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uilding enclosure assemblies is the temperature at which the moisture content, age, temperature, and serve a variety of functions RH of the air would be 100%. This is also other factors. resistance is commonly to deliver long-lasting sepa- the temperature at which will expressed using the inverse term “vapor ration of the interior building begin to occur. permeance,” which is the relative ease of environment from the exteri- The direction of vapor diffusion flow vapor diffusion through a material. or, one of which is the control through an assembly is always from the Vapor-retarding materials are often Bof vapor diffusion. Resistance to vapor diffu- high vapor side to the low vapor grouped into classes (Classes I, II, III) sion is part of the environmental separation; pressure side, which is often also from the depending on their vapor permeance values. however, vapor diffusion control is often warm side to the cold side, because warm Class I (<0.1 US perm) and Class II (0.1 to primarily provided to avoid potentially dam- air can hold more than cold air (see 1.0 US perm) vapor retarder materials are aging moisture accumulation within build- Figure 2). Importantly, this means it is not considered impermeable to near-imperme- ing enclosure assemblies. While resistance always from the higher RH side to the lower able, respectively, and are known within to vapor diffusion in wall assemblies has RH side. the industry as “vapor barriers.” Some long been understood, ever-increasing ener- The direction of the vapor drive has materials that fall into this category include gy code requirements have led to increased important ramifications with respect to the polyethylene sheet, sheet metal, aluminum insulation levels, which in turn have altered placement of materials within an assembly, foil, some foam plastic insulations (depend- the way assemblies perform with respect to and what works in one may not work ing on thickness), and self-adhered (peel- vapor diffusion and condensation control. In in another. Improper use and placement of and-stick) bituminous membranes. Class III particular, for construction in cold , vapor-impermeable materials within a wall (1.0 to 10 US perm) vapor retarder materials these changes have led to the widespread can lead to condensation and, potentially, are considered semipermeable, and typical use of exterior-insulated and split-insulated to damaged materials wall assemblies (see Figure 1). and fungal growth.

PrinCiPles of VaPor diffusion Controlling Fundamentally, vapor diffusion is the Vapor diffUsion movement of through Vapor-retarding porous materials (e.g., , insulation, materials are used drywall, etc.) as a result of differences in to control vapor dif- . Vapor pressure differences fusion through wall occur as the result of variations in air tem- assemblies. All build- perature and sources of , such as ing materials provide occupants, showers, pools, plants, etc. The some resistance to commonly used term “relative humidity” vapor diffusion, and (RH), which is expressed as a percentage, the amount of resis- refers to the amount of water vapor in the tance varies depend- Figure 1 – Exterior-insulated (left), split-insulated (middle), and air (i.e., the vapor pressure) divided by the ing on the properties stud-cavity-insulated (right) steel-stud walls are three ways maximum amount of water vapor that the of the material. These to insulate the building enclosure, but these walls can provide air could hold at the same temperature (i.e., properties can change significantly different performance with respect to vapor diffusion the saturation vapor pressure). point with the RH and and condensation.

a p r I l 2 0 1 7 I n t e r f a c e • 3 3 Figure 2 – Example of wall assembly showing outward vapor drive Figure 3 – Schematic vertical cross section showing how a vapor for a cold climate (interior on the left and exterior on the right). barrier on the interior (left) side of a wall assembly can control vapor diffusion through the assembly (interior on the left and materials that fall into this category include a wall assembly where exterior on the right). latex paints, plywood, OSB, and some foam it can restrict vapor plastic insulations (depending on thick- diffusion through the wall and potentially within assemblies. ness). Materials greater than 10 US perms create a condensing plane within the wall A potentially damaging example of are considered vapor-permeable. assembly (see Figure 4). restricted drying of an assembly can be Figure 3 illustrates how a vapor retarder caused by a condition referred to as a “dou- can be used in a cold climate to control the Wetting Vs. drying ble vapor barrier,” in which a vapor retarder diffusion of vapor through the wall assembly. Vapor diffusion is typically thought of is installed at two different locations in as a negative phenomenon—one that needs an assembly such that any moisture that When does Condensation oCCur? to be completely stopped. In reality, vapor manages to get between the vapor retarder Condensation occurs within a wall diffusion is a positive mechanism that can materials is unable to dry effectively. When assembly when the temperature of a mate- be used to a designer’s benefit, and is a very the materials between the vapor retarders rial in the assembly is lower than the dew important drying mechanism for an enclosure are moisture-sensitive, this trapped mois- point temperature of the air at that location. assembly. In fact, vapor diffusion is the only ture can lead to damage. Moisture between This will typically occur when the tempera- process through which the interiors of most the vapor retarders may be the result of ture of the cold side of the assembly is lower in-service wall assemblies can dry. The con- air leakage, rainwater ingress, or built-in than the temperature of the warm trol of vapor diffusion within a wall assembly construction moisture. Figure 5 illustrates side, and the materials on the cold (low is therefore a balance of minimizing or man- a schematic double vapor barrier situation vapor pressure) side provide greater resis- aging wetting sources and maximizing drying restricting the drying of a wall assembly. tance to vapor diffusion than the materials potential, should the wall be constructed wet on the warm (high vapor pressure) side of or somehow be wetted in-service. This is par- Wall assemBly design an assembly. An extreme example of this ticularly important with highly insulated wall For the design of durable wall assem- condition arises if a vapor retarder material assemblies, as more insulation means less blies, the placement of insulation and vapor- is placed on the low vapor pressure side of heat energy is available to dry moisture from retarding materials needs to be carefully

Figure 4 – Schematic vertical cross section showing condensation Figure 5 – Schematic vertical cross section showing air leakage (red of moisture on a vapor-retarding material placed on the wrong (cold arrow) from the interior to the exterior, causing moisture to become side) of a wall assembly (interior on the left and exterior on the trapped within a wall assembly due to the presence of a two vapor right). barriers (interior on the left and exterior on the right).

3 4 • I n t e r f a c e a p r I l 2 0 1 7 considered. Many build- ing codes and building enclosure design pub- lications provide guid- ance for the selection of appropriate vapor control layers within wall assem- blies in North American climate zones, based on the class of vapor retard- er (I, II, or III). This guid- ance is also based on the anticipated indoor condi- tions for certain building types, which is related to exterior climate, indoor moisture generation rates, and ventilation rates. Walls with insulation between the studs are pervasively used in North Figure 6 – Stud-insulated wall assembly using Figure 7 – Split-insulated wall assembly using semi- American construction mineral wool batt insulation is relatively standard rigid mineral wool insulation to provide a combination (see Figure 6). In cold and straightforward. of the performance of stud-insulated and exterior- climates, interior vapor insulated walls. These walls can provide good per- control is often provided by a polyeth- sheathing will be and the lower the formance with respect to vapor diffusion and air leak- ylene sheet vapor barrier, although other risk of condensation. This can also age, but is important to use the correct type of insul­ options—such as vapor barrier paint, Kraft apply to the steel studs, as when ation on both the exterior and within the stud cavity, paper, and smart vapor retarder prod- insulation is only provided in the and also the correct type of sheathing membrane. ucts—can also be used. This interior vapor stud cavity, these studs create retarder limits the diffusion of moisture significant thermal bridges that can also some drying to the interior is possible in the through the wall assembly toward the exte- potentially create condensation locations. event of moisture entering the stud cavity rior. Outward vapor diffusion drying can A special condition exists when a rela- (see Figure 8). In terms of balancing the occur from within the wall cavity to the tively vapor-impermeable insulation prod- wetting and drying potential, split-insulat- exterior through the sheathing, membrane, uct, such as many foam plastic insulation ed walls with vapor-impermeable exterior and cladding. products (i.e., extruded polystyrene [XPS], insulation are generally more sensitive than In some cases, it can be advantageous to polyisocyanurate, medium-density spray walls with permeable exterior insulation. use split insulated wall assemblies; that is, polyurethane foam) is used on the exterior. Recently, a number of projects have assemblies where insulation is provided both These insulation materials can be consid- occurred on which a vapor-closed insulation in the stud cavity and on the exterior of the ered Class I or Class II vapor retarders, (XPS) has been placed over a high- sheathing (or less commonly as the sheathing depending on type, density, thickness, and performance, vapor-permeable, self-adhesive itself). Usually, split-insulated wall assem- facings. This relative- blies are used because they can provide the ly vapor-impermeable necessary R-value in a relatively compact (i.e., layer on the exterior of thinner) assembly (see Figure 7). the wall assembly can Split-insulated walls can also include potentially lead to a an interior vapor retarder to control out- double vapor retarder ward vapor diffusion, but there is the situation when anoth- potential that the addition of exterior insu- er vapor retarder, lation will impact the diffusion of moisture. such as polyethylene Specifically, insulation on the exterior of sheet, is installed on the sheathing keeps the sheathing closer the interior of the wall to the interior temperature (i.e., warmer in assembly. To avoid a cold climate), generally reducing the risk this, a more perme- of condensation at the sheathing plane. In able interior vapor a split-insulated assembly, the more insu- retarder, such as a lation placed outboard of the sheathing smart vapor retard- Figure 8 – Schematic vertical cross section of a split-insulated compared to the insulation within the stud er, would typically be wall located in a cold climate with impermeable exterior insulation cavity, the closer to interior conditions the recommended so that (interior on the left and exterior on the right).

a p r I l 2 0 1 7 I n t e r f a c e • 3 5 sheathing membrane. Unfortunately, in this using vapor-permeable materials to the vapor-impermeable insulation such as foam arrangement, the vapor resistance of the exterior to encourage drying. plastics. Consequently, when a vapor-im- insulation negates the potential benefits of When insulation is added to the exterior permeable exterior insulation is used in cold the vapor-permeable sheathing membrane. of the walls (as in the case of split-insulated climates, an interior vapor barrier should be Vapor-impermeable and vapor-perme- or exterior-insulated walls), this insulation avoided to prevent trapping moisture within able exterior insulation can both be used maintains the temperature of the stud cav- the wall assembly. A more permeable (i.e., successfully to provide robust performance ity and exterior sheathing closer to interior Type II or III) vapor retarder or an adaptive, with respect to vapor diffusion when used conditions, reducing the potential for vapor permeance-smart vapor retarder material in fully exterior-insulated assemblies. These diffusion (and air leakage) condensation to may be appropriate. types of wall assemblies place all the insu- occur within the cavity. However, in cold Overall, the correct selection and place- lation on the exterior of the wall and move climates, the type of insulation installed ment of vapor-impermeable materials with- the vapor retarder from the interior of the outboard of the sheathing (or as the sheath- in wall assemblies is fundamental to their studs out to the exterior of the sheathing. ing) has an important impact on the vapor durability. Failure to correctly account for Bitumen-based, peel-and-stick-type mem- diffusion-drying capability of the wall. the impacts of vapor diffusion can lead branes are commonly used for this applica- Vapor-permeable insulation allows for great- to damage and premature failure of wall tion. By moving the insulation to the exte- er outward drying than can be achieved with assemblies. rior of the sheathing, all materials inside of the vapor retarder are kept warm, and all Lorne Ricketts is Marcus Dell is a materials outside of the vapor retarder are an associate and professional engi­ durable to wetting. Thus, the risk of damage building science neer who special­ from condensation is substantially reduced. engineer with RDH izes in practical Building Science solutions to build­ summary Inc. He specializes ing enclosure prob­ The control of vapor diffusion within in new construc­ lems. He combines walls is a balance between minimizing tion, investiga­ his academic train­ wetting and maximizing drying potential. tion, and research ing with over 20 Correctly placed vapor control layers pre- work, with typical years’ professional vent excessive moisture from diffusing into projects including experience to offer wall assemblies and potentially condens- Lorne Ricketts designing building Marcus Dell comprehensive ing, while vapor-permeable materials allow enclosure systems, knowledge of the moisture to diffuse outward and are benefi- hygrothermal and thermal analysis, and application of building science principles to cial to drying performance of an assembly. testing and monitoring of building perfor­ North American buildings. His focus at RDH In the design and construction of walls in mance. His combination of practical and the­ is on existing buildings and repair, renewal, cold climates, it has been common practice oretical expertise provides him with a unique and rehabilitation projects. He has published to install a polyethylene sheet vapor barrier perspective on building enclosure consider­ numerous papers on building envelope top­ at the interior of the insulation to control ations, which allows him to deliver innova­ ics. Marcus is a member of the RCI Interface vapor flow (and often air flow) and, there- tive practical solutions to complex problems. Editorial Board and a member of the Western fore, limit vapor diffusion wetting, while Canadian Chapter of RCI.

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