Fiber cement siding has a longer history than you might suppose, with asbestos-based cement siding products first coming onto the scene at the beginning of the 20th century. These products were phased out in the 1980s, for obvious reasons, and replaced with cellulose-based fiber cement.
Since then, this siding option has seen a huge increase in popularity year over year (at least, according to U.S. figures). One of its big draws is its ability to mimic other traditional siding options like clapboards and shingles without the upkeep required for wood or the cash outlay required for brick.
Another draw is its longevity, which could be from 25 years to a lifetime, depending on who’s manufactured it and what kind of warranty they’re offering.
And a third big draw is its reputation as a sustainable building material. Proponents of fiber cement claim that it’s less toxic to produce than vinyl and that it reduces the impact of unsustainable forestry practices that can come along with wood siding.
Does it really have a green leg up, though? This post will explore how it stacks up.
What Is Fiber Cement Siding Made Of?
To know whether something is “green” or not, we need to know what it’s made of. Fiber cement products are made of a mix of water, cellulose fibers (often wood pulp), cement and fly ash or silica sand. Each of these materials has its own environmental impacts and considerations. Let’s look at each of them in turn.
Cellulose Fibers
Cellulose fibers are harvested directly from plants or from recycled materials like newspaper. The most common type of cellulose fiber is wood pulp. Wood pulp is made by breaking down the fibers of a tree, either chemically or physically. On its own, pulp is biodegradable, but won’t be, of course, once it’s mixed with cement.
Perhaps the biggest consideration with wood is how it’s harvested. Wood pulp that comes from Forest Stewardship Council (FSC) or Sustainable Forestry Initiative (SFI) certified forests will be more sustainable, although if the pulp comes from recycled sources, that’s even better.
Cement
Cement is made of limestone, clay and sand (although it could contain other ingredients like shale or blast furnace slag). These materials are ground together and heated in giant kilns to produce something called “clinker.” The clinker is cooled and then ground and mixed with gypsum and limestone to produce cement.
Cement production is extremely energy and water intensive. Quarrying is also an issue, but the biggest problem is that cement production alone generates 8% of the world’s CO2 emissions. Production practices are changing, slowly, but for right now, this is not a sustainable material.
Fly Ash
Fly ash is the ash leftover from coal combustion. This fine substance is recovered from coal-burning furnaces and used as a filler in a variety of construction applications.
As you might imagine, fly ash contains toxic elements and is not great for humans or the environment. Its environmental benefits mainly appear when it’s recycled instead of being held in a pond or disposed of in a landfill, both of which practices put soil and water at risk of contamination. The EPA ruled that fly ash was safe to encapsulate in building materials in 2014.
Silica Sand
Silica sand is the sand typically used on golf courses and in water filtration systems. The environmental considerations kick in when this sand is mined. Mining processes carry risks for ecosystems across the globe. Crystalline silica, that is, airborne silica dust particles small enough to be breathed in, is also a well-known occupational health hazard for miners and others who work with the substance.
When fiber cement products are cut, for instance, they release these respirable crystalline silica (RCS) particles into the air. Long-term exposure can result in lung damage, silicosis and cancer. This exposure is one of the reasons manufacturers recommend professional installation. Pros have access to safety equipment the average DIYer won’t.
Water, as a building material, is benign. What matters most with water is how it’s used, which leads us to manufacturing.
How Is Fiber Cement Siding Made?
Generally speaking, a mixture of fibers, cement and sand or ash is pressed into sheets or molds, depending on the manufacturer’s technique, and then cured. Once cured, the product is machined into its final form and finished. Manufacturers might paint the product themselves or leave it primed for builders or homeowners to do.
Because of the cement involved in the making of these products, manufacturing is generally energy intensive and water intensive.
Specific manufacturing processes will differ from producer to producer. That means that if you want to know exactly how green a certain siding product is, you’ll need to have a look at how the company makes it. Some manufacturers pay a great deal of attention to sustainable production. Others, not so much.
To give you a few examples from major manufacturers, some companies, like Nichiha, use fly ash to replace some, or all, of the silica. Nichiha also uses low-VOC materials. Some of their products are locally sourced, with water recaptured during processing.
TruExterior makes its products with fly ash instead of silica in a LEED Silver manufacturing facility. They use 70% recycled materials, as verified by SCS Global.
James Hardie, the original maker of cellulose fiber cement siding, has one line of products that’s GREENGUARD Gold certified. GREENGUARD Gold is an independent certification standard for low-emissions products.
James Hardie also uses local production practices that minimize transport emissions from raw material to finished product, which is important because this siding is heavy and transporting it can raise its overall embodied energy quite a bit.
End of Life
Since it’s a recently introduced product with a long lifespan, we don’t have enough information to tell how much of an issue end-of-life is going to be with this siding.
It isn’t, however, recyclable. Since it’s inert, there’s not a danger of toxicity, but we could still end up with a lot of it in the landfill if recycling programs aren’t put in place.
Conclusion
The environmental think tank Dovetail Partners published an environmental assessment of siding products in 2019, primarily based on data published by the Building for Energy and Environmental Sustainability (BEES 2.0) program in their much heftier report.
Dovetail found that in terms of both global warming potential and overall environmental impact, fiber cement scored worse than cedar and, to their surprise, vinyl. They cite the energy consumed in manufacturing fiber cement as the main reason for its larger negative impact, along with the human health concerns that come with the product.
In case you’re curious about the greenest siding options, Dovetail suggests that over “an assumed building life of 60 years or less,” the “environmental performance of vinyl and wood-based products is generally better than that of available alternatives.”
Ideally buildings last a little longer than that, however, and when we assume a longer lifespan, the top choices shift. With a longer lifespan, Dovetail states that aluminum and stucco become more eco-friendly contenders, with brick likely winning out over the long haul due to its durability.
Feature image: Roger Starnes Sr; Image 1: Geneviève Tremblay