The stabilization of lipid-monolayer-covered water films on a solid substrate (called asymmetric soap films) under controlled humidity is investigated to determine the conditions of stabilizing these films in air. A Langmuir trough is described which simultaneously allows high precision control of the relative humidity of the atmosphere, control of the monolayer density, and optical observation of the evolution and stabilization of the asymmetric soap film. Film properties can simultaneously be measured and observed optically by use of imaging and null ellipsometry. The influence of the DLVO forces in stabilizing the film, as well as the influence of the humidity and the connected stationary flow of water into and out of the film, is described theoretically. It is found that the dispersive force changes direction if a free water film becomes covered by a lipid film. Although experimentally the thermodynamic force is dominant in determining the thickness of the film, electrostatic repulsion surprisingly has significant influence on the stability and also on the thickness of the film.