In this paper the effects on the heating and cooling load resulting from the use of building thermal mass in Cyprus are presented. This is achieved by modelling and simulation with the TRNSYS program of a typical four-zone building with an insulated roof in which the south wall of one of the zones has been replaced by a thermal wall. Despite the fact that the diurnal temperature variations in Cyprus are ideal for the application of thermal mass, no such application is presently available. Therefore the main objective of this paper is to investigate the possible benefits resulting from such an application. The results of the simulation show that there is a reduction in the heating load requirement of the zone by about 47%, whereas at the same time a slight increase of the zone-cooling load is exhibited. Optimisations of the various construction parameters have also been carried out. The optimum overhang size is found to be equal to 1.2 m with minor variations in the range of 1 to 1.5 m. The effect of the air gap size between the glazing and the thermal wall is insignificant. The optimum value of wall thickness obtained is equal to 25 cm. The effect of roof insulation is investigated and it is found that insulation is a must for better comfort conditions. Also, the effect of applying ventilation whenever the ambient temperature is lower than the indoor temperature during summertime is investigated. A reduction of 7.5% is obtained when air at 3 air changes per hour is directed into the house. In conclusion it can be said that the thermal wall offers some advantages and should. be used whenever buildings are erected with south-facing walls. (C) 2002 Elsevier Science Ltd. All fights reserved.