Vapor Barrier In Roof Assembly

The dedicated vapor retarder and the roofing membrane.

Vapor barrier in roof assembly.

Vapor barriers are not typically intended to retard the migration of air. The best place to start on understanding this wall is look at walls with double vapor barriers that do not work. Therefore you will often see it installed directly on a roof deck such as a concrete or wood deck or on a. A vapor retarder is typically installed on the warm interior side of a roof or wall.

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. Everyone knows that water in roof systems is never a positive. Materials have a moisture vapor transmission rate that is established by standard test methods. 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.

Typically a vapor retarder has to have a perm rating of 1 0 or less to be successful. In a roof assembly the vapor retarder is normally installed under the primary roof insulation. 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 this assembly it is also acting as an air barrier air control layer.

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. Technically many of these materials are only vapor retarders as they have varying degrees of permeability. One common set of units is g m day or g. 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.

One should note that this assembly exhibits two vapor impermeable i e 0 1 perms or less components. Interior polyethylene vapor barrier the sheet polyethylene on the interior is clearly a vapor barrier.