Dug Out Tipi

I've been considering the challenges of living in a tipi and one concern is how to keep it well insulated and warm in the winter to avoid burning more soft wood than necessary. I came up with a possible solution, and am wondering if anyone can point out any issues and improvements.
The idea is to dig a few feet down in the tipi, potentially using the ground as a thermal mass. Other things could be added like a pipe pulling air from outside up through the fire, a wood stove or a rocket mass heater but for now I want to focus purely on the dug out aspect of things and how it would affect the temperature inside.  

My most immediate concern would be moisture, but that issue varies from region to region. Here, simply digging underneath would set you up for a very wet winter. Depending on the severity this could be remedied by digging in an additional foot or so from where you want the floor to be and setting up rocks and a drainage pipe going downhill, then putting the floor on top of that. The actual temperature would probably be somewhat improved as long as this is adequately addressed.

Howdy Phin, welcome to permies!
Have you seen this thread ?

lab Tipi

I've known several people who have wintered comfortably in Tipis up in the Canadian Rockies and in the Yukon.  What they did for insulation was either loose straw up against the Tipi for 5 feet with plastic over it to keep the straw dry, or spruce/fir boughs heaped up against it, with tarps keeping the wind out of the boughs.  A floor of pallets is handy, but not necessary, according to them.   Both of these tipis had a tarped off entrance way/mudroom/fire wood storage area.  Both burned far less wood in the stove than most homes because the stove is central and the space is small.  

I see few reasons why you should not try your design, besides the labor of digging the pit.  Obviously there will be potential issues of airflow for the fire, and the moisture issues which can be dealt with as mentioned above....  

............The other concern that I need to mention is that combustion gasses (poisonous) tend to sink into low areas and collect, and kill people.  Keep your sleeping area higher than the low pool area, like the Inuit do with their sleeping platforms in their igloos.    

instead of trying to dig down to a warmer soil, how about using rigid insulation? I'm thinking something like what is done for shallow foundations in cold climates to reduce the use of concrete. There is a vertyical portion of rigid insulation that extends a few inches below grade, protecting the interior space; then a horizontal section, perhaps 18-24 inches that keeps the frost from driving under the protected interior space. The challenge is doing this in a curve to follow the tipi.