The role of RH (relative humidity) during the formation of templated silica thin films has been investigated using real-time grazing incidence small-angle x-ray scattering. A detailed analysis of the evolution of the lattice parameters as a function of RH is presented. It is shown that in the modulable steady state, the lattice parameter parallel to the surface is pinned after a transient regime is reached while the parameter normal to the surface can still vary. In the initial stage we find that the film can take up to one layer of water per micelle. Interferometric measurements confirm that swelling occurs in the entire film. An explanation of the pinning effect is presented.