Build your foundation at the very least 8 inches higher than ground level. Make it higher if you live where it's wet, where there's a lot of windblown rain, or if you simply love stone work. If you are making a two-story building, make the foundation at least 16 inches high. A common height for a foundation is 18 inches above ground level. You can check with the local builders to find out what they suggest in your area.
Mortar, concrete and earth slowly draw up moisture. One would think that the obvious solution to this would be to put something waterproof somewhere in the wall to block the moisture from the ground. Easier said than done! Wherever there is a moisture barrier and temperature differences, moisture condenses. Moisture barriers limit the ability of the wall to breathe. If moisture gets in, as it invariably does, a barrier will slow down the drying process.
I haven't noticed any moisture problems in the structures we've built with stone foundations, thanks to their good drainage systems. Creating a good drainage system is the best way to keep your foundation, floor, and walls as dry as possible.
Another minus to using a moisture barrier is that it can weaken the connection between the cob and the foundation. There are two schools of thought in the natural building fields on how to think about moisture barriers.
Some say the breathability is vital, never use a moisture barrier. Others say they work OK on top of the foundation to keep the moisture from wicking up into the wall.
The most natural type of barrier I've heard of is flat stones embedded on the top of the concrete or soil cement foundation to stop the upward flow of water from getting into the wall. Remember to slant the stones toward the center of the wall and towards each other so the cob doesn't slide off the wall. (See illustrations on page 49.)