Roof Vapor Retarder Purpose

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.

Roof vapor retarder purpose.

Its purpose is to help prevent water vapour from reaching building walls ceilings attics crawlspaces or roofs where it can condense and cause building materials to rot or grow mould. This eliminates the need for a separate air barrier that could potentially prevent the moisture from escaping. When we use a vapor retarder in a roof system it will also act as an air barrier as long as it is sealed at all perimeters and penetrations and is tied to the wall air barrier. The purpose of vapour barriers.

In other words it is used to prevent the formation of condensation in a low slope roof or wall system. Specific vapor retarders are often unnecessary if appropriate air movement control is provided or if control of condensing surface temperatures is provided. Considering that some deck board products may be vapor semi impermeable omitting this component will save cost and may foster greater drying potential toward the conditioned building interior. The purpose of a vapor retarder is to minimize or reduce water vapor diffusion into a low slope roof or wall system.

And since most vapor barriers are not obvious to the homeowner they are an often neglected part of many do it yourself construction projects. The function of a vapor barrier is to retard the migration of water vapor. Vapor barriers are not typically intended to retard the migration of air. Everyone knows that water in roof systems is never a positive.

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. Damage from water condensation due to water vapour movement called water vapour drive can wreak havoc on even the sturdiest of built structures and threaten the effectiveness of insulation. Vapor diffusion should be considered as a secondary moisture transport mechanism when designing and building roofs. Without a vapor retarder the deck board sometimes called a roof board or substrate board under the insulation layers has much less utility.

A vapour barrier is an important component in building construction.