The ways moisture moves through a building enclosure are complex and collectively comprise a phenomenon that affects the way buildings are designed and constructed.
Two major modes of moisture travel—gravity and capillary action—are related to bulk water management, which is essential to the longevity of any assembly consisting of porous materials.
However, two other modes of moisture travel—vapor-laden air infiltration and vapor diffusion—relate to how water vapor works through an assembly. The former can be mitigated by an air barrier; the latter can be addressed by a vapor barrier.
Moisture Management and the Unique Function of Vapor Barriers
A vapor barrier, sometimes referred to as a vapor retarder, is a material in a building assembly characterized by low vapor permeance and impeded water vapor diffusion.
Similar to heat, moisture constantly seeks equilibrium. The interplay of heat and humidity has a dynamic effect on the "drive" of heat and vapor through any building assembly. As such, the predominant directionality of vapor drive can shift daily and, in a predominant sense, seasonally.
It's important to understand vapor drive and always afford an assembly the opportunity to eventually dry. Trapping moisture may lead to condensation and the build-up of bulk water content.
Distinguishing Vapor Barriers from Air Barriers
Sometimes, air barriers and vapor barriers are erroneously conflated. They are distinct:
- An air barrier is a dedicated component of a building enclosure that seeks to establish greater air tightness through a comprehensive system of coordinated building products and materials that, in concert, establish a continuous "barrier" to air impedance.
- A vapor barrier is a dedicated component of a building enclosure that seeks to mitigate vapor diffusion through a comprehensive system of coordinated building products and materials that, in concert, establish a consistent level of permeance throughout the enclosure.
These terms are often conflated because certain products and materials can effectively serve both functions. However, one should never assume this to be the case.
What Is the Difference Between Permeance and Permeability?
There are two measures used to gauge the rate of water vapor transmission through building materials:
- Permeance indicates the water vapor transmission rate over the course of one hour through one square foot of a material of a given thickness at a specified vapor pressure, expressed in "perms" (gr/hr × ft2 × inHg).
- Permeability is essentially the permeance per unit of thickness—or perm-inches. This measure is useful when considering building materials that come in a variety of thicknesses within an assembly, such as insulation.
Be careful not to confuse permeance (perms) and permeability (perm-inches).
Vapor Barrier Classifications
In the 2024 International Building Code and 2024 International Residential Code, the International Code Council (ICC) uses permeance to categorize vapor retarders by three classes using the desiccant method with Procedure A of ASTM E96 as follows:
- Class I: 0.1 perm or less (e.g., sheet polyethylene)
- Class II: 0.1 < perm="">< 1.0="" perm="" (e.g.,="" kraft-faced="" fiberglass="">
- Class III: 1.0 < perm="">< 10.0="" perm="" (e.g.,="" latex="">
The three classifications are widely recognized; though, in many instances, they are reconciled with a more descriptive terminology:
- Vapor impermeable: 0.1 perm or less
- Vapor semi-impermeable: 0.1 < perm="">< 1.0="">
- Vapor semi-permeable: 1.0 < perm="">< 10.0="">
- Vapor permeable: greater than 10 perms
As such, the ICC defines a vapor retarder as having a perm rating of 10 or less (e.g., Class I, II, or III). Though vapor barriers are not defined by the ICC, generally, one might reasonably presume the term "barrier" to infer a relatively lower perm rating among the three classifications—perhaps 1.0 or less (e.g., Class I or II).
Different Types of Vapor Barriers
A range of building products is available that can serve the function of vapor barriers, including:
- Sheet products. Polyethylene is relatively inexpensive and is common in residential construction. Foil-faced membranes are frequently applied in wall and roof assemblies; they are often reflective and can double as a radiant barrier.
- Fluid-applied products. Applied to sheathing or concrete surfaces, these products may be spray-applied or rolled on to establish a joint-free, seamless membrane.
- Coated or impregnated paper products. Asphalt-impregnated kraft paper is common. Continuity can be a challenge when such products are integrated with fiberglass batts for infill solutions with framing.
- Spray foam and foam board insulation. Closed-cell spray polyurethane foam is a common example of this type, which at two inches, may achieve a Class I classification.
- Paints and coatings. Epoxy coatings and certain low-permeance latex paints can achieve at least a Class III classification.
- "Smart" membranes. A twist on the sheet product type, these products exhibit an altered permeance at different humidity levels, which is ideal for mixed climates.
- Integral vapor barriers. Certain building enclosure solutions, such as insulated metal panels, structural insulated panels, and certain sheathing products, have such low permeance that they function as vapor retarders—intentionally or not.
Key Design and Detailing Considerations of Vapor Barriers
Do Not Trap Moisture
Every building material exhibits some degree of permeance, certainly when installed as a system. As such, make sure to select appropriate materials and avoid creating two separate vapor barriers in the same assembly.
A 4-inch brick might be 0.8 perms; plywood sheathing could be 0.7 perms; extruded polystyrene might be 1.2 perms. It is important to understand the total vapor transmission resistance of an assembly. It is also vital that a designer determines if a vapor barrier is necessary and, if so, ensures that the assembly does not unintentionally establish a second barrier in addition to a "designated" barrier.
Recognize When Double Vapor Barriers May Be Permissible
In certain circumstances, an assembly may exhibit back-to-back low-permeance materials that both technically qualify as vapor barriers—an insulated low-sloped roof assembly that includes an air barrier below the insulation is a common circumstance where this is encountered.
If both materials serve the common function of bulk water resistance, then they may collectively serve as a more robust water-resistive barrier. However, as noted previously, trapping moisture could be problematic. As such, avoid back-to-back Class I vapor barriers.
The Standard of Care Might Require Hygrothermal Modeling
Keep in mind that codes, standards, and guidelines typically offer broad-based criteria for vapor barriers/retarders based on climate zones. A hygrothermal analysis based on exterior climate, interior climate, solar absorptance, rainwater absorption, and the vapor and thermal resistance of all components in the assembly can help designers better assess both the need and effect of establishing vapor barriers.
Placement Matters
The rule of thumb is to locate the vapor barrier on the "warm side" of the thermal barrier. In cold climates, that is toward the interior; in hot climates, that is toward the exterior. In many locations, the "warm side" changes throughout the year. Perform a hygrothermal analysis and choose the best product for the specific circumstance.
Detailing and Field Verification
Always ensure continuity. This entails sealing all joints and penetrations—using compatible tapes, sealants, or gaskets to seal edges, overlaps, electrical boxes, pipes, and fasteners—and field verifying that the installation was executed properly.
Do Due Diligence
Confirm that the solution neither runs afoul of regulatory requirements nor runs the risk of voiding a product warranty. If it does, engage stakeholders and pursue an evidence-based resolution.
Selecting the right solution depends on the specific vapor control needs of the project, the local climate, and how the overall assembly manages moisture.
Sources:
- ICC, 2024. "2024 International Building Code." Washington, DC: International Code Council.
- ICC, 2024. "2024 International Energy Conservation Code." Washington, DC: International Code Council.
- Lstiburek, J., 2006. "Builder's Guide to Cold Climates." Somerville, MA: Building Science Press.
