Water diffusion at the surface of a silica glass is increased when surface cracks are present which were inadvertently introduced by the surface treatment of samples. Due to the larger true surface exposed to the humid environment, an apparently increased diffusion constant must occur. With the help of a simple model the influence of the cracks for small water penetration depths is described. Simplified approximations are given which allow a continuous interpolation of two limit solutions over greater penetration depths or diffusion times. In the present report we restrict our considerations on unloaded cracks.