The contribution of surface roughness of nonporous polymeric membranes to their gas separation and mechanical properties was studied in terms of surface free energy. The membranes samples were prepared based on glassy polymers with different chain rigidity, namely polysulfone (PSU), cellulose triacetate (CTA), and poly(vinyl alcohol) (PVA). The results were obtained by atomic force and scanning electron microscopy (AFM and SEM) with individual gas permeation, wettability, and mechanical testing. The specific surface free energy (as well as its polar and dispersive components) for the polymers was calculated by the Owens-Wendt method. It was proven that the surface roughness of the polymer membranes affects both energy components; however, the degree of this influence depends on the chemical nature of the corresponding polymer. Moreover, it was assumed that the dispersive energy component is inversely correlated with any gases' total permeability. In contrast, the polar one is inversely correlated with the permeability by gases with the ability for site-specific interactions. The gas separation results confirmed this assumption. It was also shown that the mechanical properties of the polymer membranes are also influenced by the surface energy, namely, its dispersive component.