Heat flows (gains or losses) in a building are affected by solar radiation, internal gains and temperature differences. A prototype of a tool that evaluates solar radiation on building envelopes has been developed as an applet to run on the Internet. This tool analyses the relationship of the solar position with the surfaces of complex envelopes by evaluating the potential amount of direct solar radiation that each surface section receives as a function of the angle of each surface with the sun for specific days and latitudes. A fitness function rates the overall performance of the proposed building envelope at specific hours or whole days, permitting to compare alternatives. Because of its characteristics, this tool is a Very Simple Design tool that works interactively with the user during the design process helping to design solar responsive building envelopes.