The role of advanced isothermal heat storage systems in buildings is discussed. A storage system encapsulated with phase change materials in which energy is absorbed in the hot period and released in the cold period is analyzed. The thermal behaviour of isothermal heat storage composites is examined using numerical techniques. Two methods of heat transfer with latent heat storage are described in the first part. Based on the initial results, the "effective heat capacity" method was selected and implemented into ESP-r. Numerical studies on the effect of isothermal storage of solar energy in specific building material components are discussed in the second part. Numerical simulations were conducted for two cases of multi-zone, highly glazed and naturally ventilated passive solar buildings. PCM-impregnated gypsum plasterboard was used as an internal room lining in the first case study and transparent insulation material combined with PCM was applied for the external south-oriented wall in the second case study. The behaviour of a TIM-PCM wall and its influence on the internal surface temperature are estimated. Air, surface and resultant temperatures are compared with a "no-PCM" case for both case studies and the diurnal and the seasonal latent heat storage effect is analyzed. (C) 2009 Elsevier Ltd. All rights reserved.