Underfloor air distribution (UFAD) is an air distribution strategy for providing ventilation and space conditioning in buildings. UFAD systems use the underfloor plenum beneath a raised floor to provide conditioned air through floor diffusers that creates a vertical thermal stratification under cooling operation. Thermal stratification affects energy, indoor air quality and thermal comfort performance. The purpose of this study was to characterize the influence of linear bar grilles and VAV directional diffusers on thermal stratification in perimeter zones by developing theoretically and empirically based prediction models. Forty-seven laboratory experiments were carried out in a climatic chamber equipped with a solar simulator. Linear bar grilles tend to produce less stratification than VAV directional diffusers and, in some cases with high airflow rates, may generate reverse stratification. Lowering internal blinds causes an increase in thermal stratification. Models to predict temperature stratification for the two tested diffusers have been developed. (C) 2014 Elsevier B.V. All rights reserved.