Current design trends often result in extending curtain wall systems beyond the thermal boundary of the exterior enclosure. A common example of this condition is where a curtain wall system extends above the roof and becomes the parapet. Other examples of this type of situation include where the curtain wall extends past a floor line to create a soffit enclosure or past an adjacent perpendicular wall to create what is often referred to as a “wing wall.” These types of conditions are not considered in the standard test methods that define the system's thermal performance and condensation resistance and can often lead to installations with increased heat loss and reduced condensation resistance. These types of conditions may deviate from the published performance of the system and impact the overall energy efficiency of the building enclosure. The current industry standard computer modeling methods to determine the thermal performance and condensation resistance only include two-dimensional computer modeling. Although this may be sufficient for the performance in the field of a curtain wall system, it may not adequately address these unique conditions. The two-dimensional nature of the evaluation cannot include the linear transfer of heat flow in the third dimension through the mullions extending beyond the thermal envelope. Laboratory testing of these complex project-specific conditions is often impractical due to cost and schedule. Additionally, once the condition is installed it is often too late to change the design. This paper evaluates the three-dimensional effects of heat flow through curtain walls using three-dimensional modeling. The relative impact on thermal performance and condensation resistance is compared with the published performance of standard curtain wall systems. The paper also reviews installation methods to decrease the heat loss and produce more resilient systems. Two-dimensional computer modeling is utilized to compare the results with the three-dimensional modeling procedures.