Exterior insulation thickness and vapor permeance

Is there a good resource online to understand how the thickness of exterior insulation (or any material, for that matter) affects the vapor permeance of an assembly?I hear things like… Is there a good resource online to understand how the thickness of exterior insulation (or any material, for that matter) affects the vapor permeance of an assembly?I hear things like…Read MoreGreenBuildingAdvisor