The onset for the formation of two-dimensional Faraday waves in a liquid with a viscous surface is numerically studied. The viscous behavior of the interface due to the presence of an insoluble surfactant, affects the surface traction and this, in turns, induces changes in the flow field. It is shown that the formation of waves of a given wavenumber in the absence of elastic effects requires a force that when plotted versus the Boussinesq number increases, producing a sigmoid curve. A detailed analysis of the interfacial variables and flow fields is carried out in order to understand this behavior.