The extreme-ultraviolet (EUV) signatures of heated material in the solar atmosphere have been continuously monitored by the Atmospheric Imaging Assembly (AIA) on board the Solar Dynamics Observatory (SDO) for over five years. These high temporal and spatial resolution images combined with the computational fast regularised method (Hannah & Kontar 2011, 2012) allow maps of the underlying emission measure distribution (EMD) to be obtained. From this the thermal evolution (both temporally and spatially) of non-flaring active regions can be studied. Here we present an initial analysis of the evolution of EMD maps in non-flaring active regions to probe the heating mechanisms in the solar atmosphere.