What are the thermal R-values for wood?
USDA Forest Products Lab Wood Handbook, Table 4-7 titled “Thermal conductivity of selected hardwoods and softwoods” lists thermal properties for various species of wood. The table is available online as a PDF file at:

The Canadian Wood Council has a technical bulletin on the thermal performance of light-frame wood assemblies. It discusses an R value of 1.5/inch of thickness for wood products:

Also, from 1993 ASHRAE standard, the R value per inch of thickness is about 1.1 F*ft^2*h/BTU per inch of thickness.