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.

Where does the polyethlene vapor barrier go on a roof.

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. 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. This barrier is meant to keep moisture from getting to the insulation in the walls and ceilings and it is required by building codes when insulating most houses.

Vapor barriers are sheets of plastic or other material placed on one side of insulation sheets. Everyone knows that water in roof systems is never a positive. Plastic vapor barriers should only be installed in vented attics in climates with more than 8 000 heating degree days. For example the national roofing contractors association nrca recommends vapour barriers on the interior side of a roof in any climate where the outside average january temperature is below 40 f 4 c degrees and the expected interior winter relative humidity is 45 percent or greater.

Not every wall does. 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. 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 any case the vapor barrier must point to the warm side.

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. 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.