The objective of this research was to determine the actual heat loss into the subsoil from a massive slab-on-ground structure in a low temperature floor heating system. The main objective was achieved by field test measurements of an actual new building in Southern Finland. The test building is a detached house including a massive concrete slab, an underneath polystyrene insulation and a crushed stone fill layer on top of the clay subsoil. The heat loss into subsoil is determined from the measured temperature difference over the slab cross-section during a 1-year measuring period. The long-term behaviour of the structure was also studied by numerical simulations using 2D FE-modelling. According to the field test results and the simulations, the increase of the slab temperature in winter increases significantly the flow rates into the subsoil, also at the central part of the slab. Theoretical calculations for a standard building show that the heat loss into the subsoil from slab-on-ground structures is a significant part of the total heat loss from a building and the intensity of the heat loss is strongly dependent on the average temperature of the slab structure. The effect of the various floor heating systems on the total energy consumption of a building should be taken into consideration when designing the thermal insulation of ground slabs. Copyright (c) 2005 John Wiley & Sons, Ltd.