Combining mass and energy balances, a differential equation for the profile of a liquid wedge underneath a vapour bubble that is growing on a solid surface is derived. It connects the spatial and temporal changes of the film (wedge) thickness with the spatial temperature changes and velocity of liquid at the interface. Specifying particular conditions, the equation reduces to those from the literature. The paper brings further an illustrative explanation of why the wall heat flux in the wedge region may reverse its direction.