In my simple straw bale cabin I did not use any clay on the inside or outside. 2x4's 2' on center on the outside then house wrap and clapboard siding. The frame was vertically boarded with the bales going on next.

I would dip the whole bale in a clay slip next time, to seal it up, it would be a messy job I would stuff any voids with wads of loose straw mixed with a slip.

Tim