Metal Architecture Home
Features

Rainscreen Primer

Rainscreen1

Types of Rainscreen Systems

There are two types of rainscreen systems: drainable assemblies and pressure-equalized rainscreen systems.

Drainable assemblies, often referred to as a "drained cavity wall" or "drained/backventilated" (D/B-V) system, take into account that some moisture will be driven past the outer skin and allow this moisture to drain harmlessly away through the use of a drainage plane.

Drainage assemblies consist of two layers separated by an airspace. The outer layer is the cladding that sheds that majority of rainwater, while the inner layer is usually the exterior surface of the structure, which must be protected by a weather barrier to shed any water that does get past the cladding. The air space serves as a place for water to drain down to the appropriate base flashings, while also allowing air to circulate behind the cladding. Additionally, this back ventilation helps dry the wall by promoting evaporation.

Meanwhile, pressure-equalized rainscreens (PERS) rely on a series of pressurized compartments that usually consist of a drainage plane vented at the top and bottom and closed at the sides. These systems are designed to eliminate or significantly reduce the water that is allowed past the outermost layer of the envelope, and also manage any moisture that does get past the outer layer.

When wind blows against a wall with a pressure-equalized rainscreen, the compartments fill with air. Because the sides are closed, and the airflow through the vents is limited, the compartments are only able to fill up until the air pressure matches the pressure of the incoming wind. Essentially, the air inside the compartment pushes back, and when the wind subsides, the pressurized air in the compartments will flow out the vent until the pressure inside again matches the outside pressure. As long as the pressure on each side of the cladding is the same, there is no driving force for air and water to penetrate the cladding.

--

Melissa Grimes, marketing manager at Laminators Inc., Hatfield, Pa.

While popular in Europe for many years, rainscreen systems are still relatively new to the U.S. Rainscreen façade systems put an air barrier between the panel system and structural wall, allowing wind-driven rain or water to escape the building envelope without entering the building.

While that seems simple enough, there are many aspects of rainscreen systems that not all architects are aware of. To help you better understand rainscreen systems, Metal Architecture went to a variety of experts to put together a list of 10 things every architect should know about rainscreen systems.

1. Rainscreen Benefits

According to Earl A. Hernandez, director of sales and marketing at Tuschall Engineering Co. Inc., Burr Ridge, Ill., one of the best benefits of a rainscreen is that it allows a building to breathe. As he explains, while in the past, architects and contractors used to try and button up buildings, it is now important to allow buildings to breathe and go through a normal wet-dry cycle. This lends itself to overall longer building life cycles that building owners now require. Additional benefits include increased energy efficiency and improvement of indoor air quality.

"We've moved away from just using recycled content and it's now much more holistic so that when you build a building it should last 100 years or more," Hernandez says. "Especially with net zero design and the 2030 challenge, a rainscreen really allows a building to breathe and achieve those goals, all while helping the building lend itself toward achieving those longer life cycles."

2. Understand the Assembly

Scott Bacon, director of the MetalWrap business unit at CENTRIA, Moon Township, Pa., says it is important to understand that the overall wall assembly and design of a rainscreen system is paramount to achieving the four critical barriers of air, water, thermal and vapor. "The very nature of a multicomponent wall assembly (with multiple products and contractors required to dry-in a building) can lead to moisture intrusion and entrapment within the wall cavity during construction," he adds. "Certainly, over time, this entrapped moisture can lead to a further degradation of the insulation, mold and mildew, and corrosion of the steel studs. A singlec-omponent system is essentially a four-in-one solution that provides the sheathing, air barrier, vapor barrier and thermal performance in one pass around the building."

Frank Resso, director of product development at IMETCO, Norcross, Ga., explains that there is a difference between a complete rainscreen wall system and rainscreen cladding. Resso says that by definition, rainscreen cladding can be any material used on the exterior of a building to shed bulk water. He goes on to say that a complete rainscreen wall system will always incorporate rainscreen cladding, but rainscreen cladding is not always part of a complete rainscreen wall system.

3. Manage the Moisture

The amount of moisture that makes its way into the rainscreen cavity depends upon many factors, the most static of which is how open the design is, says Dave Clapperton, partner, The Miller-Clapperton Partnership Inc., Austell, Ga. As he explains, AAMA 509 was designed and developed to classify drained/back-ventilated rainscreens (D/B-V) by how tightly they are designed to limit the intrusion of weather-driven moisture (rain) and how much ventilation the system promotes. "Generally speaking, the tighter the system the lower the volume of ventilation over a given period," he says. "The converse is generally true too-the more open the system is, the more ventilation it will promote. The key is to find the right balance for the given environment."

"In addition to evaporation by ventilation, D/B-V systems must also be designed to channel any penetrating moisture through channeling and gravity to flashing rails that direct the initial volume of water out of the system, leaving only residual moisture to be evaporated," Clapperton adds.

He goes on to say that the essential principle of pressure-equalized rainscreens (PERS) is to prevent moisture intrusion during a weather event by allowing the external environmental pressures to equalize to the pressures within a system's cavity in a constantly adjusting manner so that the weather dynamics will not drive rainwater into the system.

"PERS systems must also be designed with internal guttering rails that direct any intruding water to egress points to be drained out," Clapperton notes. "To limit the amount of potential water intrusion by increasing the likelihood of more active PERS balancing, well-designed PERS rainscreens are compartmentalized to help the system react to the many different pressure conditions that occur on a building's skin during a weather event."

rainscreen primer, metal architecture, august 2014, marcy marro4. Air/Water Barrier

The air/water barrier is the most critical component of the rainscreen system, explains Tom Diamond, PE, project engineer at The Garland Co. Inc., Cleveland. "Design of its application, especially around transitions and penetration, as well as its installation is essential," he says.

For a wall to leak, Resso notes that three things must be present: water, a hole and a force. "Water (rain) will always be present and penetrations in a wall are inevitable," he adds. "A continuous air barrier (properly detailed and installed) is an effective means to take away the force that occurs between conditioned and unconditioned spaces."

Resso and Diamond agree that redundancies in the air barrier layer are beneficial. As Diamond notes, high-performance building enclosures will not tolerate even moderate amounts of air leakage. Resso says to ask these questions: How will the air barrier be made continuous at joints and transitions?

Has the air barrier been clearly detailed in all architectural details on the bid documents? Can the air barrier be sequentially and contiguously installed as detailed? Will the air barrier be inspected and/or tested before covered with other components?

Additionally, Diamond says the vapor control layer, which is typically the same product as the air barrier, should be designed in accordance with Chapter 14 of the International Building Code. "A vapor open profile is beneficial when possible to allow drying of the wall to the interior and exterior of the building," he says. "However, northern climates and high interior humidity conditions may require a vapor closed profile."

5. Continuous Insulation

According to Diamond, continuous insulation on the exterior of the air/water barrier is almost always better than insulation applied in the stud wall cavity. "This allows for less thermal bridging and reduces risk of condensation from forming within the stud cavity," he says.

When it comes to determining the proper amount of insulation that should be used within a wall assembly, Diamond says that can be determined by utilizing the R-value requirements of a wall, or the overall U-value requirement of the wall assembly, as prescribed by Chapter 4 of the International Energy Conservation Code or ANSI/ASHRAE/IESNA 90.1. Therefore, Diamond recommends using extruded polystyrene (XPS) or rockwool insulation for a rainscreen assembly, since both can stand up to moisture without significantly reducing its R-value.

rainscreen primer, metal architecture, august 2014, marcy marro6. Attachment and Framing Systems

Rick Brow, director of marketing at CENTRIA, says it is important to understand how the rainscreen cladding supports are connected so they have minimal effect on the weather barriers.

Framing systems for the cladding should be designed to reduce the amount of thermal bridging as much as possible, Diamond says. "Typical z-shapes framing members with insulation sandwiched around it can reduce the effective R-value of the system by as much as 60 percent," he explains. "However, utilizing a vented hat channel system with only fasteners penetrating the insulation results in a reduction in effective R-value of less then 5 percent."

While continuous insulation helps promote thermal efficiency, Resso says it poses challenges for the cladding attachment. "A high-performance rainscreen wall system will embrace the benefits of continuous insulation by utilizing a cladding attachment system that has superior thermal efficiency properties," he says.

7. Ventilation

Proper ventilation is a key component for a rainscreen wall system to work properly, says Resso. The rainscreen assembly, Diamond adds, should provide for ventilation of the system through a drainage/ventilation cavity, a properly designed framing system and ventilated trim components at the bottom and top of the assembly.

Resso goes on to note that there is more to it than requiring an installer to drill a few weep holes. Resso suggests looking to manufacturers with integrated and tested venting details, which will include air intake openings low in every elevation and air exhaust openings high in every elevation to promote ventilation and convective drying of the wall cavity.

8. Open Joints

According to Diamond, the cladding of the system should provide open joinery, allowing for more ventilation of the overall assembly. "Relying on sealants and caulks to keep a wall system watertight creates maintenance issues and susceptibility to leakage over time," he adds.

In a rainscreen wall, you want water to develop some velocity as it runs down the building and slides over whatever joints are there, explains Robert A. Zabcik, P.E., LEED AP BD+C, director, research and development at Houston-based NCI Group Inc. If water does intrude and get into those areas, you want to catch it and drain it out through the backside of the wall. This is done primarily by making sure that if the wind blows, the air pressure is not driving the water into the wall system.

The fact that rainscreens don't rely on any sort of caulks, sealants, tapes or butyl mastic makes them a no- or low-maintenance fa.ade system, Hernandez says. As he explains, if a rainscreen system is detailed and installed properly, a building will have reduced maintenance costs, aiding in its desired longer life cycle.

However, when working with aluminum composite material (ACM) or metal composite material (MCM), Zeke Miller, president, The Miller-Clapperton Partnership, suggests avoiding curving rainscreen joints, and always planning on "skyward" joints being caulked.

rainscreen primer, metal architecture, august 2014, marcy marro9. Integrated Technology

Keith Spurlin, CSI, CDT, Assoc. AIA, business development manager at Deland, Fla.-based Kingspan Insulated Panels, notes that the industry has progressed from the over 50-year-old technology of multi-component systems to using insulated metal panels as the barrier wall portion of a rainscreen assembly. In addition to allowing for a faster build time, the panels reduce operational costs for energy and maintenance.

10. Testing

According to Brow, architects can minimize rainwater behind the rainscreen by using panels that have passed AAMA 508 and AAMA 509. "These tests specifically address water penetration under both static and dynamic pressures, as well as pressure-equalization behavious, among other moisture intrusion factors," he says.

Resso adds that AAMA 508-07 is intended for products that are defined by the manufacturer as being pressure-equalized rainscreen wall cladding systems and is based on pass/fail criteria, while AAMA 509-09 is intended for products defined as being drained/back-ventilated rainscreen wall cladding systems, and establishes a classification system based on the results of a set of performance tests.

Bacon adds that it is also important to understand how the wall assembly complies with the NFPA 285 fire code. As he explains, there seems to be a general misunderstanding of the test and when it is applicable on a project. "There is also a general misunderstanding of how different wall assemblies may, or may not, comply with the requirements of NFPA 285," he says. "If NFPA 285 is a requirement, and it is subsequently determined that a specified wall assembly does not comply, then costly testing and project delays could be a result."