Details and the building code

Good question. That layer in the wall assembly is a nail base for the wood shingles; it is non-structural. There is a note in the introduction to all of our wall assemblies that using continuous rigid insulation as exterior sheathing means an alternative approach to achieving required shear resistance: corner structural sheathing (with reduced thickness rigid insulation applied over top), let-in wood or strap metal diagonal bracing, or inset shear panels.

Also, the sheathing nail base--and any drainage mat for wood shingle siding--can be replaced with horizontal wood furring strips or drain-through rigid plastic vent strips, such as Battens Plus. These relatively new products are being introduced for drain-through venting on roofs, but many innovative builders are using them on walls as well.

GreenBuildingAdvisor.com has added information to all roof assembly details that also have wood shingle siding, responding to your question.