A.Alex Wilson responds:With plywood or foam sheathing on the exterior and a properly installed vapor barrier on the interior (all seams taped or caulked), an outside air barrier is not strictly required and will probably not be cost-effective. However, if the vapor barrier is typically installed — with unsealed seams and full of holes — an exterior air barrier is a good idea.

The research on this issue is scanty. Manufacturers have commissioned studies comparing different brands of air barriers and comparing taped to untaped installations. But no objective, third-party tests evaluate air barriers when the interior vapor barrier is properly installed.

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