There is significant in the GTA for GHG emissions reduction through energy retrofit measures of the more than 2000 post-war multi-unit residential buildings. Overcladding is an effective energy reduction strategy; however, it is crucial to properly detail window installation to avoid thermal bridging in a retrofit situation, as there may be excessive heat loss and condensation at this junction. This paper examines the thermal bridging potential at the window-wall interface in an EIFS overcladding retrofit scenario for a typical MURB retrofit. The research used the software THERM to compare influence of three typical window-wall interface on the energy performance of the window and wall. The analysis examined the position of the window within the frame, insulation placement around the window perimeter. It was found that window placement within the wall section and detailing at the opening do significantly affect the wall’s overall thermal performance, determining that design improvement should be considered and quantified in retrofit energy reduction strategies.