Where Does The Polyethlene Vapor Barrier Go On A Roof

In most locations that means installing vapor retarders on the interior side of the walls and ceilings.

Where does the polyethlene vapor barrier go on a roof. A vapor retarder is a material or system that is designed as part of the roof system to substantially reduce the movement of water vapor into the roof system where it can condense. Avoidance of the installation of vapor barriers such as polyethylene vapor barriers foil faced batt insulation and reflective radiant barrier foil insulation on the interior of air conditioned assemblies a practice that has been linked with moldy buildings lstiburek 2002. This eliminates the need for a separate air barrier that could potentially prevent the moisture from escaping. Everyone knows that water in roof systems is never a positive.

After the insulation is in place you will want to add a vapor retarder sometimes called a vapor barrier if you need one. A vapor retarder is a material used to prevent water vapor from diffusing into the wall ceiling or floor during the cold winter. However if the inside of the structure is refrigerated as in a cold storage warehouse or food processing plant vapor barriers may be more effective on the outside of the wall or ceiling assembly. In very cold climates the use of polyethylene plastic vapor barriers between insulation and interior wallboard may be beneficial provided all air gaps into any wall and ceiling cavities are also blocked.

You can forego the plastic and use a vapor retarder kraft faced insulation or latex ceiling paint in all other climates except hot humid or hot dry climates. In any case the vapor barrier must point to the warm side. Vapor barriers are sheets of plastic or other material placed on one side of insulation sheets. Not every wall does.

Typically a vapor retarder has to have a perm rating of 1 0 or less to be successful. When we use a vapor retarder in a roof system it will also act as an air barrier as long as it is sealed at all perimeters and penetrations and is tied to the wall air barrier.