Understanding rainscreen walls: Cues for today’s construction environment

Figure 2: A ventilation gap is created between the insulation and the exterior cladding panel, allowing for drying.
Figure 2: A ventilation gap is created between the insulation and the exterior cladding panel, allowing for drying.

Continuous air barrier systems

Arguably, the most significant innovation has been the widespread use of continuous air barrier systems. Rather than focusing on pressure equalization, an air barrier system, as defined by the Air Barrier Association of America (ABAA) “is a system of building assemblies within the building enclosure designed, installed, and integrated in such a manner as to stop the uncontrolled flow of air into and out of the building enclosure.” This addresses one of Garden’s three conditions (openings permitting passage of water) and controls a factor Garden did not describe. Air moving through a wall assembly brings water with it in the form of humidity, leading to potential condensation and a number of undesirable consequences. Now required by code, an effective air barrier system is fundamental to every wall design.

Two of Garden’s forces involve capillarity. Wikipedia defines capillary action as “the ability of a liquid to flow in narrow spaces without the assistance of, or even in opposition to, external forces like gravity.” It is commonly called wicking and can be seen in an oil lamp where oil is drawn by capillary action up the wick to the burning tip. Rousseau’s 115-mm concrete air barrier, which was porous by nature and prone to cracking over time, was also vulnerable to moisture entry by capillary action. This was controlled with an air space or cavity, which created a capillary break. In this way, water was managed using an exterior rainscreen (brick façade), a capillary break (air space) and weeps that allowed moisture to exit from the wall. Garden’s forces were managed.

Masonry walls of this kind represent an extreme condition. Bricks and mortar can store large amounts of water, and concrete is porous and hydrophilic, making it highly conducive to capillary water flow. As a result of this and other considerations, the air space in this wall design must be between 25 and 168-mm (1 and 6 5/8-in.) wide, though spaces less than 50 mm (2 in.) are not recommended (consult the Brick Industry Association Technical Note 28D). More recent materials such as oriented strand board (OSB) sheathing also allow capillary movement of water and are more prone to moisture damage than concrete (Figure 1).

Innovation in this area of construction materials has also been significant. Fluid-applied polymeric air and moisture barriers can effectively waterproof all code-compliant base walls, including masonry, concrete masonry units (CMUs), and both gypsum and wood-based sheathing. These materials must demonstrate the ability to resist a 500-mm (20-in.) column of water for at least five hours (this is according to the American Association of Textile Chemists and Colorists [AATCC] TM127, Test Method for Water Resistance: Hydrostatic Pressure). Unlike many building wraps, fluid-applied air and moisture barriers are fully adhered to the substrate and create an effective capillary break. In addition to sealing gaps, fluid-applied air and moisture barriers address one of the main forces moving liquid water through a wall.

This takes us back to the heated controversy over cavity wall and rainscreen wall terminology, without pressure equalization to stop the debate. Clearly, any wall design carrying the “rainscreen” designation must have a gap between the outer rainscreen and a drainage plane. In most cases, the moisture barrier within the drainage plane will also function as an air barrier assembly, continuously connected to the roof, foundation, window, and penetration air barrier assemblies. While a gap is an absolute necessity, how large must this gap be?

The answer to this key question is specific to every wall design because the gap is called upon to perform different functions. For example, all of the materials used in an exterior insulation finish system (EIFS) have very low moisture absorption. Provision for drying absorbed water is therefore less critical. Air circulation within the gap, which is sometimes associated with rainscreen terminology, is unnecessary with EIFS. What is required is a gap establishing an effective drainage plane. For this, a 1-mm (39-mils) gap has been shown to be effective (read Modeled and Measured Drainage, Storage and Drying Behind Cladding Systems by J. Straube and J. Smegal, Research Report – 0905 2009. Check out CMHC Research Highlight – Technical Series 01-104, Monitoring the Performance of an EIFS Retrofit on a 15-story Apartment Building. Research consultant: Morrison Hershfield). While larger gaps are sometimes used, they have not been shown to provide improved drainage performance (for more information, read “Rain Control Theory” by the Building Science Corporation). Utilizing a 1 to 3-mm (118-mils) gap to allow gravity to drain moisture out along a defined drainage plane, EIFS with a fluid-applied air and moisture barrier is consistent with the principles laid out by Garden and has proven itself to be effective, both in testing and in real-world service.

On the other hand, stucco has substantially higher moisture absorption than EIFS and is often applied over building wraps. This creates a greater potential for capillary water movement and the need for more comprehensive moisture control strategies. Introduction of a 9-mm (3/8-in.) drainage mat between stucco and the water-resistive barrier (WRB) within a stucco wall system has been shown to greatly reduce the incidence of moisture damage (read Building Science Insights 029: Stucco Woes-The Perfect Storm by Joe Lstiburek, 2010). Drainscreen products are often marketed as “rainscreens.” These products create a capillary break between stucco and building wraps, which is consistent with rainscreen principles. By comparison, stucco directly applied to masonry is clearly not a rainscreen wall.

Ventilated and open-joint wall systems make a further use of the air gap. By creating a convective loop outside of exterior insulation, the wall assembly dries itself continuously (Figure 2). The vast majority of water is shed by the outer rainscreen siding. Air circulation provides a means for the moisture that has entered the wall (perhaps through open cladding joints) to exit. Behind insulation, an air and moisture barrier provides a second capillary break and an additional line of defence against moisture entry. Sometimes referred to as “true” rainscreen walls, ventilated and open-joint rainscreen walls make full use of Garden’s rainscreen principles.

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