Looking for advice -Interior Wall Covering over horizontal timber

[D L Bahler]

Back to the OP

reviewing information both from the codes and regarding wood performance and application of code standards to log building (this isn't log building, it's structurally a timber frame but filled with solid wood so a log building is a reasonable analogy) I've determined the best approach for interior treatment would be to apply 1 1/2" thick t+g wood planks vertically, and possibly some sort of material over the inside as well (for looks). This, I think, should bring my walls up to the standards of fire resistance required by my occupancy. Actually it should bring me well above them. 5/8" type X drywall would perform similarly, but in this case due to the possibility of water incursion wood is better.