Vapor Barrier In Roof Assembly

In a roof assembly the vapor retarder is normally installed under the primary roof insulation.

Vapor barrier in roof assembly.

In this assembly it is also acting as an air barrier air control layer. One common set of units is g m day or g. Interior polyethylene vapor barrier the sheet polyethylene on the interior is clearly a vapor barrier. Vapor barriers are not typically intended to retard the migration of air.

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. 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. One should note that this assembly exhibits two vapor impermeable i e 0 1 perms or less components.

A vapor retarder is typically installed on the warm interior side of a roof or wall. 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. Technically many of these materials are only vapor retarders as they have varying degrees of permeability.

Typically a vapor retarder has to have a perm rating of 1 0 or less to be successful. The function of a vapor barrier is to retard the migration of water vapor. 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. 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. That reduction of air and moisture infiltration can then help improve roof longevity.