A DIY bathroom vapour barrier is one of the best projects for controlling moisture in a bathroom. Vapour barriers, also called vapour retarders, are necessary to stop water vapours from passing through and moisture from getting trapped in the walls and damaging finishing material and wood structural members. This condition also contributes to health hazards, such as mould and mildew.
Use a combination of methods to prevent the accumulation of excess water vapours in your bathroom. The most common solutions are ventilation, insulation, air barriers and vapour barriers. Before installing vapour barriers, understand the basic approach to the application of vapour barriers for your climate.
In cold climates, install the vapour barrier on the interior side of the building insulation. In hot and humid climates, apply the vapour retarder on the exterior of the building insulation. In mixed climates, install the vapour barrier according to the side that has the most moisture flow.
The general theory behind the application of vapour barriers is that the wall structures have to be designed to keep moisture and water vapour out. Heat flow from the outside (hot climates) or the inside (cold climate) must be resisted. In addition, vapour flow must be stifled from the moist side of the wall structure.
Vapour barriers are available in two categories: coating and flexible materials. Coatings are vapour barriers, such as mastic, hot-melt and primer vapour barrier. The primer vapour retarder is the only product from this category you would consider using in the bathroom
Flexible vapour barriers include foil-face papers, treated paper and plastic sheeting or polyurethane. For a DIY bathroom vapour barrier project, you will usually have a choice of working with treated paper, polyurethane or latex vapour barriers.
Make sure that you are knowledgeable about the building code requirements for the use and installation of vapour barriers in your location.
When constructing a shower from scratch, do not make the mistake of installing a drywall product, such a green board, and placing a vapour barrier behind it. The proper technique for building a shower is to insulate the walls; apply the vapour barrier and install cement board. Cement board is highly water-resistant. It is also permeable enough to allow the material to breathe properly.
Green board along with a vapour barrier can be used in other spots, including behind the walls of a shower stall. Follow the rules for your climate and the local building codes.
Consider using a high-density polythene (HDPE) polyurethane instead of using the standard polyurethane for your vapour barrier. HDPE is more expensive, but it will provide you the best protection against moisture.
Install polyurethane by starting at the horizontal top plate in the corner of the wall. Staple the material approximately every 12 inches along the top plate and the studs. Overlap the sheeting 12 to 16 inches. Pull the polyurethane tight at the sole plate where the studs rest. Use the special tape for sealing the seams and any openings in poly film.
If you're installing insulation, some products have a treated paper vapour barrier already attached. Do not install polyurethane with this type of product. If you do not want to disturb the existing bathroom ceiling or walls, apply a coat of primer vapour barrier on the plaster or drywall and apply your choice of a finish.