The heat transfer processes occurring in the earth surrounding a building have a substantial effect on the building's energy consumption. During the heating season, for example, heat loss through ground- contact surfaces may be one of the most significant contributors to building heating load. Equipment sizing procedures and building energy analyses must use some method for calculating heat exchange between the building and the surrounding earth if they are to adequately calculate the building heating and cooling loads. Most existing models of the earth and the earth-building interface are either complex models which require large amounts of computation time or very simple models which are inconsistent with the accuracy of existing detailed hourly building energy calculation programs. BLAST, for example, currently uses a single monthly average ground temperature to define all groundbuilding heat transfer mechanisms. This method does not accurately account for thermal mass effects of the ground beneath the building or the spatial variation of conduction loss due to varying ground temperatures from the surface to the deep ground. Finite element methods can be very accurate, but the required computation time makes them inappropriate for inclusion in existing detailed hourly energy analysis programs. This paper describes an earth heat transfer model of square slab floors suitable for use with detailed hourly energy programs. The model is based on transfer function methodology using multiple inputs to account for heat transfer between the building, the deep ground and the climate- affected region near ground surface. The transfer functions described in this paper can be calculated from known building and ground characteristics. This model is more accurate than the simple algorithms currently in use while avoiding the excessive computational requirements of more detailed models. The results of this model are compared to a detailed three?dimensional finite difference model described in a companion paper.