Moisture Management In Exterior Wall Assemblies
Against this backdrop, it’s not surprising to discover there is a great body of research being conducted by building scientists and industry organizations to determine how best to protect exterior wall assemblies from moisture intrusion, while at the same time allowing them to breathe so that moisture buildup on the inside can escape.
Much of what they’re finding is that the materials developed years ago, that have proven the test of time in the field, oftentimes do the best job of managing moisture in wall structures. In some cases, they do a better job than newer materials that have come on the scene in the last few years.
Building basicsTo be effective, an exterior wall assembly must embody the four Ds of moisture management: deflection, drainage, drying and durability. We will explore each of the four Ds in detail by examining a typical stucco wall assembly.
But we also need to consider the often overlooked fifth D of moisture management: do-ability. Watertight wall assemblies need to be practical in both design and execution. Regardless of how well an exterior wall assembly or building product performs in a laboratory, if those results can’t be achieved within the context of real world construction practices, the building component or assembly is of little practical use.
Deflection:Deflection is the primary means by which an exterior wall assembly protects the building envelope from moisture intrusion. While stucco will resist the majority of bulk water it encounters, under wet conditions it does absorb moisture. However, liquid water can only penetrate stucco assemblies through defects in the wall assembly, such as through cracks or failed sealant joints. The deflection of any wall assembly can be improved by limiting the exposure of the exterior to bulk water, namely through the use of roof overhangs and eaves in the exterior design. Unfortunately, current trends in construction have all but eliminated these favorable design elements from exterior wall assemblies.
Drainage: Presuming that a certain amount of bulk water will penetrate the exterior cladding through defects in design or construction, it is imperative that the wall assembly provide a means of draining away such incidental moisture intrusion. In stucco assemblies, this is accomplished by means of building paper, flashing and weep screeds. When properly integrated, these materials provide both an effective and efficient drainage plane to evacuate any water that gets behind the stucco cladding.
Recent independent studies have noted that asphalt-saturated kraft building paper is particularly well suited for this task. In comparative testing conducted by Building Science Corp. of Westford, Mass., stucco assemblies incorporating two layers of Grade D building paper over wire and open studs delivered the highest drainage performance of tested assemblies. Among the poorest performers were stucco assemblies employing 15-pound felt over OSB sheathing, and creped plastic wrap over OSB sheathing.
The reason behind the strong performance of building paper in stucco assemblies has been noted by building organizations such as the Northwest Wall and Ceiling Bureau. In testing conducted by the NWCB, it was observed that, during the stucco curing process, building paper forms natural drainage channels that provide a separation from the back of the stucco cladding, creating an optimal drainage plane. Joseph Lstiburek, principal with Building Science Corp., has noted that this separation does not occur with stucco applied directly over plastic wraps, and cautions that stucco should never be installed in direct contract with plastic-based housewrap.
Drying:Because situations can arise where moisture finds its way into the wall cavity or in direct contact with the exterior sheathing, most often due to defective or improperly integrated building materials, the wall assembly must provide an additional means for the trapped moisture to escape. Building paper provides this capability by virtue of its rate of moisture vapor transmission, the rate at which moisture vapor can pass through the material.
The MVT of building paper, combined with the natural separation of building paper and stucco cladding, is ideally suited to provide the optimum release of any moisture that finds its way behind the drainage plane. In fact, preliminary findings of a current research study indicate that building paper responds to changes in relative humidity in the most favorable way possible. In high-humidity conditions, when drying of the wall assembly is most critical, the MVT of building paper is at its highest, providing the greatest capacity for release of moisture trapped within the wall assembly.
Asphalt-saturated kraft building paper also increases the drying potential of the wall assembly. Because today’s homes are built much tighter than in decades past, exterior walls have less capacity to deal with moisture that makes its way behind the exterior cladding. If water gets into the wall assembly, it stays in, ultimately leading to deterioration of building materials. But because building paper has the ability to intercept moisture and release it over time, it is far less susceptible to moisture overload than other types of weather-resistive barriers such as plastic housewraps.
What this means is that building paper is unique in its ability to adapt to changing environments—by providing both a high resistance to moisture penetration and the highest capacity for drying—making it the right material for many climatic conditions.
Durability:The long-term success of any wall assembly is dependent upon its component building materials maintaining their integrity and performance characteristics over the long haul. Although stucco may develop hairline cracks over time, the cladding is remarkably durable, and will maintain its water-resistive qualities for tens if not hundreds of years.
That same durability can also be found in building paper, which has been commonly used in stucco assemblies for more than four decades with a proven track record of success.
Do-Ability:An exterior wall assembly that properly addresses all four desired performance criteria provides the proper foundation for long-term weather protection. However, if the design details can’t—or won’t—be properly assembled in the field, the wall assembly is likely to fail at some future point.
Consider flashingsOne key consideration is the proper integration of window and door flashings with the weather-resistive barrier and other building components. All materials must be installed in shingle fashion, with proper overlaps, so that any moisture that makes its way behind the cladding stays on top of the drainage plane until it can be discharged from the wall assembly.
With some approaches to moisture protection, such as liquid weather barriers applied directly to the sheathing, proper sequencing can be difficult, if not impossible, to achieve.
Compatibility of building components is also an important consideration. Sealants with high solvent or plasticizer content, for example, can attack bitumen flashing products. Also, some rubberized asphalt products are incompatible with single-ply flexible PVC roofing materials.
When selecting individual components of exterior wall assemblies, the operative word is caution. Before relying on any material, check first with the manufacturer to verify that the product is appropriate and compatible with all other materials used in the wall assembly. Or better yet, work with a manufacturer that provides a complete system of compatible materials.
Given the heightened risk of litigation to contractors, it is foolish to take chances with exterior assemblies, particularly when there are already a number of proven building materials and assemblies with an extensive history of success.
While “new-fashioned” building materials may have sex appeal—particularly to those manufacturers who introduce them—what is most important is that exterior wall assemblies meet the five essential criteria for moisture management: deflection, drainage, drying, durability and do-ability.
Asphalt saturated kraft building paper may not be the latest fad in building materials, but it performs the way it should—both in the lab and the field—and has been delivering this superior protection far longer than virtually every other weather-resistive barrier or housewrap using in homebuilding today.
In an environment where the failure of a wall system could mean a major lawsuit, how much do you really want to experiment? W&C