Bathroom has an exhaust fan but the walls stay damp.

No, you should not install greenboard over a poly vapor barrier on the exterior wall behind the tub. Here's why:

Code & Safety Issues:

• IRC P2715.4 requires bathroom exhaust fans to be ducted to the exterior, but doesn't address wall construction.
• IRC E3405.1 requires a vapor retarder on the warm-in-winter side of insulation in climate zones 5+.
• Installing poly on the exterior wall creates a double vapor barrier when combined with interior paint/drywall, trapping moisture.

The Real Problem: Your damp walls indicate the exhaust fan isn't removing enough moisture. Adding vapor barriers will make this worse by trapping moisture inside the wall cavity.

Correct Approach:

1. Fix the fan first - Check if it's properly sized (minimum 50 CFM for bathrooms, per IRC M1507.3) and ducted to the exterior
2. Use greenboard or cement board directly on the wall (no poly)
3. Ensure proper insulation in the exterior wall with the vapor retarder on the interior side only

If you must add moisture protection:

• Use a smart vapor retarder (like MemBrain) instead of poly
• Ensure the wall can dry in at least one direction

The dampness is a ventilation issue, not a wall construction issue. Fix the fan before modifying the walls.

No. Do not install greenboard over a polyethylene vapor barrier on an exterior wall. This creates a "double vapor barrier" (poly + greenboard facing + paint), trapping moisture in the stud bay with no drying path to the interior or exterior. In a bathroom—especially behind a tub—this guarantees mold and rot.

Code & Building Science:

• IRC R702.7 / IBC 1404.3: Requires vapor retarders based on climate zone. Poly (Class I) is rarely correct for walls in mixed/humid climates (Zones 4–7) because it blocks inward drying.
• IRC R702.4.2: Water-resistant gypsum board (greenboard) is not permitted as a tile backer in wet areas (tub/shower surrounds). It loses structural integrity when wet.
• IRC R702.7.3: Class I or II vapor retarders on the interior of frame walls in Zones 4–8 require a vented cladding or exterior insulation to allow drying.

Correct Assembly (Tub/Shower Exterior Wall):

1. Framing: 2x4/2x6 studs.
2. Cavity Insulation: Unfaced batts or dense-pack cellulose (no poly).
3. Sheathing: Exterior-grade (OSB/plywood) + WRB (housewrap/Zip System) + Rainscreen gap (furring strips) + Siding.
4. Interior Side: Cement board, fiber-cement, or coated glass-mat tile backer (per ASTM C1325/C1178) fastened to studs.
5. Waterproofing: Liquid-applied membrane (RedGard, Hydro Ban) or sheet membrane (Kerdi) over backer behind tile. This is your only vapor/liquid barrier here.
6. Non-wet areas (walls away from tub): Use mold-resistant paperless drywall (purple board, XP) painted with vapor-retarder primer (Class III, ~1–10 perms). Allows seasonal drying.

Fix the Dampness:

1. Verify fan flow: Measure CFM at grille (Home Ventilating Institute standard: 50 CFM intermittent / 20 CFM continuous for bathrooms). Clean duct; ensure <0.25" w.g. static pressure; vent through roof/wall—not soffit.
2. Run time: Wire fan to humidistat or timer (20–30 min post-shower).
3. Air seal: Seal all penetrations (plumbing, electric) at top/bottom plates with foam/caulk before insulation.

Bottom line: Strip the poly. Install proper tile backer + liquid membrane in the wet zone. Use vapor-permeable finishes elsewhere. Fix the ventilation.