Sun sails are a traditional street-scale shading strategy in Mediterranean cities to improve outdoor comfort. So far, their benefits for indoor comfort remain almost unexplored. To addresses this gap, we carry out thermal simulations using Finite Element Methods (FEM), a still novel approach in urban thermal studies that allows for highly detailed radiative computations at the building and street scales. We assess the potential of street sun sails to limit building cooling demands in a Mediterranean-climate city for twelve urban scenarios differing in orientation, façade reflectance, and sun sail color. Simulations show that the use of street sun sails allows for significant decreases in the air and surface temperature inside buildings, and consequently, in cooling demands (between 12% and 46%). Based on simulation results, we provide design guidelines for the installation of sun sails to maximize their effectiveness. This work illustrates how the rich visual feedback of FEM simulations can aid the decision-making process for urban design.