That is a very good question. I've thought about that myself. One of the few drawbacks to tarp tents is that the wind is often blowing on you. However, the way most of them are designed, I think that is the main way that moisture is removed. I think a "super-bathtub" approach might work. In other words, extend the bathtub of a tarptent a foot or so. This would stick up in the middle of the tent (blocking direct wind) but still allow wind to flow over the top. This would not be connected to any wall and simply be a redundant piece of fabric sticking up into the air (attached by cords to the wall). That being said, I think there is a good chance that such a design might still lead to too much moisture. I've been meaning to ask Ron about such a design, especially with his new Refuge (http://www.sixmoondesigns.com/shop/shopexd.asp?id=53) which has big ventilation shafts at the top which might allow such a design to work.
I wonder if anyone has created a double walled "tarptent". By that I mean: Take a nice tarp design and suspend a mesh enclosure underneath it. The bottom of the mesh enclosure would be a bathtub floor with a foot or so fabric up the sides (and mesh from there). This is sort of like the MLD Bug Bivy (http://tinyurl.com/2tjbu8) only much roomier and containing the super-bathtup. I think this would be a very nice compromise between the world of double walled tents and single walled shelters. It would have the tarptent weight advantages of:
1. Lightweight poles (usually re-using the trekking pole(s)).
2. Lightweight materials (Silnylon for the bathtub, nanoseeum for the mesh and Cuben Fiber for the tarp). Since most of the material would be nanoseeum and Cuben Fiber, the weight would be pretty low (I assume). All this and the warmth and low condensation of a double walled tent.
I'm thinking as I type here, but you might be able to design this so that it also adds another great advantage of most double walled tents, in that it you could remove the top. To do that, you would design a "cord frame" for the tarp. For example, if you look at the Lunar Duo (http://www.sixmoondesigns.com/shop/shopexd.asp?id=47). You could create a cord frame consisting of a cord connecting the two poles and then three additional cords connected from the pole to the ground. Such a cord frame could be used to suspend the inner walls as well as allow you to just lay the tarp on top (suspending it to the cords or the stakes for tension). Since cord is very light (and strong) this would have a very small weight penalty over the design that doesn't allow the top to be removed.
In general, this would not be the most ultralight shelter in the world. However, I think there are a lot of folks who like stuff which is in between ultralight and standard. I think this would be much lighter than any existing double wall while providing all the benefits.
