Vapor Barrier In Roof Assembly

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.

Vapor barrier in roof assembly. The best place to start on understanding this wall is look at walls with double vapor barriers that do not work. Materials have a moisture vapor transmission rate that is established by standard test methods. A vapor barrier is any material used for damp proofing typically a plastic or foil sheet that resists diffusion of moisture through the wall floor ceiling or roof assemblies of buildings to prevent interstitial condensation and of packaging. In a roof assembly the vapor retarder is normally installed under the primary roof insulation.

Therefore you will often see it installed directly on a roof deck such as a concrete or wood deck or on a. Everyone knows that water in roof systems is never a positive. 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. Where it is located in an assembly and its permeability is a function of climate the characteristics of the materials that comprise the assembly and the interior conditions.

Technically many of these materials are only vapor retarders as they have varying degrees of permeability. The dedicated vapor retarder and the roofing membrane. Typically a vapor retarder has to have a perm rating of 1 0 or less to be successful. A roof design that includes an adhered roof membrane with multiple layers of insulation with board joints offset and staggered over a vapor retarder air barrier helps lower the risk that air and the moisture it carries will infiltrate the roof system.

Whether considering a thermoplastic polyolefin tpo polyvinyl chloride pvc or ethylene propylene diene monomer epdm single ply roofing membrane for a 60 mil thickness you are probably looking at about 0 05 perm 0 10 max. Interior polyethylene vapor barrier the sheet polyethylene on the interior is clearly a vapor barrier. That reduction of air and moisture infiltration can then help improve roof longevity. Vapor barriers are not typically intended to retard the migration of air.

One should note that this assembly exhibits two vapor impermeable i e 0 1 perms or less components. The function of a vapor barrier is to retard the migration of water vapor.