This paper describes the development of a thermally activated ceiling panel for incorporation in lightweight and retrofitted buildings. The system allows use of renewable energy sources for the heating and cooling of office and industrial buildings. The design for the new ceiling panel exploits the properties of the phase change material (PCM) paraffin. Its high thermal storage capacity during phase change-up to 300 Wh/(m(2) day)-enables the overall panel thickness to be limited to a mere 5 cm. Active control of the thermal storage is achieved by means of an integrated water capillary tube system. The research project also included the development of a numerical model for computation of the thermal behavior of wall and ceiling systems incorporating PCMs. Simulation calculations were performed to determine the necessary thermal properties of the ceiling panels and specify requirements for the materials to be used. Laboratory tests were performed to verify the system's performance and a pilot application is soon to be tried out in practice. (C) 2004 Elsevier B.V. All rights reserved.