Vapour Diffusion Control
Water vapour can migrate through building envelope assemblies by penetrating through materials by ‘diffusion’. Many activities such as cooking, dishwashing, laundering and bathing release considerable amounts of water vapour into the air and increase its humidity. As water vapour diffuses through assemblies, it can condense on cold surfaces such as the inside face of the exterior sheathing and cause deterioration.
Source : Canada Mortgage and Housing Corporation (CMHC)