Several people have been talking about traditional Japanese-style earth walls which seems like one of the most ecological solutions. The walls are breathable and if I add some rice husk ash to the mix, (which a neighbor has plenty of), I should be able to improve on the insulation properties of the walls.
Have a look at this video (Japanese) where a Company President talks of how his love for earth walls has resulted in a unique solution to insulation. He puts foam on the outside! Company itself looks pretty innovative too! Check out the website http://www.nanbakenchiku.co.jp/