If you do not control your home's moisture content, you will eventually run into potentially costly problems. When it's cold, moisture flows from the warm interior space to the outside. When water vapor penetrates the walls, ceilings or floors, it may condense and be trapped in the wall assembly. At the least, the R-value of your insulation will be compromised. Often, it is just a matter of time before you have obvious signs of rot, mildew and mold -- and costly repairs.
The purpose of a vapor barrier is to prevent the above scenario. Vapor barriers or vapor retarders keep the moisture from penetrating your walls from the outside and inside.
Installing Vapor Barriers and Insulation
Check with your local building codes for guidance on installing vapor barriers and insulation in your area.
Insulation with a vapor barrier attached should always be installed with the vapor retarder toward the living unit. If you installed unfaced insulation, and a vapor barrier is required, you can use 4-mil or 6-mil polyurethane. Use a vapor barrier primer or paint if you want to apply a vapor barrier on existing walls and ceiling.
For attic installations, use blanket insulation with the kraft paper facing (vapor barrier) already attached. For the insulation and vapor retarder to be effective, the material must be properly secured. Fasten the insulation to the front of the floor joist. Strengthen the vapor barrier by applying a 4-mil or thicker 6-milligram polyurethane over the framing before you install the drywall.
Never sandwich the insulation between two vapor barriers. For example, you should not install insulation with the vapor barrier facing the living space and then put plastic sheeting, or some type of vapor barrier, across the attic side of the floor joists.
Some local codes may require you to insulate the attic more than R-38. In these cases, it may be necessary for you to install two layers of insulation. Install the first layer with the vapor barrier faced toward the living space. Apply a second layer of unfaced insulation perpendicular to the first layer.
Before installing vapor barriers and insulation in the basement, do a bit of extra homework. Speak with some knowledgeable contractors who have proven experience insulating basements in your location. You can also do some research the Internet. There are strategies for addressing moisture and insulation projects in basements for a variety of climates.
Some strategies recommend that you place the vapor barrier on the basement walls before framing the walls, installing batt insulation and a finish. In some areas, a vapor barrier is not recommended.