The heat transfer through the floor slab in buildings with Under-Floor Air Distribution (UFAD) systems may have a negative impact on the energy performance of these buildings, although very few studies have been reported in the literature. By using an energy simulation program, EnergyPlus, this investigation compared the energy use in a Philadelphia office building with a UFAD system to that with a well-mixed ventilation system. When the heat transfer through the floor slab was taken into consideration, the thermal load of the building with the UFAD system was higher than with the well-mixed system. On the other hand, the higher supply air temperature of the UFAD system enables the use of more free-cooling. The annual energy consumption by the chillers in the building with the UFAD system was 16-27% lower than with the well-mixed system, but energy consumption by the boiler was 12-30% higher, and the energy consumption by the fan was 22-50% higher, depending on the manner in which the heat was supplied to the floor plenum. When the UFAD system was used with an un-ducted floor plenum and without heating coils under the diffusers, it consumed slightly more energy than the well-mixed system. (C) 2014 Elsevier B.V. All rights reserved.