Closed-cell foam at 2"+ thickness IS a vapor barrier. No additional barrier needed. Open-cell foam is vapor-permeable; in cold climates, a separate vapor retarder may be required. Our Cincinnati installers design per IECC code.
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IECC Climate Zone 4 vapor management rules. Closed-cell foam at 2 inches or more achieves Class II vapor retarder status (perm rating below 1.0); 3+ inches reaches Class I (perm below 0.1). Both classes meet code for warm-side vapor control without separate poly sheeting. Open-cell foam is vapor-permeable (perm 5-15) and counts as a Class III retarder; in Climate Zone 4 walls, it requires a Class I or II vapor retarder on the warm side per IRC, typically achieved by Kraft-faced batting or vapor-retarder paint. For Cincinnati attics, open-cell on a vented attic floor needs no additional retarder because the vented attic above handles vapor migration. For unvented assemblies (cathedral ceilings, conditioned attics with closed-cell on roof deck), the closed-cell itself handles vapor. Our Cincinnati installers spec the right approach during design and document it on the project closeout package. Cincinnati Climate-Zone-4 vapor-management specifics: walls in Climate Zone 4 should have a Class I or Class II vapor retarder on the interior (warm) side; closed-cell foam at 2+ inches handles this requirement integrally. Open-cell foam in Climate Zone 4 walls is acceptable when paired with a Kraft-faced batt or vapor-retarder paint on the interior side, but adds complexity that our Cincinnati installers typically avoid by specifying closed-cell for wall applications.