Screen Your Stucco: The importance of effective water-resistive barriers

by Katie Daniel | May 11, 2015 10:18 am

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by George Caruso
When addressing moisture management it is important to recognize and accept one simple and indisputable reality: water gets into buildings. No matter how tightly a building is constructed or how well it is insulated, and no matter what type of cladding is chosen and how expertly it is installed, moisture will inevitably find a way into the building.

Moisture infiltration can undermine structural integrity, cause exterior surfaces to deteriorate, and shorten the life of paints and stains. It can also foster mold and rot that not only cause structural damage, but also pose serious health hazards. Ridding a stucco wall assembly of liquid water and water vapor as quickly as possible—before it can damage structural components—is what moisture management is all about.

Stucco builds a following
During the upturn in U.S. construction from 1992 to 2006, stucco cladding material gained extreme popularity. Valued for its versatility and relatively low maintenance schedule, stucco offered builders a product with the potential to last for the structure’s life. However, buildings were constructed faster than good oversight could keep up with. As a result, water management was often overlooked.

“Builders were opting for a quicker, cheaper, and faster method over quality, proven systems and solutions,” said Kevin Thompson, president of The Green Valley Group, which provides inspections for moisture-related problems, thermal imaging, and building envelope forensic services.

Like many cladding solutions, stucco can have its challenges. However, the majority of issues are moisture related. These issues are incredibly problematic as they are hidden from view and often build up within the wall cavity for years before they are detected.

“The awareness of the issues with stucco has led owners of facilities both young and old to become more cautious of stucco applications,” Thompson noted. “All types of buildings have problems. It is just that stucco, though aesthetically beautiful, is one of the least forgiving products on the market when it comes to bulk water intrusion.”

For successful wall systems, an effective drainage and drying system is required. In stucco, when only minimum standards are followed, there is little room for drainage, and the system readily absorbs water—a combination making the product unforgiving of installation errors and lack of attention to detail.

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Improving WRBs
Water resistive barriers (WRBs), used as part of an exterior wall system, are designed to prevent air and water from entering the stud wall cavity from the outside. They also allow the free passage of water vapor to the outside of the building so the framing and wall cavity can dry, reducing the threat of mold and rot.

In an effort to minimize the problems surrounding stucco, builders have started integrating improved moisture management solutions. There are numerous options currently on the market, including traditional building paper, rainscreen systems, sealants, and self-adhered flashing membranes. Choices are expanding, driven by advances in technology, evolving building codes, and growing customer concern with mold prevention and other factors. Each of the aforementioned moisture management products has its place, depending on local environmental conditions and project details.

“Regardless of the product, drainage and drying are the top two concerns,” explained Thompson. “You cannot stop 100 percent of bulk water. So when the water gets behind the cladding, project teams must ensure continuity within the wall system making every effort to get it out from behind the system as quickly as possible—through both drainage and drying.”

Traditionally used as an element to resisting initial moisture infiltration, building wraps go a step farther by helping to remove trapped moisture from the building enclosure. Building wraps provide a vapor-permeable layer that resists liquid water from the outside, while also allowing water vapor to escape the assembly, enabling the wall to “breathe.” (For example, if water vapor is driven from the interior to the exterior during a heating season, the building wrap will allow the vapor to escape.)

An alternative to the traditional WRB is a high-performance drainable building wrap. Offering all features of a building wrap or building paper, drainable building wraps also include a drainage system. Drainable building wraps are designed to promote bulk water removal by channeling moisture to the outside through spacers or channels or manufactured into the building wrap sheet. The building wrap’s structure is engineered to maintain a more constant drain rate despite the repeated wetting and drying cycles observed in wet or humid climates.

These enhanced products provide a much higher and more constant drain rate than standard building wraps. Compared to standard versions, the newest drainable building wraps—especially those providing a minimum of a 1-mm (0.03-in.) gap—can be as much as 100 times more effective at removing bulk water from the wall.

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Dry it out
Experts say rain is the single most important factor to control in promoting sidewall durability, therefore, understanding specific local climate conditions is key to specifying the most appropriate building envelope product.

Even the most effective WRBs may still not solve the problem of drying out moisture that remains behind cladding. In regions prone to excessive precipitation, high temperatures, and humidity, a more robust solution may be called for, and a rainscreen wall may be the answer.

Originally developed for use in masonry construction, rainscreen walls are based on the scientific fact water will infiltrate all exterior cladding over time and therefore, a more forgiving water management solution must be employed. The rainscreen system controls rain entry in an exterior wall assembly by creating a pressure-moderated air space immediately behind exterior cladding along with a WRB. The air space reduces the forces drawing water into the assembly, as well as managing intruding water by creating a space behind the cladding allowing it to drain and exit. At the same time, a rainscreen offers accelerated drying of moisture-laden vapor that accumulates in the wall assembly by moving air in a convective fashion through the cavity.

There are two ways to construct an air space as part of a rainscreen system. The traditional method incorporates nailing wood furring strips—also called strapping—over wall studs and sheathing after applying a building paper or wrap. In recent years, building product manufacturers have developed “void space” products that achieve the same effect by using a 3D plastic matrix to create a vented continuous rainscreen on a roll. Builders can choose from several different varieties—a plastic matrix that can be applied directly over a WRB or special bonded products that combine the plastic matrix with a WRB for a one-step installation.

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The biggest advantage of strapping is lower material costs. However, installing these strips can prove time-consuming and labor-intensive, making strapping ultimately more expensive than void space systems. Strapping also creates hot spots along stud locations, where moisture is trapped because of wood-to-wood contact. In contrast to strapping, the consistency of manufactured products ensures the entire wall surface is protected from water infiltration and continuous air movement is optimized. The combination-manufactured systems also make for quicker installation, as the rainscreen and WRB are applied at the same time.

In addition, considerations like the cladding type, climate, geographic orientation, and wall assembly are important when selecting a WRB. The amount of annual rainfall can be used as a guide for determining the level of moisture management needed in a wall. The Building Enclosure Moisture Management Institute recommends “any area receiving more than 20 in. [508 mm] of annual rainfall should incorporate enhanced drainage techniques in the wall system, especially if using an absorptive cladding material.”

Additional factors such as geographic orientation of the wall, amount of overhang, altitude, and even trees and buildings in proximity will help contribute to a more knowledgeable design decision. Thorough examination of the factors will help in understanding the potential for wind pressure, wetting, and drying, and therefore what level of moisture management to incorporate into a wall.

Among the factors driving the need for better moisture management solutions are the continued growth and standardization of requirements such as the International Building Code (IBC), which requires a means of draining the water entering the wall assembly.

Importance of installation
Due to the porous nature of stucco, it absorbs water and therefore benefits from air space protection. Drainable building wraps may suffice in certain drier climates, but not all enhanced building wraps optimize drying. In such cases, a properly installed rainscreen system is essential to the building’s longevity and safety.

Due to its outward visibility, mold is the most frequent problem building owners associate with stucco. As a result, stucco’s dependability as a cladding solution is often questioned. However, mold is not a stucco problem, mold is an installation problem.

Stucco is essentially a cement-building product that is practically impervious to weather conditions. But, no product on the market will do its intended job if it is not carefully and properly installed. The time between rainscreen installation and the addition of exterior cladding should be kept to a minimum. It is essential to install the rainscreen systems so the channels aimed at removing the bulk water run vertically. The rainscreen should also canvas the entire surface targeted for stucco installation.

“Issues with workmanship are common,” Thompson said. “Very seldom are two contractors, when given the exact same base materials, going to install the product the same. The knowledge levels are all different and many have picked up trades from peers. No licensing is required for installers of exterior products (as opposed to plumbers or electricians). Now, every crew has a specialty and it is common for there to be a lack of proper oversight. Proper training of installers is crucial.”

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Most commonly, inspectors see moisture management problems are caused by a lack of continuity in the water management system, lack of attention to detail, poor workmanship, and failure to meet the minimum standards. When discussing moisture, there are so many different factors that can become problems. It is important for installers to make sure the details are done correctly.

“Stucco absorbs water and there is very little air movement within the walls, a combination that makes the product unforgiving of installation errors,” Thompson added.

The progression of technology in building construction has made it possible to tailor solutions to fit each particular circumstance, enhancing the structure’s ability to guard against moisture over the long haul.

Determining the proper system for a building remains one without a set method of evaluation, but with a solid understanding of the differences in the roles and performance characteristics of rainscreen systems and the distinct types of WRBs, making the right product decision can be simplified. Ultimately, the deciding factor should be the environment, taking into consideration the geographic location and the building’s climate zone. The levels of annual rainfall, temperature, and average relative humidity (RH) will dictate the proper application. Each option for stucco specifications should be evaluated for its ability to drain bulk water and dry remaining wetness based on the environmental stresses that will be imposed on it. As the state of building science progresses, builders and architects will have better information to guide them in this decision-making process.

George Caruso has more than a decade of experience in the building products industry and serves as the directorof product development and technical support for Benjamin Obdyke. Caruso has authored several articles for trade publications, as well as being featured in a business magazine for championing an innovation initiative at Benjamin Obdyke. He has participated in the building science community and code standards committees (such as ASTM Committee E06 Performance of Buildings), while balancing that knowledge with extensive in-field work with contractors, remodelers, and builders. A graduate of Penn State University with an engineering degree, Caruso is holder or co-holder of seven U.S. and Canadian patents. He can be contacted by e-mail at gcaruso@obdyke.com.

Source URL: https://www.constructionspecifier.com/screen-your-stucco-the-importance-of-effective-water-resistive-barriers/

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