I'm attaching mine to the back of my home. Our code for foundations state the depth must be 36". I will be putting up the foam board around the foundation. The fourth side will be up against the house. I would think this to be sufficient...especially if you plan on heating the space. You could always double up on the foam boards around the foundation as well. I don't believe you would like the results of laying the boards on the ground. Just my two cents.

Thanks - I think you are absolutely correct. A friend mentioned that laying styrofoam on the ground would actually diminish the ability of the ground to cool the house in the summer and actually warm it in the winter as ground temperature is mid fifties. So putting boards around perimeter makes best sense because that will prevent the colder temps of winter from travelling in without losing the other good effects of normal ground temperatures.