CsPbBr₃ is an inorganic perovskite material that is promising for light-emitting applications. Such applications are known to benefit from an island-type active layer structure, which enhances the device's light emission efficiency. Here, the impact of the environment on the island formation in thermally deposited bilayers of CsPbBr₃/LiBr is investigated. It is demonstrated that the island formation occurs only in humid environments, leading to an enhancement of the photoluminescence quantum yield by a factor of 350. Time-resolved grazing-incidence wide-angle X-ray scattering experiments document the island growth process and reveal that the LiBr has already changed the perovskite crystal orientation prior to the island formation.