Vapor permeance describes how readily water vapor passes through a material of a particular thickness under a pressure difference. Product literature may express it in perms.
What this means at your property
The value helps describe vapor movement, not whether a seam is watertight or a wall is free of air leaks. Coatings, facings and other layers can change how the finished assembly manages moisture.
How is permeance different from permeability?
Permeability describes a material property; permeance applies to a layer with a given thickness. Ask the designer or contractor to evaluate the complete assembly and drying paths instead of choosing a product from one number alone.
Related terms
Further reading: Department of Energy: vapor barriers.