Can polyurethane serve as a vapor barrier?
Polyurethane spray foam can act as a vapor barrier since it is a closed-cell material. The minimum thickness required for this function depends on the substrate on which the polyurethane is sprayed. Non-hygroscopic materials (e.g., concrete) allow the formation of a “skin” surface, which is resistant to water vapor. When sprayed on a capillary material (e.g., gypsum) there is no formation of the surface skin.
Research indicates that greater thickness is required when the polyurethane is sprayed on a capillary material and a reduced thickness is possible when the polyurethane is sprayed on a non-hygroscopic material.
Consult the manufacturer or the product datasheet for the permeability to water vapor as a function of the substrate and the intended polyurethane thickness.