This paper presents the results of an original design process-oriented user study conducted on an interactive expert system specifically developed for full year, climate-based daylighting design support. The aim of the study was to determine how well its decision-making algorithm would work when independent human interactions and decisions were included into the process through a two-step process, first based on design heuristics then using the expert system. The results of this evaluation demonstrate that the expert system is generally successful as a performance-driven design tool and as a method for influencing and educating designers in ways that they can improve the daylighting performance of their designs. It also demonstrates the relevance of the proposed user study to validate design support tools that account for the unpredictability of the inherently ill-defined architectural design process itself.