now it's cold water which makes the occupants less comfortable.
The larger the tent space, the less heat recovery you get from the moisture condensation and the heat that the bodies in the tend put off. I've camped with 3 people in a Eureka Timberline which is a largish 2 man tent. It was quite comfortable and the temperature in the tent was tolerable. I've also camped in a 4 man Eureka Alpine Meadows which is about twice as large as the Timberline and it was no were near as comfortable. Same bags and pads in each tent.
Originally Posted by
License2Ill
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.
The tent in question adds 5 F compared to what? The open air or another tent? Even without knowing what the tent is compared to, 5 F is a huge temperature difference and just goes to show that the tent adds some value to the heat inside the tent.