Permeability describes how readily a substance can pass through a material. In building discussions, identify whether the concern is liquid water or water vapor: these move through an assembly in different ways.
Look at the whole wall or roof
A material's description alone does not determine how a finished assembly manages moisture. Layers, joints, penetrations and drying paths also matter. A product intended to control vapor is not automatically a remedy for rain entering through a damaged opening.
Questions for a repair proposal
Ask what moisture path was identified, which layer the proposed product changes and how adjoining materials will be handled. Share product information from previous work if available. Avoid selecting a coating solely because it is described as waterproof or breathable.
The Department of Energy's wall-system research distinguishes several moisture loads. Related terms: seepage and vapor barrier.