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Q. What is the difference between a vapor barrier and an air barrier? Between a "barrier" and a "retarder"? A. A vapor barrier is a material or coating that significantly reduces the passage of water vapor by diffusion through wall, ceiling, and floor materials. In general, its purpose is to keep moisture from getting into exterior wall and ceiling cavities, where it can condense on framing members and sheathing. Diffusion is the transfer of moisture through tiny pores in building materials. The amount of diffusion through a material depends on the material's permeability (perm rating) and the vapor pressure that pushes the moisture through. It's a scientific fact that moisture will always move from areas of higher temperature and relative humidity (higher