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.