Rammed earth provides a warm interior.  The addition of insulation in the core of the wall provides a thermal break that insulates a high mass interior wyth that will function as an enormous heat sink.  It is like living inside a masonry stove.  Heat from any source is stored in the walls.  The houses are so energy efficient that the sizing of the furnace may be reduced significantly.  The minimum R-value of an insulated RE wall is 25.  But that number does not reflect the additional efficiency of the thermal mass.  The mass also keeps a structure cool during the hot summer days without the need for air conditioning.