I have been reading a book about solar houses. One of the Figures in the book "Differing resistances of construction elements" shows, as an example, an uninsulated timber frame wall having an exterior temperature of 2 C and the internal temperature of 10 C.

With insulation the temperatures are 2 C and 14 C respectively. No calculations, no R values, no U values, etc.

Assuming a known outside temperature and knowing the construction materials of the wall how is the internal temperature calculated?