ABSTRACT Building Performance Simulation can record av significant victory. In the city of Curitiba, Brazil, situations of poor access to both diffuse daylight and direct solar radiation in high density zones led the City Hall to consider the adoption of a simplified solar envelope concept. In order to evaluate the effectiveness of such change over a broad range of possible situations, a computer simulation code based on the raytracing technique was developed. Results were presented to the City Council in December, 1999 and supported the issue of new zoning regulations in January, 2000, despite the resistance offered by the building industry.