To improve the energy performance of Toronto's post-war multi-unit residential buildings (MURBs), these buildings must be energy retrofitted. Here, a novel energy retrofit strategy employing air-source heat pumps (ASHPs) operating in enclosed balcony spaces is assessed. The enclosed balcony provides a thermal buffer zone (TBZ) which can improve the coefficient of performance of the ASHP compared to typical exterior operation by accessing captured solar gains. The estimated energy savings associated with implementing this retrofit strategy was determined using a hybrid modeling approach. A calibrated energy model of a 1968 MURB represented the base case energy consumption. Then, a supplementary algorithm, developed based on laboratory testing, was used to determine the quantity of energy that could be extracted from the TBZ and delivered to the north- and south-facing suites as well as to hot water storage. The modeling exercise yielded estimated whole-building energy and greenhouse gas emissions savings of 39% and 45%, respectively. Due to utility prices at the time of writing, the energy cost savings do not present a compelling financial case. However, the qualitative benefits of this retrofit strategy include suite-based control for occupants and the ability to sub-meter space heating energy use at the suite-level. (C) 2014 Elsevier B.V. All rights reserved.