An improved dry stack insulated building block and block wall system, the block having a first and a second side wall, a central web interposed between the side walls, a pair of end transverse webs, and a pair of intermediate transverse webs. The side walls, central web, and transverse webs define a first, a second, and a third cell. A first cell core, second cell core, and a pair of third cell cores of insulative material are inserted in the respective cells. The cell cores have trapezoidal shaped ear members which matingly fit in notches in the transverse webs, the trapezoidal shaped ear members providing for the creation of a notch gap between the base of the ear members and the notch bottom of the respective notches, thereby accommodating crumbing in the notch bottom.