Rough Carpentry Moisture Trap in a Coastal Build
A two-story coastal residence uses 2x6 wood framing at 16 in. on center with fiberglass batts, an interior polyethylene vapor retarder, and a vented attic. The climate is hot and humid with roughly 4,500 cooling degree days and mild winters. During rough-in inspection the superintendent notices condensation on the interior face of the sheathing in two north-facing wall cavities, and the framer has already installed the poly on the first floor. The owner wants to add a whole-house dehumidifier anyway. The drywall is scheduled in 10 days and the building has not yet been conditioned.
1. Diagnose the likely cause of the condensation and evaluate the vapor retarder strategy.
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In a hot-humid climate, the dominant moisture drive is from outside to inside, not inside to outside. An interior polyethylene vapor retarder is a vapor barrier on the wrong side of the assembly for this climate: it traps moisture that enters from the exterior or from construction moisture and prevents drying to the interior. Condensation on the interior face of the sheathing is consistent with warm, humid exterior air reaching a surface cooled by the air-conditioned interior, with no path to dry.
Contributing factors to check: air leakage at the sheathing plane, whether exterior sheathing is a vapor-retarding product, whether the cavity insulation is installed with gaps, and whether the building was closed up while concrete and framing were still wet. Unconditioned buildings in humid weather condense readily.
The correct strategy in this climate is generally a vapor retarder with low permeance on the exterior or a Class II or III interior retarder, not polyethylene, plus rigorous air sealing and a drainable, ventilated exterior. Removing the poly on the first floor is likely the right call.
2. The framer says removing the poly will fail the energy code. How do you resolve the conflict before drywall?
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Resolve it with the code text and the energy rater, not with opinion, and do it in the next few days because drywall is in 10 days.
Energy codes regulate vapor retarder class by climate zone; in hot-humid zones they typically require a Class I or II retarder only where the assembly needs it, and some provisions allow a Class III retarder or a vapor-retarding exterior sheathing in lieu of interior poly. The rater's compliance model may have assumed a specific assembly, so the fix must be reflected in the model or the compliance path fails.
Action plan: (1) get the energy rater's written position on an acceptable assembly, (2) get the building official's interpretation in writing if there is any ambiguity, (3) confirm with the designer that the revised assembly still meets the moisture and air barrier intent, and (4) document the change order and any cost impact.
Do not let the framer proceed on the second floor with poly while the question is open. The cost of removing poly after drywall is far higher than resolving it now.