Published Article / The purpose of this paper is to analyse the thermal performance of two buildings. The one has a large thermal mass and the other a highly insulated low thermal mass. A typical 120 m2 suburban building was modelled in Ecotect. As part of the model infiltration rate, wind sensitivity and a central Pretoria weather file were used. New material composites were introduced in the materials database to represent typical building materials used in the construction of heavy and light-weight buildings in South Africa. The thermal characteristics of these new materials were then calculated within Ecotect. Ecomat was used to calculate thermal lag which was used as an additional input into Ecotect. The research indicates that a low thermal mass and highly insulated building have been shown to use 18.3% less annual space heating and cooling energy when compared to the high thermal mass building. The good thermal performance results of the light-weight building will help in clearing scepticism to adopting this construction technology in southern Africa where high thermal mass masonry is still predominant.