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

stucco_Rainscreen_Stucco Installation
Installation of a rainscreen system in a stucco application.

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.

These are examples of product options to incorporate into a moisture management system: rainscreen, flat housewrap, and drainable housewrap.

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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