Where Does The Polyethylene Vapor Barrier Go On A Roof

An introduction to vapour barriers and vapour retarders.

Where does the polyethylene vapor barrier go on a roof.

Whether or not you need a vapor retarder hinges on three main factors. 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. Most authorities now agree that vapor barriers are important under certain conditions but not necessarily as a whole house solution for every home. In hot humid climates attics should not be vented and vapor.

Everyone knows that water in roof systems is never a positive. Some air barriers are vapor open and allow moisture to escape. Class i impermeable vapor proof 0 1 perm. In any case the vapor barrier must point to the warm side.

The lower the perm rating the less diffusion i e less drying occurs through a material. Some debate still occurs over how necessary vapor barriers are but a consensus is growing closer. Built up roofing membrane perm rating 0 00 0 02. After the insulation is in place you will want to add a vapor retarder sometimes called a vapor barrier if you need one.

Not every wall does. 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. Typically a vapor retarder has to have a perm rating of 1 0 or less to be successful. Vapor barriers are sheets of plastic or other material placed on one side of insulation sheets.

What is a vapour barrier. A vapour barrier sometimes referred to as vapour retarder is typically a plastic or foil sheet used for damp proofing to prevent interstitial condensation from forming in various building assemblies such as walls roofs foundations and floors. In circumstances where conditions inside a home or office are much different than outdoor conditions water. A vapor retarder is a material used to prevent water vapor from diffusing into the wall ceiling or floor during the cold winter.

Plastic vapor barriers should only be installed in vented attics in climates with more than 8 000 heating degree days. There are three classes of vapor retarders. Single ply membrane perm rating 0 03 0 06. A vapor barrier better described as a vapor retarder reduces the amount of water vapor entering your wall and ceiling cavities.

Resolution of the vapor barrier debate. 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.