Fiber cement panels as rainscreens

by Katie Daniel | September 3, 2015 11:18 am

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By Carolina Albano
Moisture intrusion in a wall system can cause building defects and health ailments for occupants, making rainscreens a very important tool in water mitigation. There are several popular types of rainscreens that manage moisture infiltration in different ways. The two most common panelized systems are drained and back-ventilated, and pressure-equalized rainscreens.

The origins of moisture in wall cavities differ depending on climate, construction practices, and occupant lifestyles. Common sources present in all climates include moisture resulting from the construction processes either due to excess water evaporating from the building materials (e.g. curing concrete and drying lumber), or from exposed conditions during the construction process (e.g. uncovered material storage and rain-soaked wall assemblies).

Another source is elevated relative humidity (RH)—the amount of water in the air compared to the maximum amount it can hold at a given temperature. Cold air cannot hold as much water as warm air. When air comes in contact with a surface that reduces its temperature so the relative humidity reaches 100 percent, dewpoint temperature has been reached; building surfaces at or below dewpoint temperature will condense water onto the surface of building materials. Keeping the RH low keeps the dewpoint temperature low, reducing the potential for condensation.

Further precipitation and groundwater lead to bulk moisture intrusion. The cause might be faulty or non-existent flashing, poor site grading, improper or non-existent rainscreens behind exterior claddings, or non-existent or poorly maintained gutters and downspouts.

Water concentrates around window and door openings, the roofline, construction joints, and the base of exterior walls. The building envelope acts as the interface between the interior and exterior of buildings; to avoid moisture problems in extreme weather conditions, the building envelope must control water from all of these factors.

Long-term effects of moisture intrusion
Moisture in envelope assemblies can cause numerous problems affecting the indoor air quality (IAQ) of a building and the longevity of its components. Internal moisture degradation is a leading cause of premature failure of building envelopes. Persistent moisture can pave the way for rot, corrosion, and other forms of deterioration. Moisture-induced degradation could include reduced thermal resistance and decrease in the strength or stiffness of materials. Moisture also supports insect infestation, ranging from mites to cockroaches and ants.

Moisture traveling through building components can cause corrosion of components and dissolve water-soluble constituents, damaging the structure.

Some common moisture-related problems include:

• structural wood decay;
• high indoor humidity;
• relative condensation;
• expanding soil that can crack or undermine the foundation;
• metal corrosion;
• ice dams;
• insect infestation; and
• mold growth.

If elevated moisture levels persist on or inside a wall or roof assembly, it can lead to the growth of microorganisms such as mold and bacteria, which can create microbiological volatile organic compounds (VOCs) that adversely affect IQA and lead to health problems, such as asthma and lung disease.

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Preventing moisture intrusion with rainscreens
Rainwater penetration is caused by the presence of water in the substrate, openings in the structural backup of the wall, and/or pressure differences (e.g. raindrop kinetic energy, surface tension, gravity, and capillary action) forcing water ingress through the structural backup.

Rainscreens are a successful method for deterring rainwater intrusion into walls. They shed most of the rain and manage the rest to prevent moisture intrusion. Rather than attacking the symptoms of moisture intrusion, rainscreens tackle the source—the forces that drive water into a building shell. By neutralizing these forces, rainscreens can withstand extreme environments. They appear to be effective in any climate and handle any weather condition short of a disaster.

Rainscreens consist of the exterior surface of the building (cladding) that has direct contact with the weather and elements, but is not directly attached to the building substructure. It is a barrier that sheds and attempts to control, but does not prevent, the majority of the rainwater intrusion into the cavity between the rainscreen and the substructure. Rainscreens stand off from the moisture-resistant surface behind it, creating a cavity or pocket of air.

According to “The Rainscreen Principle” in the ninth volume of the Canadian National Research Council (NRC’s) Construction Technology Update, two exterior walls are better than one at controlling water penetration into a building. There are three required components of a rainscreen wall assembly:

• the outer leaf or barrier, which is a vented or porous cladding (the rainscreen) that deters surface raindrop momentum;
• a few inches of air chamber or cavity separating the cladding from the support wall, reducing splashing and capillary moisture transfer—large, protected openings (e.g. vents or weep holes) positioned at the top and bottom of the wall promote convective airflow, allowing moisture to quickly drain or evaporate from the air cavity; and
• the inner leaf or barrier—comprising water-resistive barrier (WRB), insulation, and the structural wall—must act as the final moisture barrier and drainage layer protecting against any moisture bypassing both the cladding and air cavity.

The effectiveness of a rainscreen cannot be achieved without an airtight weather barrier and appropriately sized air chamber or cavity.

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The installation of weather barriers and rainscreens requires comprehensive integration with other building envelope elements such as the structure, insulation, vapor retarder, air retarder, and flashing systems.

The components of a successful building envelope design are:

• deflection—limiting structure’s exposure to rain with the use of overhangs and flashing;
• drainage—any moisture that penetrates the wall must be able to be redirected to the exterior;
• drying—any moisture that penetrates the wall should be able to dry within a reasonable amount of time before causing damage to the structure; and
• durability—only materials that are moisture tolerant should be used.

Common rainscreen claddings include brick veneers, stud backup, stucco, clapboard, and panelized wall systems such as metal and fiber cement. Rainscreens can be achieved with a variety of construction materials in various applications.

Types of rainscreens
For panelized systems, there are two primary types of rainscreens.

Drained/back-ventilated (D/BV)
The most common type of drained and back-ventilated rainscreens allow water to enter the cavity in limited amounts, and the WRB prevents it from entering the building or substructure. The cavity is vented to allow for the water to evaporate. The system must be able to vent and dry out any moisture that has entered the cavity.

Pressure-equalized rainscreen (PER)
This type of rainscreen prevents all rainwater penetration, while air is deliberately forced to penetrate the wall cavity in order to equalize pressure on the exterior and interior of the outer wall. In the PER system, static and dynamic air pressures are theoretically in equilibrium. That is, pressure measured on the exterior of the rainscreen is equal to the air cavity between the rainscreen and the substrate. Pressure equalization is important to prevent physical forces causing water to penetrate the building structure. The key to an effective and efficient PER lies in the ability to control the airflow within and through the wall assembly.

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The cavity is divided into horizontal and vertical compartments. These breaks act as vent holes, for horizontal and vertical air to flow into and out of the cavity. This also allows the air space to respond to wind gusts, reducing the rain-driving force.

The structural designer or engineer would determine the number and size of these breaks according to known, expected, and calculated pressure depending on building dimensions, height, exposure category, and basic wind speeds. The size and locations may differ within the same structure, since air pressure induced by wind can vary over the height and width of the building.

This type of rainscreen allows pressure to rapidly rise behind the panels and reach equilibrium with the pressure available in front of the panels. Compartments are required to be closed at all building corners to prevent excess wind forces on adjacent wall faces.

Applying PER technology to a wall or joint demands additional detailing care. Short-lived sealants and foam gaskets that disintegrate will decrease the effectiveness and may incur future maintenance costs. Mechanical seals such as metal flashing and gasketed furring strips offer a more permanent approach, but increase cost and complication.

Outside of these traditional rainscreens are various combinations developed by construction material manufacturers, collectively called ‘modified rainscreens.’ A variety of building envelope materials are now available, including WRBs acting as ‘drainscreens.’ A possible side effect of these face-fastened products is moisture entering through fasteners, while track and grid systems are more expensive. Cladding panels installed with a clip system are a cost-effective and minimal-maintenance option.

The most important thing to consider when deviating from the traditional rainscreen types is the system’s behavior; each part of the building envelope must be carefully studied prior to specification. Once the envelope materials are selected, they should be examined again as a system, for moisture loading on the wall, and drying forces in the cavity. If care is taken in the design stage with use of proper materials, a number of combinations are available to create a rainscreen that will offer
a lasting line of defense against water penetration into structures.

Standards
American Architectural Manufacturers Association (AAMA) 508-7, Voluntary Test Method and Specification for Pressure-equalized Rainscreen Wall Cladding Systems, and AAMA 509-9, Voluntary Test and Classification Method of Drained and Back Ventilated Rainscreen Wall Cladding Systems, can be used to determine the performance of a variety of products used in rainscreen applications.

In AAMA 508-7, the rainscreen must prevent water penetration throughout, vent water vapor, have a WRB that resists the full positive and negative wind load, not trap or conceal water, and be able to control water penetration.

For AAMA 509-9, water entry through the water barrier must be prevented. The WRB must be the primary weather protection, the system must be able to manage and drain any water entering the cavity behind the cladding and be sufficiently vented to allow the cavity to dry, and water vapor entering the cavity must be allowed to vent or drain to the exterior.

Since a D/BV system allows water penetration through the wall system, the only pass/fail criteria is the weather barrier does not permit water penetration into the structure. One should keep in mind D/BV systems allow water penetration into the cavity.

The specifier must ensure the weather barrier will perform according to the above test’s standards. When specifying a rainscreen into their projects, architects need to address all joints where water penetration is a possibility, such as butt joints and termination points.

How to use fiber cement panels as a rainscreen
Rainscreen cladding is a construction façade system consisting of the subframe and different finishes like cladding panels, fiber cement boards or panels, brick, manufactured stone, or metal. Rainscreen cladding is 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 that uses an outer layer to keep out the rain and an inner one to provide insulation, prevent excessive air leakage, and carry wind loading.

The outer layer breathes like a skin while the inner layer reduces energy losses. The structural frame of the building is kept absolutely dry, as water never reaches it or the thermal insulation. Evaporation and drainage in the cavity removes water that penetrates between panel joints. Therefore, there is no significant pressure differential to drive the rain through joints. During extreme weather, a minimal amount of water may penetrate the outer cladding. This, however, will run as droplets down the back of the cladding sheets before dissipating through both evaporation and drainage.

Some fiber cement panels act as a drained/back ventilated rainscreen. This is a paneled wall system installed to framing using clips that hold the panels away from the structure.

The clips provide different depths of air space depending on the designer’s preference, and moisture is released through this air layer. Fiber cement panels are installed over a starter track with weep holes.

Fiber cement panels are profiled along all four edges, so both horizontal and vertical joints between the installed panels are ship-lapped. A factory-applied sealant is applied to the top and right panel edges, ensuring all factory joints will contain a sealant and no additional sealant or caulk is necessary for installation, except at termination conditions.

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Design approaches to manage water-driving forces through cladding
To minimize the forces of water penetration by gravity, the following should be provided:

• drainage holes for all horizontal surfaces that can act as troughs;
• a minimum slope of two percent on horizontal surfaces to prevent flow to the interior;
• gaskets or sealants for closed vertical joints within a two-stage joint; and
• shielding for open joints.

To manage water penetration through capillary action:

• drainage and vent holes should be at least 11 mm (7/16 in.) wide to avoid bridging by water; and
• thicker materials should be chosen to delay or minimize water absorption.

To minimize water penetration because of air pressure difference:

• some degree of pressure equalization must be achieved across the cladding, its joints, and junctions; and
• air pressure across the cladding is a function of the effectiveness of the WRB system, the size of the venting in the cladding, the volume of air chamber between the weather barrier, and the stiffness of the chamber.

To manage water from the driving forces of surface tension, one should add drip caps under any projecting horizontal surface, such as windowsills, balcony floors, or soffits. Also, a drip edge should be specified for flashing.

To manage water from the forces of kinetic energy of raindrops, openings can be protected from direct rain entry by overlapping materials, sealant, or gaskets.

At the end of the day, the industry has come a long way in developing performance criteria for rainscreen systems that should be used when designing a rainscreen. A rainscreen must be viewed as a system and not as its individual parts. Guessing must be taken out of the design stage, and all parts have to be tested to make sure they will work well together in every possible design combination.

Fiber-cement Rainscreens at Work in Minneapolis

[6] For the Third North 204-unit apartment complex in Minneapolis, Minnesota the architects designed the façade to respect the existing aesthetics of surrounding historic buildings while still offering a modern look. A similar warehouse massing and the structure’s position abutting the sidewalk on three sides reflects the frontage of nearby warehouses, many of which have been adapted into office spaces or condos. The building’s U shape conceals residential features—including green space, a dog run, and pool—in the center and rear. Cladding selection also played a key role. The designers specified 457-mm x 1.8-m (18-in. x 6-ft) architectural panels in a blend of six colors—divided into swaths of reds and grays to mirror nearby buildings’ brick and concrete—using both color and shape. Simultaneously, the panels’ large, smooth scale and nod to metal help the overall look tilt toward the contemporary. However, specifying the material was not completely straightforward. Prior to the Third North project, the city of Minneapolis did not allow fiber cement to cover more than 30 percent of a building’s façade. The architects submitted for an exception, and the panels’ commercial look, aided by its hidden fastening system and the performance brought by an integrated rainscreen, helped it receive an allowance. In the end, the city and the community were equally pleased with the finished product, potentially paving the way for similar applications in the future.

Carolina Albano has served as technical manager at Nichiha USA. since 2010. She holds degrees in mechanical engineering from the Georgia Institute of Technology. Albano is a member of Structural Engineer’s Institute (SEI), and American Society of Civil Engineers (ASCE), and active in several committees in ASTM. She can be reached at calbano@nichiha.com[7].

Source URL: https://www.constructionspecifier.com/fiber-cement-panels-as-rainscreens/

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