It has long been determined that incidental moisture can find its way past the cement stucco membrane on framed walls and with proper design, detailing and installation, the assembly works. However, excessive water can infiltrate stucco walls through larger cracks, penetrations and separations at accessories, and this can lead to problems. Water that enters through openings will generally be absorbed by the cement or run down on the face of the water resistant barrier and exit at the building base within the stucco foundation screed. Excessive water entering the stucco system typically can become a problem on walls that are over one-story in height. Water that infiltrates through cracks, trim accessories, building penetrations, and when not allowed to drain or “weep” can create a hydrostatic head, which can eventually leech the bitumens from the paper and thus compromise the integrity of the WRB and can lead to interior water infiltration.
Continuing research and development of stucco accessories are occurring at this time. This research and development is leading to an enhanced management of water within the stucco wall assembly. Through a simple modification of current stucco accessories, and the implementation of some new improved ones, water can be diverted out of the stucco wall before it gets to the building base. This can alleviate many concerns and make cement stucco a true high-performance cladding.
IntroductionTwentieth-century stucco wall claddings did not face the type of performance expectations today’s stucco walls face. To amplify the matter, today’s building materials, which include fast growth Douglas fir, paper-faced sheathing and fully insulated wall cavities create the ideal conditions for mold growth from minimal water infiltration. The transition to energy efficient buildings has also created building construction with less airflow through the structure, creating “tighter” buildings and less opportunity for infiltrating moisture to evaporate from the exterior claddings, further exacerbating the conditions for mold growth.
In a design driven construction industry, the traditional stucco systems have been forced to adapt. The introduction of new finish coatings, foam (EPS) plant-on shapes, aluminum reveals and custom architectural designs have led to a greater consideration for water management within stucco wall systems. In today’s designs, the challenges for the construction industry to prevent water infiltration problems associated with utilizing the standard technologies for stucco systems requires good design, good planning and qualified contractors.
Despite industry advances in technology of building materials, buildings continue to leak. Most experts have learned that many water infiltration problems can be remedied by limiting entry and then providing drainage points for water built into the stucco accessory. Focusing on water management is important and how to detail stucco accessories to function as a flashing, as well as a termination point for stucco.
Design Function Considerations for Stucco SystemsThe construction industry has adopted multiple variations and stucco-like assemblies and systems. The introduction of EIFS, finish coats and foam (EPS), and plant-on shapes over stucco systems, are examples of the continuing advancement to meet designers’ needs and wants. The original EIFS is a system unto itself in which the water resistant barrier is on the outer layer, which is commonly called lamina, a fiberglass mesh embedded into a polymer enriched cement coating. This system is heavily dependent on perimeter and termination sealant joints, as well as a properly installed system according to the manufacturers guidelines. Commonly called a “barrier” system, designers and installers must adhere to that principle.
In a stucco system, the water resistive barrier is behind the stucco, sometimes called a “concealed barrier system.” The cement coating provides strong water resistance and protection for the WRB. Since the cement stucco membrane is made from sand and cement it is accepted that the membrane will eventually crack. Large cracks have been proven to allow water (via rain) into the stucco cavity. Small amounts of water entry will be absorbed by the cement and diffuse, but water in volume that enters the assembly relies on draining out by force of gravity along the outer face of the WRB; most common of which is building paper.
The designer should consider this when merging EIFS finish coats with traditional stucco systems and the ramifications. Although they can work together successfully, the following designs and concepts should be considered.
Acrylic/Elastomeric Finish Coats. Acrylic finish coats have water repellency and should be verified they are vapor permeable prior to use. Elastomeric finish coats have extreme water resistant properties, which can provide additional water intrusion protection, but have vapor drive and other issues. Stucco experts always anticipate cement stucco walls to develop a crack, most are hairline, but some will allow water entry. Once water enters through the crack, the assembly should allow the water to drain down the face of the water resistant barrier and ultimately out at a weep point.
Aluminum Reveals. The installer should be careful to insure all fasteners go into solid backing, this will require extra backing for all intersections, which often times gets missed. Aluminum reveals should not be considered as a flexible control joint because there are very minimal movement capabilities in the rigid aluminum profile. Another consideration is the expansion and contraction of the aluminum as compared to cement. This can result in cement stucco pulling from aluminum leaving large enough gaps for bulk water to enter the stucco assembly.
Pressure Differential. When air pressures are lower inside a structure than on exterior areas, water can literally be sucked into a building. Interior spaces may develop a negative differential pressure with respect to outside ambient conditions, or they may develop positive differential pressures. The former condition may arise due to greater volume of air being exhausted from the space than is being supplied. Under certain conditions, a slightly negative room pressure can result in excessive air and dust infiltration, as well as direct water leakage through penetrations in the exterior wall assembly. Wood framing and other building materials within the wall cavity are storage mechanisms to hold this moisture.
Stucco Accessories. The type of stucco accessories offered by the construction industry has changed very little since they were first introduced. Until recently, the primary functions of stucco trim accessories were a termination point, thickness control, expansion and weeping capabilities. Although some manufacturers have recently modified their trim accessories to divert water away from the water resistant barrier, many of the original accessories are still available and in use today. The following potential failures should be considered when installing trim accessories:
Control Joints (also known as Trim Item #15): This particular trim is installed for the purpose of controlling the location and amount of cracking that might occur due to stucco shrinkage. The installer must be aware that these trims have the potential of allowing water into the stucco assembly via separation cracks and unsealed butt joints within the accessory. Once the water gets in, it becomes dependent on gravity and the nearest weep screed to exit the stucco cavity. In a structure that is several stories high, the foundation weep screed may be at a great distance. A better design for areas with significant rainfall is to allow for a weep point at various height locations. The intent is to avoid trapped moisture that will eventually deteriorate the WRB.
The designer/installer should be aware that the water traveling down the face of the WRB can enter through these holes and travel along the stucco of the returned soffit. If this stucco system is coated with an elastomeric coating, moisture will become trapped in the soffit, which can damage the soffit framing over time.
Standard Casing Bead (also known as #66): This accessory is used for a termination point for stucco systems.
The manufacturers of this accessory provide perforated nailing holes along the leg that attaches to the substrate. The installer must be aware that the open holes can provide access for water infiltration to the interior. This accessory also provides a very limited area for topical sealant joints with its 1/8 inch return edge when installed on edge. The result has been sealant applied over stucco, which does not provide a reliable sealant joint.
Consideration for Stucco Screed Flashing IntegrationDespite following industry guidelines and recommendations for stucco wall assemblies, designers can elect to provide additional water management capabilities within the stucco system. Currently, design consultants are working with manufacturers to provide plaster trim accessories that will also work as flashings. The designer/installer should be aware that any stucco system can ultimately have an elastomeric coating placed over the stucco. Water behind the stucco with an elastomeric coating will have a reduced ability to evaporate because of reduced vapor presence on the wall assembly. With this in mind, it is generally recommended a cement stucco assembly in wet areas should have a horizontal drainage point spaced no further than every other floor. In wind-driven areas with rain, it may be advisable to have drainage points at every floor. Discuss the design and options with your local plaster bureau and/or contractor.
Expansion/Control Joints: Drainage PointsIn multi-floor construction, where the designer/builder needs a two-piece expansion joint, rather than using the traditional two-piece #40 that requires sealant, consider the following graphic M-Slide Expansion Joint :
This two-piece expansion joint allows water to exit the top piece similar to a standard weep screed, while the bottom is sloped to divert moisture. Providing a bond breaker to the sloped edge of the accessory such as WD-40, will greatly improve drainage within the system. In the past acrylic finish coats that bond to the stucco accessory have restricted drainage and cause moisture to become trapped behind the stucco .
In low-rise construction where a one-piece control joint will be used, the installer should consider the noted detail:  CEMCO #15 Solid Leg
By incorporating a solid 3-inch flange to the upper portion of the accessory, water can exit the top slope of the control joint the same way it would with a standard weep screed. The bottom portion will still have the expanded metal lath incorporated into the one-piece accessory. This portion will be wire-tied to the lath installed on the wall, which will allow movement for stucco shrinkage.
Sealant JointsSealant joints that are installed directly over stucco do not provide continuity from the WRB to the sealant joint. Due to the fact that bonding to cement plaster is precarious and water can bypass the topically applied sealant altogether it is preferred the sealant be installed to a non-absorptive material.
The designer/installer should have an understanding of the location of the sealant joint and make an attempt to see if it can be installed to solid metal of the stucco accessory. The extended flange casing bead is an example of new products that easily solve the age old problems.
Closing RemarksInterior moisture penetration issues related to cement stucco assemblies can be minimized with good communication and proper detailing before the job gets started. Mock-ups for the purpose of understanding sequencing are vital for the installer and designer for all construction parties to have an appreciation for the installer’s ability and the designer’s intent.
The construction industry is constantly reviewing new design elements as well as new building materials. Some of these changes are good, some are not so good, but understanding water management is key to distinguishing between the two.
One way to better understand a cement stucco assembly over framed walls is to think of when, not if, water gets to the water resistant barrier. Where will it go? Does it have drainage points in place? If an elastomeric coating is placed over the top of the stucco, will water still be able to escape the assembly? Is there proper integration between the stucco and all penetrations i.e. windows, vents, doors, pipes, electrical, etc.?
It is hoped that the concepts and the new products discussed in this article provide better management of water and the beginning of good dialogue between designers, installers and manufacturers.
References: Details from “Portland Cement Plaster” Stucco Resource Guide, published by Northwest Wall and Ceiling Bureau, Seattle.
 “Moisture Control Handbook, Principals and Practices for Residential and Small Commercial Buildings,” Lstiburek, Joseph, Carmody, John, copyright 1994.
 “Technical Manual No. 15 Evaluation of Three-Coat Portland-Cement Plaster (Stucco),” The Association of the Wall and Ceiling Industries International.
 “Stucco as a Weather Protection System,” Westcon Annual Seminar, November 2, 2006.
 CEMCO Water Management www.cemcosteel.com.
 NAAMM Standard ELMA 920-09