INSTALLING VAPOR BARRIERS
Moisture in the air in the form of vapor is transferred along with heat. This is especially common in humid environments and in certain areas inside a home -- such as bathrooms, kitchens and laundries, in addition to homes with interior hottubs or swimming pools. When moisture vapor becomes trapped, mold and mildew growth can result. These can damage the home and present a potential health concern to the home's residents. Vapor barriers keep the moisture in the air in your house from condensing in the insulated cavities.
Insulation facing usually acts as a vapor barrier. If you need a vapor barrier and your insulation is unfaced, you must cover it with a polyethylene film (visqueen). Note: This type of installation will prohibit you from the ability to glue your drywall or gypsum board to the face of the studs, in which case it is recommended that you screw the drywall at twice the number of screws per square foot than in a normal application. This will help prevent nail pops.
Whether your vapor barrier is a facing or a film, it is usually placed on the warm side of the wall. If you live in a cold climate, place the vapor barrier between the interior of your home and the insulation. If you live in a hot, humid climate, you may want to place the vapor barrier toward the outside of the wall cavity.
How to Apply a Vapor Barrier
To apply the poly film, start at the top plate in one corner of the room.
Pull the film tight and staple at least every 12".
Drive staples at the center of every stud and around openings, working around the room.
Overlap the sheets by one complete cavity to minimize leakage. Then staple evenly through fastening both sheets to the studs at 1' to 2' intervals.
Pull the film tight along the sole plate and staple in the same manner, making sure the staples are driven flat, flush to the stud surface.
Trim out the poly from over the windows, doors, and electrical boxes.
NOTE: In some areas of the country where vapor barriers are not required, bathrooms would require unfaced insulation and no poly film covering to allow moisture to escape.