Convection in a layer of fluid with a free surface due to a combination of thermocapillary stresses and buoyancy has been studied extensively under atmospheric conditions. However, recent experimental results have shown that removing most of the air from a sealed cavity significantly alters the flow structure and, in particular, suppresses transitions between different convection patterns found at atmospheric conditions. On the other hand, removing air has a very small effect on the flow speed, while a simple analytical estimate predicts that complete removal of noncondensable gases such as air should reduce the flow speed by an order of magnitude. To understand these unexpected results, we have formulated and numerically implemented a detailed transport model that takes into account mass and heat transport in both phases in the absence of noncondensables. The model was used to investigate how the flow is affected by the magnitude of the (poorly defined) accommodation coefficient and by the temperature jump across the liquid-vapor interface predicted by some phase change models. Our results eliminate both effects as possible explanations for the unexpected experimental observations, suggesting that the small amount of air left in the cavity in the experiments is the most likely, albeit somewhat unexpected, explanation for the observations. (C) 2014 Elsevier Ltd. All rights reserved.