Various super-insulated retrofit strategies with high Rvalue wall assemblies have been proposed and investigated in North America since the energy crisis in the early 70s. Even today, research and development activities in high thermal performance building envelopes have been very popular due to increased concerns on issues related to installed costs, environmental impacts (sustainable or green construction), saving additional building energy consumption (zero net energy building), and higher thermal and occupant comfort expectations. In order to reduce the heating and cooling demand in structures, it is often more cost effective to retrofit existing building envelopes with the addition of improved fenestration. This paper explores the optimum location of replacement windows in retrofit applications with thicker building wall systems due to the addition of insulation to existing walls. Replacement windows may be installed at different depths in the wall aperture (e.g., close to the interior, in the middle, or close to the exterior of the window opening). This work discusses the best thermally performing locations to install the replacement windows in two cases of high R-value wall retrofits: those using vacuum insulation panels and those using the thick layer of conventional plastic foam insulation (e.g., extruded or expanded polystyrene). Finite difference and finite element computer modeling is utilized to analyze the thermal zone of influence around the window openings and to determine the most energy-efficient position to install the window. In order to differentiate between the different window installation systems, the linear thermal transmittance of the windowwall connection, as determined by ISO 10211 (2007b) is also calculated and presented.