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Understanding Vapor Diffusion and Condensation

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Understanding Vapor Diffusion and Condensation uilding enclosure assemblies temperature 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. Vapor resistance is commonly to deliver long-lasting sepa- the temperature at which condensation 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 pressure 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 water 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 climate 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 climates, 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 water vapor molecules through Vapor-retarding porous materials (e.g., wood, insulation, materials are used drywall, etc.) as a result of differences in to control vapor dif- vapor pressure. Vapor pressure differences fusion through wall occur as the result of variations in air tem- assemblies. All build- perature and sources of humidity, 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). Dew 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 dew point 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 still 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.
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