Vapor Barrier (Vapor Retarder)
A material that restricts the passage of water vapor through building assemblies. Classified by permeance: Class I (≤0.1 perm — impermeable), Class II (0.1–1.0 perm), Class III (1.0–10 perm — semi-permeable).
In Detail
IRC R702.7 governs vapor retarder placement by climate zone. Cold climates (zones 5–marine 4) require Class I or II on the warm-in-winter side (interior). Warm-humid climates may require the vapor retarder on the exterior side, or none at all. 6 mil polyethylene (0.06 perm) is Class I. Kraft-faced insulation (1 perm) is Class II. Latex paint on drywall (3–5 perm) is Class III. NEVER install vapor barriers on both sides — trapped moisture causes mold and rot.