The development of a variable density precast concrete panel system is described that significantly improves the thermal efficiency of concrete buildings by simultaneously improving insulation and increasing thermal mass. Concrete from a single mix is stratified (defined as controlled segregation under vibration) to produce a lightweight, top layer and a dense, bottom layer. When erected, the panels have the lightweight layer on the outside providing insulation while the inner layer provides structural strength and improves thermal mass. Concrete mixes were stratified using different density aggregates, ensuring the rheology was in the correct range and using moderate levels of vibration. Waste materials such as fly ash, slag and expanded glass were used together with either Portland cement (PC) or inorganic polymer (IP) binders. Initial findings suggest that there is significant potential in using variable density concrete in buildings to improve thermal efficiency and comfort.