A simplified building simulation tool to evaluate energy demand and thermal indoor environment in the early stages of building design is presented. Simulation is performed based on few input data describing the building design. HVAC systems and control strategies. Hourly values for energy demand and indoor temperature are calculated based on hourly weather data. Calculation of the solar energy transmitted through windows takes into account the dependency of the total solar energy transmittances on the incidence angle. shades from far objects and shades from the window recess and overhangs. Several systems including heating, cooling. solar shading. venting. ventilation with heat recovery and variable insulation can be activated to control the indoor temperature and energy demand. Predicted percentages of dissatisfied occupants are calculated for a given time period to support decisions concerning the thermal indoor environment. The simplified building simulation tool gives reliable results compared to detailed tools and needs only few, input data to perform a simulation. The tool is therefore useful for preliminary design tasks in the early design stages where rough estimates of the building design are given and rough estimates of energy use and thermal indoor environment are needed for decision support. (C) 2004 Elsevier Ltd. All rights reserved.