Windows affect energy consumption for heating and cooling of buildings and their thermal indoor environment. Standard energy calculation programs do not enable an easy comparison of different window designs, even though this is very important in the early stages of the building design process. A user-friendly calculation program based on simple input data has recently been developed to assist engineers and architects during the process of selecting suitable windows for residential building design. The program is organised in four steps, which together represent an analysis of how windows in a specific building design perform with regard to energy consumption, thermal indoor environment, and cost. The analyses in the steps gradually increase in level of detail and support the design decisions throughout the design process. This document presents work done to validate the program and demonstrates that the effect of orientation and window size on heating demand has decreased in well-insulated dwellings of today. Windows can be positioned in these dwellings with a considerable degree of architectural freedom with no significant increase in heating demand. However, an even window distribution is recommended, because this provides a generally improved daylight level, a better thermal indoor environment, and a reduced cooling demand.