We investigate the effects of an opposing buoyancy force on the performance of an air curtain in the doorway which separates a warm indoor environment from the cold exterior. Such an opposing buoyancy force arises for example if a downwards blowing air curtain is heated. We conducted small-scale experiments using water, salt and sugar solutions as the working fluids. The effectiveness curve of a downwards blowing air curtain as a function of the deflection modulus was measured for situations in which the initial density of the air curtain was less than both the indoor and the outdoor fluid density, which corresponds to the case of a heated curtain. It was found that the effectiveness of the air curtain starts to decrease if it is heated beyond a critical temperature. We also discuss the question whether it is more energy efficient to use a heated air curtain or an air curtain operating at room temperature. Based on our experimental results we conclude that a heated air curtain is likely to be less energy-efficient. Further, we propose a theoretical model to describe the dynamics of the buoyant air curtain. Numerical results obtained from solving this model corroborate our experimental findings. (C) 2015 Elsevier B.V. All rights reserved.