Log Cabin Heat load design

Wayco Wayne_2

I;ve been asked to help design an old log cabin's heating and cooling system. It may be 200 years old but has been updated with drywall on the inside and siding on the outside. There is also a kithcen addition on the back where they left the beautiful old cabin wall exposed. Very nice. I've reead that the R-value of wood can be anywhere from 1 to 1.5 per inch, but that the excellent thermal mass properties of wood should also be considered. I own Wrightsoft heat load software and am wondering how to accurately simulate the R-value plus the thermal mass of these walls. Should I go the the masonry screens for wall materials? Inquiring minds want to know. Thanks for your shared knowlege and opinions in advance.