June 29, 2017
by Michael Radigan
One of the most successful technologies for deterring rainwater intrusion into a building’s exterior wall and protecting the envelope is the rainscreen system. Introduced more than 20 years ago in the United States, this technology has continued to advance, along with its popularity among building owners who desire a ‘stylish raincoat’ for their structure.
Walls with rainscreen systems tend to be more durable and less prone to costly repairs resulting from water damage. In addition to water infiltration, rainscreen systems are also effective at managing air infiltration, negative wind pressures, heat transfer, and vapor transmission into and out of a building. The only real drawback to a rainscreen system is the added cost associated with its more complex design and installation.
Numerous terms have been used in the past to describe rainscreens, which has caused some confusion as to the nature of these assemblies. In basic terms, a rainscreen comprises the attachment of an outer skin of rear-ventilated cladding to a new or existing building. The system is a form of double-wall construction employing an outer layer to keep out the rain and an inner layer to provide thermal insulation, prevent excessive air leakage, and carry wind loading.
Cladding that does not stand off from the wall sheathing to create a cavity is not considered a rainscreen. However, a masonry veneer can be called a rainscreen wall if it is ventilated.
The purpose of using an insulation in the exterior cavity is it moves the dewpoint to outside the air/vapor barrier in rainscreen systems, virtually eliminating moisture from the interior wall construction, unlike conventional construction. This reduces the chance of mold developing within the interior wall. These systems are nicknamed ‘healthy wall systems,’ and increase the R-value due to the addition of an exterior insulation board within the cavity.
The exposed outer layers of a rainscreen façade may consist of many different materials, such as glass, terra cotta, fiber cement, metal, glass-fiber-reinforced concrete (GFRC), wood, fiberglass, or concrete.
The three main types
There are essentially three types of rainscreen systems available.
This type of rainscreen cladding system is not recommended for buildings more than four stories high—taller structures would not be sound because of the wind load. It utilizes a warm cavity (with no ventilation), open only at the bottom. The warm cavity provides a dewpoint similar to the outside, reducing the chance of moisture entering the system.
Drained and vented systems
This type of system is also not recommended for use on buildings more than four stories high; otherwise, there is a danger of condensation buildup. Not watertight, this system requires a cavity opening at the top and bottom providing drainage and promoting convection ventilation. An overhang is generally used to protect the cavity from the elements.
This type of system allows for ventilation openings large enough for air pressure to nearly equalize on both sides of the cladding. It requires extensive design to balance the amount of air flowing into and out from behind the rainscreen panels.
The only system that can be built above six stories, these assemblies’ height limitations depend on the structure calculation and are determined by geological location and climatic condition. So far, this author has experienced no code limitations locally.
Which buildings are ideal for rainscreens?
If a building is located in a relatively wet climate with an average annual rainfall of 1524 mm (60 in.) or more, it is a good candidate for a rainscreen system. Conversely, in some cities, a rainscreen system is required by local building codes.
Generally, most structures could benefit from a rainscreen system, which provides greater durability and moisture protection to an otherwise ordinary wall. A compromise by installing a rainscreen system will result in far less damage than a wall facing the elements without one.
Rainscreen systems can be complex. It is always recommended a facility manager or owner work with a reputable specialty contractor experienced in the particular system’s installation.
Michael Radigan is the senior operations manager for Western Specialty Contractors’ façades division in Ridgefield, New Jersey. He has been with Western for 34 years, and previously served as a regional manager, branch manager, and superintendent.
Source URL: https://www.constructionspecifier.com/superior-protection-with-rainscreens/
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