by David Hohenstern
Fiber cement can increase the resilience of homes and other structures in the face of environmental threats posed by the sun, precipitation, wind, and fire. In order to maximize the performance of fiber cement against the elements and to successfully install them, owners and builders must be aware of clearances, flashing, placement of joints, proper installation tools, and other requirements provided by manufacturers.
Physical properties of fiber cement
Fiber cement consists of fibrous materials such as wood pulp mixed with Portland cement, water, and silica or fly ash. By virtue of its physical properties, fiber-cement siding is less susceptible to common environmental threats compared to traditional wood cladding. For example, in coastal regions, natural wood is vulnerable to airborne salt water that may accelerate deterioration. Elsewhere, wood siding may be compromised with rot caused by common fungi. Given that fiber cement is at least 70 percent inorganic, neither of these environmental features can significantly impact its longevity or appearance.
This increased durability does not require any aesthetic sacrifices. Available as boards and architectural wall panels, fiber cement is versatile and can be processed and pressed to create products of varying sizes, lengths, thicknesses, and textures. It can also be molded to closely resemble natural products, such as wood and stone, or fashioned to foster a clean, ultra-modern look.
Unlike vinyl, both wood and fiber cement can be painted. To keep its appearance, wood siding needs to be repainted every three to five years. According to the Portland Cement Association (PCA), fiber cement siding can retain its painted appearance for 15 years.
Sun and heat
In California, heat and thermal expansion pose regular threats to the longevity of cladding materials. Inland, it is common to experience 30 consecutive days of dry, hot conditions—temperatures can often reach 38 C (100 F) or higher. In other areas, such as in the Central Valley, the temperatures swing from hot during the day to cold at night.
Fiber-cement siding resists the impact of thermal expansion. This is relevant to areas experiencing significant heat and temperature fluctuations, as heat increases the movement of matter’s constituent atoms and molecules, meaning, as the particles move, they occupy more space, resulting in expansion of the material. Cold temperatures have an opposite effect, causing matter to contract. The rate at which different substances expand and contract varies based on the strength of the forces bonding the atoms. Fiber cement has stronger bonds than wood, making it a more stable material and resistant to expansion and contraction in conditions where wood siding might crack or chip.
Precipitation and moisture
Real wood is a hygroscopic material, meaning it expands as it absorbs moisture and contracts as the wood dries. Therefore, humidity is another environmental element impacting fiber cement to a lesser degree than wood.
Manufacturers use ASTM E331, Standard Test Method for Water Penetration of Exterior Windows, Skylights, Doors, and Curtain Walls by Uniform Static Air Pressure Difference, and the American Architectural Manufacturers Association (AAMA) 509, Voluntary Test and Classification Method for Drained and Back Ventilated Rain Screen Wall Cladding Systems, tests to prove that properly installed fiber-cement sidings have the abilty to resist water.
Compared to vinyl and wood siding, modern fiber cement is impact-resistant. Falling hail, snow, ice, and even harsh torrential rain have minimal effects, and the panels can successfully resist water penetration. The previously mentioned tests simulate rain driven by 121 km/h (75 mph) winds, and pressures of 575 to 766 Pa (12 to 16 psf). A properly installed fiber cement system should allow minimal moisture to reach the weather barriers.
Some assemblies are equipped with an additional gasket to prevent infiltration of water and air at the top and sides. With shiplap edges, panels fit together so joints are concealed, and all the four edges effectively form a gasket on their own. Manufacturers take additional precautions by including the drainage channels between the sheathing and back face of the panels. These channels ensure water penetrating the system drains out quickly.
Building owners can take additional precautions, such as properly maintaining weather barriers as a backup for moisture-draining systems and installing kick-out flashings to divert rain away from wall faces, maintain clearances, and keep landscaping clear.
In cases of snow accumulation, the thickness and flexural strength of fiber cement allows it to withstand the physical force exerted by the snow that piles up against a structure. Residential-grade fiber cement is made using a wet process for mixing water with the fibrous material, silica, and Portland cement. This means residential-grade products have some sensitivity to thaw and pre-thaw in cold climates. For this reason, fiber cement made without water might be more appropriate for northern Midwest states as it is less vulnerable to cold conditions.
As high-velocity hurricane zones (HVHZ), Florida’s Miami-Dade and Broward counties have established strict building codes to ensure structures can resist potentially severe wind loads. Architects across the country look to these counties when specifying for structures that must be resilient to storms.
Some manufacturers offer commercial-grade fiber cement panel systems, able to withstand pressures of up to 4549 Pa (95 psf), for use in HVHZs. Miami-Dade maintains a checklist for fiber cement products approved for use, including detailed product and installation information along with test results and reports. The Florida Building Code (FBC) requires approved products to pass the following tests in compliance with the Testing Application Standard (TAS) 301, Testing Laboratory:
- ASTM C1185, Standard Test Methods for Sampling and Testing Non-Asbestos Fiber-Cement Flat Sheet, Roofing and Siding Shingles, and Clapboards;
- ASTM E84, Standard Test Method for Surface Burning Characteristics of Building Materials;
- ASTM E136, Standard Test Method for Behavior of Materials in a Vertical Tube Furnace at 750 C;
- TAS 202, Criteria for Testing Impact and Nonimpact Resistant Building Envelope Components Using Uniform Static Air Pressure;
- TAS 201, Impact Test Procedures;
- TAS 203, Criteria for Testing Products Subject to Cyclic Wind Pressure Loading; and
- TAS 100, Test Procedure for Wind and Wind Driven rain Resistance of Discontinuous Roof Systems.