[Cob] Wall thickness

[Gene Forster]

Hello all
My project this summer is building a roughly 6foot x 8 foot greenhouse using cob for the walls.
It would be around 7 feet tall at its highest point. The shape can be described as a rectangle with curved
walls and corners, but not quite oval.

My questions are 1)How thick should I make the walls at the base?
 and 2) Could a person build two thinner cob walls and stuff straw in between for insulation?
Gene