Josh Carel and Alex Daniel
March 2, 2023
The building industry is responsible for 37% of global carbon emissions.1 For us at Plural, for our fellow architects and building industry colleagues, that’s a sobering statistic. It tells us that, collectively, we hold over a third of the responsibility for current trends in global warming, the ones that are raising the levels of our seas, sparking wildfires, inflating our energy bills, and paving the way for a host of natural disasters on a scale unprecedented in human history.
It also translates to an enormous amount of power. By changing our practices, conventions, and notions of common sense, we have the potential to deescalate the climate emergency. If we all agreed to put a moratorium on new construction2 we’d take an enormous stride toward healing the planet, but that’s not practical. People need homes, companies need space to conduct business, and we have livelihoods to earn. Humans have needs, and we fulfill those needs by building—nice though it would be if we could all simply call it a day, go home, and do nothing.3
Thanks to advances in construction technology, however, we can design buildings that do even less. Doing less than nothing is counterintuitive. Seemingly impossible. But counterintuitive, impossible thinking is exactly what’s needed in times of unprecedented emergency. Where climate is concerned, if releasing carbon into the air equates to doing something, and refraining from releasing carbon into the air equates to doing nothing, then capturing carbon, taking it out of the atmosphere, removing it from the very air we breathe? That equates to doing even less.
Removing carbon from the air is exactly what cross-laminated timber (CLT) does. CLT is a building material composed entirely of wood and adhesive, engineered to maximize strength. CLT is manufactured by laminating two-by-threes, two-by-fours, or two-by-sixes together in alternating directions to form panels roughly eight feet wide by fifty feet long. Depending on the needs of a structure made with CLT, each panel may be composed of at least three but up to as many as seven layers of lumber. These panels can act as load-bearing walls, roofs, or floor components. The end result is an atmosphere-cleansing product with enormous strength and tolerance—enough to replace carbon-intensive materials like concrete and steel in mid- and high-rise structures.
This article demonstrates how engineered wood technologies like CLT are a higher-performing replacement for concrete and steel in many contexts, advocates for the sustainable sourcing of CLT, and explains CLT’s many benefits to human psychological and physical health, as well as the health of the planet. Ultimately, we point to a flexible path forward with CLT, one that’s open to how and where it’s integrated into the design and construction processes.
CLT panels have myriad benefits over traditional construction. Because the panels are cut and laminated in the factory according to the needs of the specific structures they form, CLT reduces waste and ensures more precise tolerances.4 Onsite, CLT panels can be assembled into a built structure six times faster than a standard build.5 As illustrated below, they can perform several essential functions simultaneously, acting as structure, surface, and insulation, reducing the number of layers and components in a wall assembly, ultimately simplifying construction.
With a strength-to-weight ratio higher than structural steel6 and a tested record of fire resistance,7 CLT allows for wood buildings taller than five stories. The Ascent building in Milwaukee, pictured below, places twenty timber floors atop five floors made from concrete. Feats of wood construction like these were unachievable until recently because, for a long time, most municipalities capped light-frame wood buildings at five stories. But CLT structures are stronger and more resilient than their predecessors. CLT’s distinguished status as a safe, biobased construction method for tall buildings has made it one of the most exciting new tools in the construction industry’s arsenal against climate change.
Concrete and steel are carbon-intensive materials. Cement, the primary ingredient in concrete, makes up about 8% of global carbon emissions. To put that number into context, “if the cement industry were a country, it would be the third largest emitter [of carbon] in the world—behind China and the U.S.”9
CLT on the other hand, when sustainably sourced, actually removes carbon from the air. As a wood product, CLT is made from trees, and trees by nature absorb carbon dioxide. In essence, when humans emit CO2 trees breathe it in, thus taking it out of our atmosphere, storing it, helping us manage it, and ultimately mitigating global warming.
When we cut down trees to turn their wood into construction materials, that carbon stays within the wood and out of the atmosphere. This natural process of carbon storage is known as carbon sequestration, and experts take sequestered carbon into account when considering how much carbon is emitted by a building made from wood. The manufacturing and construction processes for a mid-rise, five-story CLT building may release over a thousand tons of carbon into the air, but if its CLT panels have already stored five thousand tons of carbon, that building maintains a negative balance in terms of carbon accounting.10 Rather than worry about the ton of carbon the building emitted, we can celebrate the net four tons of carbon it removed.
That’s how structures made from CLT are able to do less than nothing: when their CLT panels are still just trees in a forest, they do an incredible amount of prework, by the mere act of breathing and growing, to clean our air. To highlight this phenomenon, the engineering firm Arup compared the global warming effect of a wood structure to a concrete one for a 12-story building planned in Portland, Oregon.11 The graph below charts the effect of the two structures throughout the life of the building. Notice that the wood building’s concrete emissions are below the x-axis, in the negative, thanks to the carbon sequestered during the growth of its constituent trees. Even when taking into account the manufacture and transport of its CLT panels, the net carbon emissions remain far below zero.
Another study by researchers at the University of Washington conducted in 2021 compares timber relative to concrete buildings at different scales and in different regions around the U.S.10 In all cases the wood buildings required considerably less carbon to construct, as charted in blue, as compared to their concrete counterparts, charted in orange. When the carbon sequestration of the wood buildings, charted in purple, is taken into account, the wood buildings actually act as carbon sinks, removing more carbon than they use.
It’s been estimated that by adopting wood-centric construction methods like CLT usage on a mass scale, by 2050, the construction industry could store and avoid the emissions of thirty-six gigatons of carbon––6.6 times the amount of annual U.S. greenhouse gas emissions. Further, assuming that this paradigm shift in construction methods would incentivize rigorous afforestation and reforestation practices, widespread CLT adoption could produce as many as 205 additional gigatons of carbon storage.12 Conventional methods of construction are a massive contributor to climate change––but through the adoption of more sustainable materials like CLT, the building industry has enormous opportunity to alleviate current and future environmental disasters.
Greenfield sites that are clear cut to make way for new suburban housing, even if constructed with biobased materials, do more harm than good to the environment. In one study, the Timber City research team compared the carbon sequestering potential of three typologies: the single-family residence, the conventional light-framed wood multi-family housing development at five stories, and a mass-timber multi-family development at ten stories.13 As seen in the diagram above, the sequestration benefits of timber structures increase with density at an exponential rate: A ten-story structure has nearly three times the sequestration potential of a five-story building, and over twenty times that of a single-family development. Furthermore, when CLT is deployed at a scale that would otherwise necessitate concrete or steel construction––five to six stories depending on municipality––it both sequesters more carbon and avoids a higher number of future emissions.
CLT’s rare ability to serve as both the structure of a building and the interior finish of its walls provides added benefits to those who occupy CLT enclosures. As you might expect, walking into a building where the walls, floors, and ceilings are clad in smooth panels of light-hued wood is a peaceful and calming experience––an impression that results from the natural, organic appearance of CLT panels, the acoustically pleasing character of the sound they reflect, and the freshness of their scent.
Recent studies have indicated that wood interiors have positive effects on occupants, similar to those induced by spending time in nature14. These studies note improved vital signs, including drops in heart rate and blood pressure, as well as improved recovery times in healthcare settings. Because CLT interiors remove the need for paint and gypsum board, CLT improves air quality in these spaces. Low VOC and formaldehyde-free adhesives should always be specified for mass timber to ensure these benefits are realized.
CLT also makes for a warmer, more natural interior aesthetic, which architect and educator Kiel Moe attributes to wood’s thermal effusivity.16 To understand thermal effusivity, imagine touching two walls in a room, one made of wood and the other made of steel. Though part of the same enclosed space, and therefore the same temperature, the one made of steel would feel colder. That’s because, compared to steel and concrete, the human body loses less heat to wood––or in other words, wood has a lower thermal effusivity. Because of that quality, to many, rooms made from CLT not only feel more organic, but also cozier.
CLT and other engineered wood technologies necessitate the cutting and harvesting of more trees than other forms of building. For her 1,500-square foot CLTHouse project, architect Susan Jones calculated that 184 trees were required.18 A light-framed wood house of the same size would require the felling of just 30-75 trees.
That’s why CLT must be sourced from sustainable systems: forests that are harvested and replanted at a rate that doesn’t decimate them, but instead allows them to thrive. Sustainable forestry practices require an understanding of the species of trees native to a given region (the Sitka Spruce, Douglas Fir, and Western White Pine are at home in western North America, for example), a knowledge of which forests in those regions might benefit from thinning, coordination with local markets that might be ready to purchase the lumber that comes from that thinning, and a commitment to replanting and managing forests according to forestry service recommendations and centuries-old Indigenous knowledge of land management.
If CLT is not sourced from local, sustainably managed systems, the carbon emitted during CLT production and transportation will nullify or even regress the environmental benefits of CLT construction. As architect Kiel Moe notes, “[a] truck can drive only so far before it has emitted as much carbon as that locked in the logs it is transporting.” It’s not only the production of a material that makes it sustainable, but also the transporting and processing of that material.16
Arup charted the impact of timber and CLT if wood is not sourced from sustainably managed forests (11). As illustrated in the chart below, the sequestering capacity of buildings is nullified if their wood is sourced unsustainably. They become carbon emitters instead of carbon sinks.
If CLT is the ecological and economical wonder that it claims to be, why is its adoption not ubiquitous? Susan Jones describes the construction industry as “perennially risk-averse” and notes that “to challenge existing notions of wood design and construction is to challenge the very bedrock of the American residential construction industry.18 It will take time to replace conventional construction methodologies that are rooted in existing economies and preconceptions about fire risk and life safety.
Further, the municipalities that could incentivize and standardize the use of CLT are difficult to convince.10 Though some work has been done in this area, codes and regulations still need to change to permit CLT construction on a wider scale, not to mention the construction of high-rise CLT buildings. And of course, monied interests are always an issue: many gain financially from the widespread, unquestioned use of materials like concrete, and they put substantial lobbying effort into keeping things the way they are.18
It's also true that CLT may not be right for all parts of the world or all project types. Areas with large nearby timber stocks and sustainable forest practices are most practical for CLT implementation, while more arid regions may not benefit as much from timber construction. Timber City, a leader in CLT research, advocates for a mixture of biobased materials that may include earth, straw, and hemp methods alongside timber to create the most ecologically restorative combination for a given climate.19 Such strategies could produce structures that use CLT where it makes the most sense, in floor, roof, and party wall conditions, with exterior and partition walls infilled with other biobased materials.
Implementing timber in combination with other biobased materials is particularly sensible for the single-family market. Single-family structures don’t need to handle the same stresses as larger buildings, so the application of CLT and other engineered wood construction technologies are often overkill for smaller homes. Conventional, sustainably-sourced light-framed wood construction still sequesters carbon, and when paired with other biobased building materials like hemp wool insulation, home assemblies can be cheaper, with higher R-values and increased carbon sequestration capacities.
Because of its benefits to the environment, human health, and the integrity of structures––inside and out––a year ago, we at Plural were ready to pitch CLT to clients as the choicest of materials for even the smallest of single-family projects. We visited the under-construction CLT pilot project residence of our friend Greg Esparza at Cross Cabin Build & Supply , found ourselves awed by the elegance of its design, the organic quality of its aesthetic, and the fresh forested aroma of its interior.
But after attending last year’s Humid Climate Conference, we pumped the brakes. There, a healthy dialogue was growing around the caveats of CLT’s sustainability claims––namely, as noted above, that CLT is only as sustainable as the forestry practices that produce it, and that to remain sustainable, the panels that compose a structure must stay local.Further, as we were advised by Sherry Mundell of SmartLam North America, the conventional process for building single-family residential homes––light-frame wood construction––is already wood-centric.
That’s why, from the Plural perspective, the best way to put CLT to use is:
Hybridized urban structures of this nature are already popping up around Texas and the nation at large. A concrete core provides the shear walls of the Soto Building in San Antonio, for instance, while the majority of the structural beams, floors, and ceilings surrounding that core are composed of mass timber.
Of course, we’d love to see an abundance of buildings composed primarily or even entirely of sustainably sourced CLT––like the DC Southwest Library––around Austin’s downtown, or in any of the many states where Plural currently has projects in development.
Until then, though, we remain open to incorporating CLT and other forms of mass timber into our designs, even if that means hybridizing our projects and specifications, and we welcome new information and research that suggests better approaches to sustainable design.
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