Vapor Retarder Membranes, What is a vapor retarder membrane

Vapor retarder membranes speaks to what is a vapor retarder membrane and explain vapor retarder membrane.

It is also commonly known as a vapour retarder or vapour barrier is a material used to slow the diffusion of water vapour through a building assembly. Vapour retarders are continuous sheets or coatings made of plastic, metal foil, coated paper or any other material resistant to the transfer of water vapour.

Vapour retarders are located toward the warmer side of the insulation in a building assembly. In this position, they can restrict the diffusion of water vapour into the assembly from the side of higher vapour pressure, limiting chances for dew point conditions and condensation to occur within the structure’s cooler portions.

For buildings in most parts of North America, where winter heating conditions predominate, vapour retarders are placed toward the interior, heated side of insulation in the building. In humid regions where warm weather cooling predominates within the building, the vapour retarder should be located toward the exterior side of insulation. In relatively mild or balanced climates, or where assemblies are designed to minimize condensation conditions, a vapour retarder may not be needed.

The better a material’s resistance to water vapour diffusion, the lower its vapour permeance and the more effective the vapor retarder. Permeance is measured in “perms” as definedby ASTM E96 as the passage of one grain of water vapour per hour through 1 square ft of material at a pressure differential of 1 inch of mercury between the two sides of the material. In metric units, a perm is measured in grams per second square meter per Pascal of pressure difference. One US perm equals 5.72 X 10 to the minus 8 metric perms.

When evaluating vapour retarders, vapour permeance should not be confused with vapour permeability, Vapour permeability is defined as a material’s vapour permeannce for a unit of thickness. The vapour permeance of some materials varies. Materials that absorb liquid moisture, such as plywood or the kraft paper facing found on much glass fiber batt insulation, tend to increase in permeance when damp.

Commonly used vapour retarder materials are polyethylene plastic sheet, kraft paper facing on glass fiber batt insulation, aluminum foil facing on various types of insulation, and special paint primers with low water vapour permeability. Some foam insulation materials, depending on their formulation and thickness, can also act as vapour retarders. In low slope roof construction, where vapour retarders are often installed as part of the roof membrane system, vapour retarders are often made of roofing felts layered in hot asphalt or of adhered rubberized asphalt sheets.


Design for water vapour control should consider all components of an assembly. The permeability of exterior sheathing, cladding or roofing, provisions for ventilation and the location and types of insulating materials within an assembly can all influence the choice and placement of vapour barriers.

Vapor retarder membranes are also for the control of water vapour. Under many circumstances, air leakage can transfer many times greater amounts of water vapour into a structure than can vapour diffusion.

Building structures must be able to dry out when wet. Water introduced into a building, such as penetrating rain or from construction with wet building materials must have a means to escape so that the assembly can dry.

The temperature and humidity conditions to which a building structure exposed over time are not static. A wall system designed exclusively for winter heating conditions may perform properly when vapour migrates from the exterior inward during summer months with cooling in operation. Solar heating of a rain soaked exterior cladding system can cause strong inward water vapour movement at any time of the year.

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