April 9, 2021
by Peter Barrett
One of the most reliable ways of keeping the exterior walls dry or allowing them to dry out when they do get wet is to construct an assembly with an outer protective shell, also known as a rainscreen. The assembly comprises, at minimum, an outer layer, a protected inner layer, and a cavity between them sufficient for the passive removal of liquid and water vapor.
Although the term “rainscreen” emerged in the 1960s, earlier examples date back to the 19th century. They are effective at managing moisture and provide exceptional opportunities for energy-efficient performance through continuous insulation (ci) and reducing thermal bridging. According to the Green Built Alliance, rainscreens are one of the best ways to increase durability and prevent water damage to buildings.
Rainscreen walls have had a few different definitions over the years. In the past, it generally referred to a façade material. The term now typically defines an entire system supported by the exterior wall and designed with products and details that assist in creating pressure-equalized and ventilated systems. Ultimately, a rainscreen is the protective exterior barrier wall system. Some rainscreens and components are engineered to function as a complete system: the material, the type of channel and clip arrangement used to fasten the rainscreen veneer, the dimension of the air gap, the air- and water-resistive barrier, and flashings.
Building researchers and scientists through the 1960s and 1970s tested and validated open rainscreen systems as well as simple vented and drained systems. The American Architectural Manufacturers Association (AAMA) published the first guide for pressure-equalizing designs in 1971. Today, while research and testing on rainscreens continues, there is a growing initiative to standardize rainscreen specifications and criteria.
To understand the importance of industry standardization, let us review the role of a rainscreen in detail.
One of the most important aspects of building enclosure durability is moisture control. Since many wall cladding systems are not watertight, managing water, as either vapor or liquid, is a critical design goal. The issue is not if water will penetrate, but how to handle the water when it does.
The three major categories of commercial wall assemblies (mass walls, barrier walls, and drained or screened walls) handle moisture differently. Drained or screened walls are the most popular assemblies and differ from mass and barrier walls in that they contain an air space behind the cladding. Moisture that gets past the cladding is stopped by a water-resistive barrier (WRB) and drained away from the building, and moisture is not drawn into the interior structural assembly.
A rainscreen should be viewed as a building envelope protective mechanism, adding reserve to the ability to manage moisture. Contrary to popular belief, the primary function is not to act as a barrier against water penetration—that is the WRB’s role. A rainscreen is designed to limit the amount of water that could potentially come into contact with the primary building envelope’s moisture barrier. This reduces the likelihood of water entering the wall assembly by resisting the five forces that can drive moisture into buildings. Those forces are kinetic energy, gravity, capillary action, surface tension, and pressure gradients.
Rainscreens add two key performance features to a wall system: drainage and ventilation. The drainage space allows the passage of bulk water to the exterior. Ventilation provides enhanced capacity to dry and moderate humidity within the cavity.
All rainscreen systems rely on the air gap between the cladding and exterior wall to prevent water from penetrating the structural wall assembly. A rainscreen typically consists of five components:
In its strictest definition, a rainscreen is not dependent on having an air barrier; just a WRB. An air barrier is a critical component of the wall system, but may or may not be part of the rainscreen system.
Rainscreens can reduce the risk of structural deterioration and improve the durability of the enclosure wall but, like all wall assemblies, they are susceptible to damage if exposed to excessive moisture at vulnerable construction details. Also, ventilation behind the cladding, water barrier/sealed penetrations, insulation placement, exterior and interior environments, and details have a large influence on wall assembly performance.
Water and vapor management
Moisture issues, especially in exterior walls, are a growing concern as building envelopes have become tighter and incorporate higher levels of thermal insulation as a result of more stringent codes and a rising demand for energy-efficient buildings.
Moisture control is fundamental to the proper functioning of any building, yet moisture problems are far too common.
Many of these problems can be traced back to poor practices in design, construction, or maintenance. A position paper by the American Society of Heating, Refrigerating and Air Conditioning Engineers (ASHRAE) said several independent decisions made by different professionals can produce enough moisture accumulation in the wall cavities for a long enough period to create an issue. Rarely can one profession, acting in isolation, take all the actions that lead up to either producing or preventing a moisture problem.
Effective moisture management means designing and building wall systems that have a greater “drying potential” than “wetting potential.” Façades are not expected to keep 100 percent of rain out of the wall. In fact, Dr. John Straube suggests 10 to 20 percent of wind-driven rain ends up behind the cladding. When specifying a rainscreen, it is important consider the following:
The famous architect Louis Kahn was onto something when he said, “The sun never knew how great it was until it hit the side of a building.” Once the plans have been envisioned, drafted, approved, and finally realized, an architect’s best friend or worst enemy is the sun, especially when it comes to moisture. This lesson is best learned before breaking ground on even the most innovative commercial architectural feat: make the sun your best friend with intelligently engineered rainscreen design.
Solar-driven moisture can penetrate many building materials and accelerate the growth of mold. However, the materials of the greatest concern are highly absorptive claddings (also known as reservoir claddings), such as conventional stucco, adhered stone veneer, and cementitious siding. Solar-driven moisture occurs when water that has soaked into the cladding is forced further into the wall by the sun’s heat. The wall looks like it dries to the outside, but it does not.
It is important to note the WRB will not protect walls from solar-driven moisture, as it is moisture-vapor permeable. The thing about moisture-vapor permeable assemblies is that they are permeable both ways, meaning without any protection, the sun-driven moisture can be pushed all the way into the wall. When moisture gets to the cooler, air-conditioned interior, one risks condensation in the wall cavity. Moisture in the wall cavity can have huge repercussions for the durability and lifespan of the building.
Solar-driven moisture can be managed with the use of drained and back-ventilated engineered rainscreens. This will allow the majority of water penetrating the cladding to drain efficiently, and through strategically designed openings at the bottom of the wall, minimize moisture accumulation. Since the rainscreen is open at both the top of the wall and the bottom, accumulated moisture can be ventilated quickly before it can contribute to rot and mold growth. This ‘bonus’ drying energy comes from the buoyancy of air through the space as the wall is heated by the sun (hot air rises) and through air-pressure differential as the wind blows against the structure.
Several types of engineered rainscreens are available today. The more familiar may be the open entangled meshes. Another is the solid-faced dual-chamber dimpled version. Each will give drainage, ventilation, and a capillary break. Only the dimple version gives the full barrier, including protection from solar-driven moisture.
Value of ventilation
Today’s building enclosures have lower moisture storage capacity and drying potential than in old times.
Not long ago, framed buildings had better drying capacity. Since there was little vapor resistance, less insulation, and more air leakage, walls dried just from the energy used to heat them, even though they had minimal moisture storage capacity. As energy efficiency improved over time, enclosures began to retain more moisture. When insulation was added for greater comfort, it led to moisture problems in colder climates, most noticeably in the 1930s, because the heat energy was no longer acting as drying energy.
In order to improve enclosure design, building professionals learned the importance of:
The newer technical construction of rainscreens allows the surface of the cladding to be separated from the wall, enabling the continual flow of air, leading to significant benefits in drainage and ventilation and a decrease in thermal load. The air exchange in rainscreen walls is expected to provide ventilation drying if excess moisture is absorbed in the wall construction. Using heat and air movement helps reduce the mass of water more quickly than no ventilation, just air movement, or just heat (e.g. warming from the sun).
Extensive research has been done on evaluating the airflow rate in the air cavity and the moisture removal by cavity ventilation through laboratory testing, field measurements, and simulations. Findings indicate rainscreen ventilation helps drying (read Bassett and Mcneil 2005a, 2005b; Davidovic 2004; Hansen et al. 2002; Hazleden 2001; Straube and Burnett 1995; VanStraaten and Straube 2004; Onysko 2003). Panels with both bottom and top vents dried faster than the comparable panels with only bottom vents (‘drained screen’), and the walls with a 19-mm (3/4-in.) cavity dried faster than panels with a 10-mm (3/8-in.) cavity.
Current practices vary in terms of specifying cavity depth and slot vent heights for panel systems. In 2012, a team at Lund University in Sweden conducted a ventilated rainscreen cladding study that focused on the drying process. They found the cavity design is of major importance for the drying rate if the material adjacent to the cavity is wet over its entire extension. Of significance, a small cavity depth (<10 mm) prolongs the ventilation drying process.
The cavity depth and vent size have a positive impact on the airflow rate in the cavity. However, the amount of moisture removed by the ventilation is governed by the properties of the sheathing membrane once the airflow rate reaches a threshold value. For panel systems, an air cavity depth of 19 mm provides higher airflow and drying rates compared to a 10-mm cavity. For cavity depths greater than 19 mm, higher ventilation rates can be achieved through larger vent openings, but the drying rates are not significantly influenced.
When designing a rainscreen, it is important to understand why one is putting a space behind the cladding and what performance criteria can be met. A common question is just how big that gap needs to be. If the gap is for drainage only, it may be quite small. Even the space between two layers of a WRB is adequate to provide drainage. However, the gap must be bigger to include ventilation.
To better understand why rainscreen systems include drainage and ventilation cavities, one must understand the different expectations for each. The drainage cavity will remove much of the bulk moisture by gravity. However, moisture can still remain adhered to or absorbed in materials within the wall assembly. The amount of moisture that can be safely absorbed or stored depends on the material properties. Drying can occur by vapor diffusion, evaporation, desorption, or by air convection (i.e. ventilation).
Vapor diffusion is a slow process, particularly when low-permeance materials are used within the wall assembly. Evaporation or desorption can only occur when moisture is able to get to the surface of the material, often only at the cladding or interior surface, and removed by the flow of air.
According to a study conducted by Dr. Achilles Karagiozis and Hartwig M. Kuenzel, published by the Journal of ASTM International, on the effects
of air cavity convection and wetting and drying behavior on building envelopes, a well-ventilated exterior cladding also negated the impact of solar-driven moisture transport for all International Energy Conservation Code (IECC) climate zones.
Allowing evaporation or desorption to occur at layers within the wall assembly and removing the excess moisture by ventilation to the exterior provides an effective means to remove additional moisture directly from sensitive materials and improve the drying potential of wall assemblies. This practice is an extremely valid method to create moisture-resilient construction.
The use of a rainscreen system does not just decrease expenses over time by protecting the building from the damages of uncontrolled moisture-causing mold or mildew, but can also result in immediate cost savings through increased energy efficiency. The outer layer protects against atmospheric effects, avoiding humidity and temperature drops, and the air cavity of the system provides the building construction envelope with a more stable temperature. This thermal isolation can reduce energy consumption by 30 to 40 percent.
When it is hot outside, the rainscreen acts as a buffer zone that cools off the air, thus reducing thermal bridging and allowing air exchange behind the cladding as the warm air rises to the top, replaced by cooler, drier air at the bottom. The same effect works in reverse during cold weather—the ventilation manages both heat gain and heat loss during all seasons.
Thermal conductivity is the ability of materials to transmit energy, heat, or cold. The best-designed rainscreen systems have as little thermal conductivity as possible to minimize the thermal bridging of energy between the structure and the outer layer of siding.
While common in Europe and Canada, rainscreens are only now beginning to make inroads into the American construction scene. In 2020, a diverse community of industry professionals passionate about supporting performance-driven rainscreen assemblies joined forces to create the Rainscreen Association in North America (RAiNA).
With airtightness and insulation building codes becoming more prominent across North America, it is more important than ever to include rainscreens, particularly in coastal areas with high precipitation. However, whether or not the code requires a rainscreen, it is a better way to detail the exterior cladding of buildings. After all, moisture problems correlating to incorrect cladding installation details have been discovered even in dry climates.
These systems offer exciting solutions and opportunities to architects and contractors who are interested in not only solving moisture penetration, but also improving wall performance.
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