I don't know the R value of wood but its generally acceptable in construction. Log homes for example.
I believe R value is defined as the insulating equivalent of so many inches of wood (pine maybe?). So R-10 is the equivalent of 10 inches of wood. In log homes (I have one), additional insulating effect comes from the thermal mass of the log walls. When I submitted the plans for a building permit, I had to include additional information, provided by the kit manufacturer, to show that the insulating equivalent met local code requirements for exterior walls.