Abstract The design process for an intelligent, energy conscious building which was built, along with the design tools that were applied, is presented. The building, situated in the hot–humid climate of Rehovot, Israel, houses the laboratories and offices of the Weizmann Institute's Environmental Science and Energy Research Department. Alternative bio-climatic design options were proposed and evaluated throughout the detailed design stage. A building energy performance index (BEPI) was established for each alternative. This index reflects the total amount of energy consumption for heating, cooling, ventilating and lighting used per square meter of floor area. Thermal modeling for the different design alternatives were carried out by means of an hourly dynamic simulation model. The model solves simultaneously the heat transfer equations through all exterior walls, taking into account the thermal mass of each external wall as well as internal partitions. The model was extended to include hourly calculations of daylighting and geometrical shading coefficient of the windows, as well as automated and `smart' control strategies.