Where Does The Polyethylene Vapor Barrier Go On A Roof

The process of water vapor passing through building materials is called vapor diffusion is it a vapor retarder or an air barrier.

Where does the polyethylene vapor barrier go on a roof.

A vapor retarder is a material used to prevent water vapor from diffusing into the wall ceiling or floor during the cold winter. 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 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.

The lower the perm rating the less diffusion i e less drying occurs through a material. Built up roofing membrane perm rating 0 00 0 02. An introduction to vapour barriers and vapour retarders. What is a vapour barrier.

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. Do not confuse vapor retarders with air barriers. 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.

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. 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.

Typically a vapor retarder has to have a perm rating of 1 0 or less to be successful. 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. Class i impermeable vapor proof 0 1 perm. Whether or not you need a vapor retarder hinges on three main factors.

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. Not every wall does. In any case the vapor barrier must point to the warm side. Some debate still occurs over how necessary vapor barriers are but a consensus is growing closer.

Single ply membrane perm rating 0 03 0 06. In hot humid climates attics should not be vented and vapor.