If you are talking straw/clay mix, I would go with cob ( included sand too ). You don't really need to use anything else unless you want to seal it, then you could use an earthen plaster. Like I said, check out the book I suggested or one called, hold on now, Earthen Plasters. A clever name, huh? Your walls are pretty thick, and you should cover it with a plaster that would harden well. If you have a good foundation and a good roof (could make it a really cool living roof), then your walls should be safe from invasion. Another nice thing is that is a wall that breathes, so you don't have to worry as much about the "sick house" effect. We actually thought about building a house of cob (okay, my hubby did) but it would take us for-ev-er. But I would like to try doing our own little spa or meditation room, or maybe just a garden wall to start off. There are a lot of people in the northwest who have done this. If you ever get down to Eugene, there is a place called Cottage Grove and there are people there who have workshops to teach this. I would suggest checking out a structure made of cob before doing this though. It gives great opportunity to be creative and unique.
It is cheap, but don't expect it to be easy.
