A. Martin Holladay, editor of Energy Design Update,responds: Your insulation contractor is correct: An interior polyethylene vapor retarder should not be installed in North Carolina. During the summer months, when interior drywall is cooled by a home's air conditioner, condensation can occur on the back (exterior side) of the polyethylene. This phenomenon — called inward solar-vapor drive — occurs after a rainstorm, when sun shines on damp siding. As the siding warms up, water vapor is driven into the wall, allowing condensation to form on the cool polyethylene. This is particularly a problem in homes with so-called "reservoir" claddings — such as brick, stucco, and adhered stone veneer — and in homes without foam sheathing.
Evidently, your local building code does not reflect recent changes adopted by the international codes. The 2004 supplements to the IRC and the International Energy Conservation Code (IECC) abolished all vapor-retarder requirements in climate zones 1, 2, 3, and 4. Moreover, the most recent versions of these codes (the 2007 supplements) provide...
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