Thin layers of a covalent organic framework (COF) have been synthesized on a flexible polymeric support using a new diffusion-induction method under ambient conditions in reaction times as short as 3 hours. The layer is uniform and has a nodular crystalline morphology, evolved after the initial stages of amorphous fiber formation. COF nano-sheets were formed in the internal cavities of the support. The resultant composite membranes had an ultrahigh water vapor permeance and negligible liquid water transport, properties required for application in dehumidification devices.