Many areas that have a building code that recognizes straw bale construction call for protection against rain splash. The most common form of protection comes in the use of roofing felt or building paper. Many jurisdictions require that the protection be wrapped around the bottom courses of the bales in a wall assembly, usually extending up to the third course of bales. The biggest problem with this requirement is the wrapping of the bales with the paper.
If moisture should get behind the protective paper, it will be trapped inside the roofing felt/building paper and will be absorbed into the bales. To avoid this problem, do not wrap the bottom edge of the protective paper. Instead, allow it to extend into the weep screed (base plaster flashing) so that any moisture on the paper’s surface can drip out of the plaster and out of the house.
If your local code does not require the use of a rain splash protection, I suggest you leave it off the building and simply design so that the overhangs will be adequate to provide protection against rain and then use gutters to minimize rain splash. In fact, I strongly recommend the use of gutters and larger overhangs regardless of whether you use roofing felt/building paper or not as they are by far the best defense against rain.