A modular insulated wall system for attaching to a wall having a plurality of insulated wall panels. Each panel has a top-side surface and a substantially parallel bottom-side surface. The top-side surface and bottom-side surface each are complementary stepped and substantially parallel in a first direction. The panels have a front surface and a substantially parallel back surface. The wall system also has a plurality of substantially parallel z-girts. The z-girts are engaged with the stepped surfaces of the plurality of panels. The wall system also has a plurality of substantially parallel support members in a second direction. The support members affixed to the z-girts and abutting the back surface of the panels.