Water Vapor Diffusion Control: Do Your Homework…Before You Build

Water vapor diffusion speaks to the movement of water in its gas or vapor form into and through materials. Thus, water vapor control involves selecting materials and assemblies with appropriate properties to manage, not eliminate, the effects of water vapor.

The goals of water vapor control are:

• To prevent excessive vapor movement into assemblies,
• To allow vapor to move out, and
• To control temperatures within the assembly to keep it in the vapor form (e.g., prevent condensation). When organic materials like wood are exposed to excessive water vapor or condensation, they will increase in moisture content to a point where damage may occur.

Thus, water vapor control is a climate-dependent balancing act to control the accumulation of moisture to tolerable levels and ensure that the ability of an assembly to dry out exceeds its potential to get wet. In other words, moisture that accumulates to tolerable levels due to diffusion in one season must dry out by diffusion in the next season to avoid a long term cycling of moisture that results in a gradual year-to-year accumulation. In addition, to prevent mold growth moisture accumulation within organic materials (e.g., wood, paper-faced drywall) or high surface RH levels on these materials (e.g., greater than 80%) must be prevented from occurring over an extended period of time.

To the extent that diffusion drying (moisture removal) exceeds diffusion wetting (moisture accumulation) over the course of a year, an assembly is considered to be more or less able to tolerate unintentional and uncertain amounts of moisture that may be introduced from other sources such as rainwater leaks (Section 4.2) or moisture-laden air leaks (Section 4.3.2). Building materials such as dimensional lumber, which have the ability to “store” moderate amounts of water vapor, can also serve as a “buffer.” They can absorb moisture during seasons with greater levels, and then release it when conditions are more conducive to drying. However, no reasonable approach to water vapor control is able to make up for leaks of rainwater or moist air due to poorly installed water-resistive barriers, flashing, and air barriers. Therefore, the recommendations for water vapor control in this section are predicated on a quality execution of recommendations in Section 4.3.2 and Section 4.2. Similarly, excessive indoor relative humidity can overwhelm a reasonable vapor control strategy; control of indoor relative humidity is addressed in Chapter 5.

As shown in Figure 4–18, adequate moisture-vapor control is much like the well-known fire prevention triangle. If any point of the triangle is not properly addressed, water vapor condensation and moisture accumulation problems are more likely to occur. A balanced design approach will consider all of these factors.

Regarding the “peak” of the triangle in Figure 4–18, an optimal range for indoor relative humidity is no more than 40% in the winter and no more than 60% in the summer. In particularly cold climates (e.g., Climate Zones 5 or greater), even lower indoor relative humidity levels during winter are recommended and may be needed as part of a reasonable water vapor control strategy. This recommendation is based on balancing an acceptable range of indoor relative humidity for occupant comfort (30% to 60% RH) with the need to protect the building envelope from excessive water vapor flows during the winter heating and summer cooling seasons. Controlling indoor relative humidity is addressed in Chapter 5. In addition, construction moisture (Section 4.3.4) and foundation moisture (Chapter 3) can contribute significant amounts of water vapor to indoor air if not adequately addressed.

The base of the triangle in Figure 4–18 involves the control of water vapor diffusion and internal surface temperatures. These relate to design of the building envelope assembly and require an integrated approach. There are three commonly accepted practices that, when done correctly, provide walls with a greater tendency to remove moisture by diffusion rather than allowing moisture to accumulate via moisture diffusion.

One method controls moisture condensation or accumulation by reducing the entry of water vapor into the assembly (i.e., more emphasis is placed on vapor retarders). This strategy (Figure 4–18 lower left) also uses a vapor permeable wall exterior to allow outward drying during the winter. This assembly assumes vapor permeable cavity insulation.

A second method (Figure 4–18 bottom center) incorporates air and vapor impermeable (low permeance) insulation materials in the cavity to control vapor flow through the assembly while still allowing materials on each face to dry to the interior or exterior, respectively. Assemblies using this strategy include SIPs panels and wood-framed assemblies with the cavity filled with air and vapor impermeable spray foam insulation.

A third method (Figure 4–18 lower right) uses exterior insulation to keep the temperature of the assembly warmer. This temperature control approach prevents condensation from occurring on a surface within the assembly that might otherwise drop below the “dew point” temperature.17 This assembly assumes vapor permeable cavity insulation.

Finally, “vapor open” walls that allow water vapor diffusion wetting and drying in both directions may also be considered, but usually work best in dryer climates and those that are not very cold. Again, balance is the key.

The principles behind the vapor control approaches described above are incorporated into “three rules” for water vapor control:

1. Maintain the envelope assembly’s ability to dry in at least one direction by not installing low-perm vapor retarders (e.g., vapor barrier) or other low-perm vapor retarding materials onboth sides of an assembly.
2. Do not put a low-perm vapor retarder (e.g., vapor barrier) or vapor retarding materials (e.g.,vinyl wall paper) on the inside of an assembly in a hot/humid climate.
3. Depending on climate zone and insulation strategy, seek to optimize the assembly’s ability todry and limit the potential for wetting by vapor diffusion.

The first two rules are pretty straightforward, yet deserve mention because failures to follow these two rules have resulted in significant moisture damage to buildings. The last rule, however, is a good bit trickier because it is a matter of optimization and there are many ways to achieve an optimal or at least acceptable assembly (and also many more ways to achieve one that is not).

Tables 4–11 and 4–12 are offered as aids in applying the above three rules in most climate zones. Climate Zone 8 (subarctic) is excluded from this document due to its limited application in the United States and, in such severe conditions, the reader is encouraged to use a specially designed solution that meets or exceeds locally applicable code requirements. The provisions summarized in Tables 4–11 and 4–12 are based on a compilation of requirements in Part 9, Sections 9.25.4 and 9.25.5 of the 2010 National Building Code of Canada, Section 702.7 of the 2015 International Residential Code, Section 1405 of the 2015 International Building Code, and various substantiating technical resources.18, 19, 20, 21, 22, 23, 24, 25, 26, 27 Also included are a few modifications intended to enhance performance, offer guidance, or draw attention to considerations where the building code may be silent, incomplete or vague. However, it should be understood that these recommendations are based on various sources and assumptions, and do not preclude alternative solutions or exclusively define acceptable solutions. Therefore, the user is encouraged to use good judgment, seek professional assistance, verify the suitability of these requirements, and confirm compliance with the locally applicable building code and energy code.

The application of Tables 4–11 and 4–12 requires an understanding of the following code definitions:

VAPOR RETARDER CLASS. A measure of the ability of material or an assembly to limit the amount of water vapor that passes through that material or assembly. Vapor retarder class shall be defined using the desiccant (dry cup) method with Procedure A of ASTM E 96 as follows:

Class I: 0.1 perm or less (e.g., sheet polyethylene, unperforated aluminum foil)
Class II: 0.1 < perm ≤ 1.0 perm (e.g., kraft-faced fiberglass batts)
Class III: 1.0 < perm ≤ 10 perm (e.g., latex or enamel paint appropriately rated and installed)

VAPOR PERMEABLE. The property of having a water vapor permeance rating of 5 perms (2.9 x 10-10 kg/Pa · s · m2) or greater, where tested in accordance with the desiccant method using Procedure A of ASTM E 96. For the purposes of this document, vapor permeability also is permitted to be assessed using the wet cup method (Procedure B) of ASTM E96.

The vapor permeance of specific vapor retarder materials should be verified by checking product-specific test data in the process of applying the requirements of Tables 4–11 and 4–12. In addition, other wall material layers such as exterior sheathing material, building wrap, spray foam, and foam sheathing also should be verified. While generic data is available from various sources, such as the ASHRAE Handbook of Fundamentals, product-specific test data from the product manufacturer is usually the best source as properties can vary significantly even among like products. It also is important to consider the product manufacturer’s design data and technical resources which may contain alternative solutions than those included in Tables 4–11 and 4–12.

While the vapor retarder class as defined above is based on vapor flow under relatively dry conditions, it also is important to know if the material’s properties change when tested under more humid conditions (i.e., perm rating per the wet cup or Method B of ASTM E 96). For example, uncoated wood sheathing products may be rated as a Class II vapor retarder under the “dry cup” condition, but can become vapor permeable (> 5 perm) under the “wet cup” condition. Similarly, some common vapor retarders, such as kraft paper, and other proprietary “smart vapor retarder” products, exhibit an ability to restrict vapor flow under low humidity conditions and permit vapor flow when drying conditions are needed. For example, the ASHRAE Handbook of Fundamentals reports that kraft paper has an ASTM E96 dry cup perm rating of about 0.3 perms (Class II vapor retarder) yet also has an ASTM E96 wet cup perm rating of about 1.8 perms or greater (Class III vapor retarder)—a factor of 6 difference in vapor permeance due to difference in ambient air humidity. Thus, the selection of a smart vapor retarder can help optimize an envelope assembly to resist wetting during one period of the year and yet also promote drying during another season.

Example: Using insulation ratios to check energy code solutions and alternative wall insulation strategies for adequate moisture durability

Given: Assume the energy code requires R20+5 (2×6 wall with R20 cavity insulation and R5 continuous insulation).

Find: What is the maximum (coldest) permissible climate zone for this wall with a Class II or Class III interior vapor retarder?

Solution: First, determine the insulation ratio, Re/Ri = 5/20 = 0.25. In accordance with Table 4–12, the maximum/coldest climate zone is 6 with a Class II interior vapor retarder and Climate Zone 4 with a Class III interior vapor retarder. While this example assembly may be permitted as a prescriptive solution in the energy code, the insulation ratio should be checked as demonstrated in this example. Consequently, the insulation locations and amounts may need to be adjusted to achieve moisture control while also still complying with the required energy code thermal performance. Some creative options are discussed next.

For example, changing to a R13+R10ci insulation strategy using a 2×4 wall which is thermally equivalent will increase the insulation ratio to 10/13 = 0.77, providing much improved water vapor control or the ability to tolerate higher indoor RH conditions.

Alternatively, closed-cell spray foam may be added to the cavity to achieve required thermal and moisture control performance (e.g., see Note c in Table 4–12). This is often efficiently done using a “flash and batt” approach (which may also be combined with exterior continuous insulation) as a means of economizing insulation cost while also creating an air barrier.

Example: Getting More Creative with Insulation Ratios

Given: Consider a wall assembly comprised of R15 high density batt insulation in a 2×4 wall, the use of exterior continuous insulation, and R2 insulating (foam backed) vinyl siding.

Find: What would be the required R-value (and thickness) of the exterior continuous insulation to use this assembly in Climate Zone 6 with a Class III interior vapor retarder (latex paint on drywall)?

Solution: In accordance with Table 4–12 a minimum Re/Ri ratio of 0.5 is required. Thus, the exterior continuous insulation amount must be at least R15 x 0.5 = R7.5. Because the insulating siding provides at least R2 of this exterior continuous insulation, the insulated sheathing only needs to make up the difference of 7.5R – R2 = R5.5.

Thus, the following insulated sheathing options are possible:

• 1.5” of EPS foam sheathing (R6),
• 1” of XPS (R5) plus an insulated vinyl siding of R2.5 instead of R2, or
• 1” of foil-faced polyisocyanurate foam sheathing (R6)

While the above wall meets the moisture control objective, energy code compliance must also be checked.

For conditions, materials, or envelope strategies not addressed by the recommendations in Tables 4–11 and 4–12, it is especially important to consult a building science professional to conduct a hygrothermal analysis of the proposed envelope assembly using material-specific properties and site-specific climate data—especially if a project involves risk factors such as anticipated high indoor RH levels or extreme temperature conditions.

Other Considerations in the Use of Foam Sheathing (Continuous Insulation): As with most sheathing materials, there are factors that must be considered other than moisture control or thermal performance. For example:

• Where foam sheathing is not used as “oversheathing” (e.g., installed over a separate structuralsheathing material), its wind pressure capability should be verified (refer to Section 316.8 of the2015 IRC and the manufacturer’s code compliance data).
• Where vinyl siding is installed over foam sheathing alone (e.g., without a structural sheathingmaterial), vinyl siding wind pressure ratings may require adjustment in higher wind loading conditions (refer to Section R703.11.2 of the 2015 IRC). This has the benefit of encouraging the use of “above-minimum” vinyl siding products resulting in improved wind performance (durability) and also may result in installations with a smoother and straighter finished appearance.

Alternatively, proprietary composite structural insulated sheathing products may be considered as a means of addressing the above considerations.

Where thick foam sheathing is used, additional corner framing members or wider furring materials may be required to allow attachment of furring and/or cladding through thick foam sheathing at these locations. For additional guidance on these and other matters related to the use of foam sheathing refer to Additional Resources in Section 4.4.