Where to Install a Vapor Barrier in a Bathroom

Whether you are remodeling a bathroom or installing a new one, you need to know where to install a vapor barrier. The primary purpose of a vapor barrier, also called a vapor retarder, is to keep water vapors from passing through walls. Moisture flows from a warm space to a cold one, and if moisture penetrates a wall, condensation will form. This leads to damaged drywall, deteriorating wood trim, mold and other decay.

Towels on bathroom shelf
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Planning

Water vapors pass through plaster or drywall, studs, sheathing and exterior siding. Where you install a vapor barrier in your bathroom will be based primarily on the climate in the area where you are live. If you live in a hot and humid location, the vapor barrier must be installed on the outside wall. If you live in a colder climate, you must install your vapor barrier on the inside of the wall.

Installation

You must install vapor retarder properly, or you will end up trapping moisture in unwanted places. An accumulation of condensation may cause damage to your home's insulation, wood structure or exterior sheathing.

Custom showers require a vapor barrier, such as 6 mm polyurethane. However, you cannot install a vapor barrier behind a drywall product, including green board. If you were to install a vapor barrier behind it, water vapor would build up between the vapor barrier and the drywall. It would be just a matter of time before your walls began to crumble.

In the shower area, use cement board, which is durable enough to resistant breakdown due to moisture and can properly breathe. Install plastic sheeting or polyurethane on the studs to prevent vapor from penetrating.

If you use batt insulation, save yourself some work by choosing a product with the vapor barrier already attached. For unfaced insulation, secure the insulation first, then attach the polyurethane vapor barrier. Any time you use plastic sheeting as a vapor barrier, seal all seams, gaps and rips in it using the special tape made for that purpose. Any openings in the vapor barrier allow wetness to penetrate it.

For projects where vapor retarders need to be installed on the interior of bathroom walls or ceilings that have plaster or drywall already in place, consider using a latex vapor barrier primer. This barrier is designed to be used on a wide variety of interior surfaces, including plaster, drywall, brick and masonry.

Latex vapor barrier primer may be brushed, rolled or sprayed on like a standard primer. If you are planning to refinish the walls and ceiling anyway, using latex barrier will save time and money.