Use the right vapor barrier behind a tub surround to control moisture and prevent wall failures.

The most common usable materials are polyethylene sheeting at six mil thickness, reinforced foil faced kraft paper, and vapor retarder paint applied to cement backer boards. Six mil polyethylene is inexpensive, widely available, and should be hung continuous behind the stud plane with seams overlapped at least six inches and taped with waterproof construction tape; fasten it to studs with staples or cap nails and extend it down into the tub flange area but avoid stuffing it into the flange jamb where it can trap water. Foil faced kraft paper combines a paper layer with a reflective foil and performs as a class two vapor retarder; install the kraft side toward the interior room and overlap seams by at least two inches, sealing with compatible tape and leaving the paper layer flat so tile mortar adheres properly over the cement board. Vapor retarder paint is applied directly to cement backer board or gypsum boards and creates a class three retarder that lets the assembly dry to the wall cavity side; apply two liberal coats and allow full cure between coats according to the product directions for reliable performance. When working over an existing undisturbed fiberglass-faced vapor barrier, do not remove it if it is clean and intact; instead you can install the new barrier over it but score or slice the old paper sparingly to create a small airflow gap so the cavity can dry if necessary while avoiding a double sealed pocket. For all installations keep the vapor control strategy consistent with your climate zone; in cold climates avoid interior class one barriers on the warm side of the assembly, and in warm humid climates prefer materials that allow drying to the interior to avoid trapping summer moisture.

Three frequent mistakes are sealing the barrier on the wet side of the substrate which traps moisture and causes mold, relying only on a surface membrane like a paint without addressing seams and junctions, and layering two impermeable barriers without a drying path which creates a moisture sandwich. If the barrier is placed on the wet or tile side, remove it and reinstall the control on the warm inboard side of the assembly or use a retarder paint on the backer board instead so moisture can move out. If seams and transitions are ignored, go back and tape all seams, seal around plumbing penetrations with a flexible sealant, and integrate flashing at the tub flange to direct water out. If you find two impermeable layers, open the cavity at a nonvisible location, create a small vent or gap by slicing the inner layer to permit drying, and replace finishes only after confirming the assembly dries within acceptable time frames.

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