III. WHAT DOES A VAPOR RETARDER DO?
A vapor retarder slows the rate of water vapor diffusion but does not totally prevent its movement. Building occupants, certain appliances, and plumbing equipment generate moisture that is carried in the air as vapor. As water vapor moves from a warm interior through construction materials to a cooler surface, the water vapor may condense as liquid water that could damage the building. It is for this reason vapor retarders that retard the flow of moisture through construction materials are installed in buildings.

IV. PLACEMENT OF VAPOR RETARDERS
The International Residential Code (IRC Section R322) specifies that a vapor retarder must be installed on the warm-in-winter side of the insulation (see illustration) with the following exceptions:

Exception 1:  In construction where the accumulation of moisture or freezing of moisture will not damage the materials.
Exception 2:  Where the framed cavity or space is ventilated to allow moisture to escape.
Exception 3:  In counties identified by footnote (a) in Table N1101.2 of the IRC standard.

Very few situations occur in residential construction that would permit the builder to select Exception 1.

Ventilated cavities and spaces (Exception 2) would include ventilated attics, properly designed and vented cathedral ceilings, and vented crawl spaces.

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