Understanding 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 download our full guide.
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.
Understanding Air Leakage in Walls
While vapor diffusion is a relatively slow process which can take extended periods to accumulate enough water to be of concern, air leakage in wall assemblies can act much more quickly and deposit large amounts of water in a short period of time.
To control air leakage in building enclosure assemblies like walls, an air barrier is installed. Unlike diffusion, air leakage is not typically governed by material properties of this air barrier. Instead, air leakage most often occurs at discontinuities (i.e. holes) in the air barrier. Consequently, prevention of air leakage depends primarily on detailing, material compatibilities and quality control during both the design and construction process.
When air leakage does occur, it can carry moisture with it, and if this air then comes in contact with a surface that is below the dew point temperature of the air, condensation can occur. Condensation from air leakage can deposit significant amounts of moisture within a wall and potentially lead to fungal growth and/or degradation.
Schematic vertical cross-section of wall with air leakage condensation occurring on the interior face of the exterior gypsum sheathing.
Wrong Side and Double Vapor Retarders
While vapor retarders can be used to control vapor diffusion and consequently prevent condensation, installation of these types of materials at the wrong location within an assembly can also cause significant moisture problems. Typically, there are two conditions which can be created which are detrimental: wrong side vapor retarders, and double vapor retarders. Wrong side vapor retarders refers to when a vapor retarder is placed on the low vapor pressure side (typically the cold side) of a wall assembly.
Schematic vertical cross-section showing condensation of moisture on a vapor retarding material placed on the wrong side of a wall assembly.
Double vapor retarders refers to when a vapor retarder is installed at two different locations in an assembly such that any moisture which manages to get between them is unable to dry effectively.
Schematic vertical cross-section showing moisture trapped within a wall assembly due to the presence of two vapor barriers.
Vapor Diffusion Drying
Vapor diffusion is a positive mechanism that can be used to a designer’s benefit, and is a very important drying mechanism for a wall assembly. In fact, vapor diffusion is the only process through which the interiors of most wall assemblies are able to dry in service.
In a cold climate, the more vapor permeable the sheathing, sheathing membrane and cladding layers, the faster this moisture can dry out, and consequently the risk of damage is reduced. This is why in some cases walls without vapor barriers can perform adequately – the drying ability exceeds the wetting.
Vapor permeable insulation such as ROXUL® stone wool insulation will allow for greater outward drying than can be achieved with vapor impermeable insulation such as foam plastics. This greater drying ability generally results in improved durability of the wall assembly.
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