Fiber cement panels as rainscreens

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.