This paper discusses the development of a methodology to quantib windowperformance on human thermal comfort with eight case studies of different window systems. The methodology is based on the Building Comfort Analysis Program that was developedduringASHRAEresearchprojects6 57and907. Human thermal comfort is achieved by maintaining a heat balance between the human body and the environment. This is accomplished by convection and radiation. In a room there are other associated issues@ such as conduction through the walls@ convection past the walls@ and radiation between the various surfaces@ that will influence comfort. Thepresence of a window system adds complexity to the problem as they have different optical and thermal properties. This paper expands on the existing three-dimensional mathematical model by incorporating a method to read in a user-de$ned window and also read in the properties obtained from the widely available Window 5.I TMprogramA. representative case study is analyzed to illustrate the means of quantifying the results. Results have been quant??j??ed as percentage of comfortable floor area.