The magnetostatic energy of a cycloidal spin-spiral configuration is calculated. The free-standing spiral is compared to the case of spirals that are brought in contact to a magnetically polarizable substrate. While a free-standing layer is energetically degenerate with respect to the spiral's sense of rotation, it is shown that a polarizable substrate breaks this symmetry and lifts the degeneracy. Consequently, a strongly polarizable substrate can stabilize (destabilize) a spin spiral that would be unstable (stable) without considering the magnetostatics of the substrate.