Vapor Diffusion


Ever more stringent energy code requirements have necessitated increased insulation levels for non- combustible (i.e. steel stud, CMU, concrete) walls, and in many cases this includes adding exterior insulation. Additional insulation thickness and changes to the insulation location require reconsideration with regards to vapor diffusion and condensation control.

Understanding Vapor Diffusion

Vapor diffusion is the movement of water vapor molecules though porous materials (e.g. wood, insulation, drywall, concrete, etc.) as a result of vapor pressure differences. Vapor pressure differences occur as the result of temperature and water vapor content differences in the air. Vapor diffusion flow always occurs through an assembly from the high to low vapor pressure side, which is often from the warm side to the cold side because warm air can hold more water than can cold air. In cold climates, this means that vapor diffuses primarily from the heated interior to the colder outdoors, whereas in hot climates, the vapor drive is reversed and instead is primarily from the warm humid exterior to the air conditioned interior. The direction of vapor diffusion can also be reversed when the sun heats up damp, absorptive wall claddings like masonry and drives water vapor inward.


Example wall assembly showing inward (left) and outward (right) vapor drive for hot and cold climates, respectively.

Overall, the direction of the vapor drive has important ramifications with respect to the placement of materials within a wall assembly. What works in Toronto or New York likely won’t work in Miami. Improper use of vapor impermeable materials within a wall can lead to condensation on colder surfaces and lead to damaged materials and fungal growth.

For detailed information on vapor diffusion and condensation control for commercial wall assemblies view our full guide under the Technical Documentation tab below.

For assistance with building modelling, contact Technical Innovations at 1-877-823-9790 or at

Vapor Retarders

Understanding Vapor Barriers & Retarders

To control vapor diffusion within wall assemblies, vapor retarding materials are used. All building materials provide some resistance to vapor diffusion that varies depending on the properties of the material.  Vapor resistance is commonly expressed using the inverse term “vapor permeance” which is the relative ease of vapor diffusion through a material.

The metric units for vapor permeance are “ng/Pa∙s∙m²” or in IP units are “grains/inHg∙ft²∙hr”, the latter of which is more commonly known as a “US perm.” One US Perm is the same as 57.4 ng/Pa∙s∙m².(Classes I, II, III) depending on their vapor permeance values. Class I (<0.1 US perm), and Class II (0.1 to 1.0 US perm) vapor retarder materials are considered impermeable to near impermeable, respectively, and are known within the industry as “vapor barriers.”

The following graphics illustrate how a vapor retarder can be used in a cold or a hot climate to control the diffusion of vapor through the wall assembly.



Schematic vertical cross-section showing how a vapor barrier on the interior or left side of a wall assembly (left) and on the exterior or right side of a wall (right) can control vapor diffusion through the assembly in a hot a cold climate, respectively.