Vapor barriers are sheets of plastic or other material placed on one side of insulation sheets.
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.
Typically a vapor retarder has to have a perm rating of 1 0 or less to be successful.
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.
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.
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.
Everyone knows that water in roof systems is never a positive.
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.
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.
A vapor retarder is a material used to prevent water vapor from diffusing into the wall ceiling or floor during the cold winter.
In any case the vapor barrier must point to the warm side.
Plastic vapor barriers should only be installed in vented attics in climates with more than 8 000 heating degree days.
This eliminates the need for a separate air barrier that could potentially prevent the moisture from escaping.
Not every wall does.
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.