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