Fiber cement panels as rainscreens

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

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.