sustainable and resilient design
A Smaller Carbon Footprint
mass timber is a truly renewable category of construction characterized by the use of large solid wood panels for floors, walls and ceilings.
By Bill Hensley
WHAT IF WE HAD A BUILDING MATERIAL THAT WAS TRULY RENEWABLE, with superior carbon sequestration properties, plus the ability to reduce construction waste. What if it also helped us feel more emotionally connected to our environment. And what if this same material was so plentiful it grew on trees.
What would we call this superior new material? First, we wouldn’t call it new at all. It’s been used for building as long as there have been buildings but is making a resurgence in a new form. That material is wood, and the new format that has designers, architects, and builders so excited is called “mass timber.”
what is mass timber?
Mass timber is far more than a large version of the stick and frame and plywood construction that is the staple of the residential building industry. It is a category of construction characterized by the use of large solid wood panels for floors, walls and ceilings. Of particular focus in the mass timber family is Cross-Laminated Timber. CLT is kiln-dried lumber aligned in multiple layers, each at a right angle to the preceding layer and glued together under significant pressure. The result is a structural panel with exceptional biaxial strength, rigidity and stability.
As the term “mass timber” implies, CLT segments can be large; up to one foot thick and 10 feet by 40 feet or larger in width and length. Panels can be manufactured to custom sizes, which reduces on-site cutting to speed on-site construction. CLT’s strength makes it a viable option for floors, walls and roofs, and its natural aesthetics enable it to be used fully exposed.
Other mass timber types include Glued-Laminated Timber (Glulam), Dowell-Laminated Timber (DLT) and Nail-Laminated Timber (NLT). Glulam is typically used in beams and columns, with the grain of all planks running parallel. DLT is planks (2 x 4 to 2 x 8 or larger) with dowels generally of hardwood friction-fit to provide the finished mass timber with dimensional strength and stability. NLT is likewise created from lumber stacked on edge but fastened with nails or screws.
CLT and other mass timber options are changing the way buildings are imagined, designed and constructed. One company making substantial investments in CLT is the technology-driven construction innovator, Katerra, based in Menlo Park, California. Katerra is developing end-to-end mass timber, from R&D and engineering through design, manufacturing, and construction. “We envision CLT becoming the backbone for future generations of high-performance, low-carbon footprint buildings,” stated Katerra’s Director of Public Affairs, JZ Rigney.
Katerra’s mission is to help transform construction using technology in every process and product. In 2019, the company opened a state-of-the-art 270,000 square foot mass timber manufacturing facility in Spokane, Washington. The facility features one of the largest CLT presses currently in operation globally, able to produce 1,500 12′ x 60′ master panels—over one million square feet per month.
Developed in Germany in the early 1990s CLT soon became popular in Europe and is now gaining traction in the U.S. In 2015, CLT was incorporated into the American Wood Council’s National Design Specifications and into the International Building Code (IBC), which is the default code adopted by jurisdictions across the U.S. While stick and frame construction is embedded into the large-scale home developers’ production models, CLT is making inroads into commercial, multi-unit residential, and custom home projects with its combination of design aesthetics, environmental advantages, and the reduced on-site construction time enabled by prefabricated panels pre-cut for window and door openings.
Biophilic design is a theory used by architects, designers and builders to increase the building occupants’ connection to a more natural environment, satisfying our intrinsic affinity to nature and natural environments. In standard stick and frame and plywood home construction, much of the wood is hidden within the walls behind gypsum wallboard, or in the sub floor below tile or carpet. In a CLT building the wood can be front and center, demonstrating the texture, scent and a visual warmth that appeals to us on a primal level, with a positive impact on health and well-being.
smart environmental choice
Beyond its aesthetic advantages, CLT delivers environmental advantages over concrete, where cement is a key component. The cement industry generates roughly eight percent of total global CO2 emissions. If it were a country, the industry would trail only China and the U.S. as a top carbon emitter. Of course, concrete requires structural steel, increasing the carbon footprint even more. As economic development increases world-wide construction demand, the concurrent demand for concrete and steel will grow, creating an environmental challenge at a global level if alternatives are not developed.
As a wood product, Cross-Laminated Timber is approximately 50 percent carbon, making it an effective carbon sequestration option. The amount of CO2 produced in the CLT manufacturing process is only a fraction of what CLT sequesters, resulting in a net positive removal of carbon from the atmosphere. The CO2 emissions released in the production of CLT is substantially less than that of concrete or steel. And CLT is only about one-fifth the weight of concrete, meaning the CO2 released during transportation of CLT is also significantly reduced.
Traditional stick and frame and plywood houses are susceptible to fire, and large fires like those recently in California and Australia make fire safety a logical question regarding any timber building material. CLT performs well in fire resistance tests, meeting all fire safety codes. As a solid, compressed mass of wood, CLT is relatively slow to ignite, and during a fire its outer layer will char in a predictable way. This charring shields the interior wood, enabling it to retain its structural integrity for several hours during a fire.
Of course, the net carbon sequestration benefits of CLT are negated if the timber is not responsibly harvested. And this is where CLT delivers yet another benefit; expanding the market for less-than-perfect boards. As CLT is made of smaller boards laminated together, CLT manufacturers can use small-diameter and already-dead trees, making tree-thinning a more profitable forest management technique. This can contribute to healthier forests, even helping them be more resistant to fire.
Several organizations are dedicated to advancing the benefits of sustainably managed forests. The Sustainable Forestry Initiative (SFI) promotes sustainable forest management in North America and responsible procurement of forest products from around the world. The Program for the Endorsement of Forest Certification (PEFC) implements a sustainable forest management certification to assure that forests are managed in line with challenging environmental, social and economic requirements. And the Forest Stewardship Council (FSC) has developed a set of principles and criteria that apply to FSC-certified forests around the world. Forward-thinking leaders in the construction industry are responding: “At a minimum, the wood Katerra sources for our Cross-Laminated Timber is SFI or PEFC certified,” Katerra’s Rigney reported. “And if the customer requests, we will also supply FSC certified wood. Sourcing lamstock from well-managed forests is an important element of our sustainability commitment.”
Most of the labor and fabrication for the CLT used in buildings can be done at the factory, leveraging computer numerical control technology to achieve precision cuts. This creates labor and cost advantages on the job site, by simplifying assembly and reducing or eliminating on-site cutting of materials delivered to the site in bulk quantities.
CLT is gaining wider adoption by delivering multidimensional benefits that span the entire construction ecosystem. For designers, architects and engineers, the aesthetic and biophilic characteristics enable a greater blend of form, function, user experience, and sustainability. For builders and developers, CLT enables moving labor upstream into the controlled environment of the factory, resulting in faster on-site build times which reduces labor hours, improves scheduling, and manages costs. For owners, the operational efficiencies inherent in CLT construction enable design differentiation that attracts and retains tenants, which drives long-term asset value growth. And for tenants, CLT-constructed buildings deliver physiological and psychological benefits to the people who live and work in them.
catalyst for the future
One example of how CLT-enabled construction benefits the entire ecosystem is the Catalyst Building in Spokane, Washington. With architectural design by Katerra and Michael Green Architecture and engineering and construction by Katerra, this will be the first office building in Washington State constructed out of CLT. The Catalyst is the result of a unique cross-industry team of partners including Avista, McKinstry and Katerra. The project will blend commercial space with co-working spaces, private sector offices, and higher education classrooms and labs. The building’s primary tenant will be Eastern Washington University, which will move its Computer Science, Electrical Engineering, and Visual Communication Design programs to the new location.
The goal is to create a wood building that will match or exceed the performance and ROI of a comparable steel and concrete building, while showcasing CLT benefits related to aesthetics, build efficiency, and environmental impact. The 150,000 square foot five-story building will have a smaller carbon footprint than that of comparable buildings built with steel and concrete. In fact, the project team estimates the mass timber in this project will store over 3,700 metric tons of carbon dioxide equivalent and avoid estimated emissions of over 1,400 metric tons of CO2. This is the equivalent to taking roughly 1,100 cars off the road for a year.
By prefabricating as much as possible in its Spokane facility, Katerra is able to significantly reduce on-site construction hours and minimize the number of trades required to install or construct the various building components. With CLT’s natural fire resistance, the building was designed to leave much of the wood exposed, creating a visual “warmth” with the biophilic aesthetics discussed earlier. This also eliminated many steps of framing, decking, topping, fireproofing and final finish.
The CLT shear walls are substantially lighter than concrete, reducing the gravity load and bracing requirements. The Katerra “curtainwood” system — a 3-ply CLT exterior curtainwall — offers a number of advantages, including fast assembly and dry-in of the interior space. CLT delivers better thermal and acoustic performance than an insulated cold-formed steel wall, and its natural hygrothermal properties optimize the movement of heat and moisture through the building envelope.
The benefits go beyond the building itself. “In this project, we are pairing the legacy industries of the region, in particular the timber industry, with the innovation happening in the construction and education sectors,” Katerra’s Rigney concludes. “The building demonstrates the possibilities converging at the intersection of economic vitality, regional sustainability, and energy efficiency.”
form. function. forever?
Cross-Laminated Timber is ready to deliver benefits across the design-through-construction ecosystem. Its biophilic properties add to quality of life for those living and working in CLT buildings. Meanwhile, the global climate insists that we focus on renewable and sustainable solutions for an industry that is both vital to quality of life as well as a current major source of greenhouse emissions. Coupled with responsible and sustainable forest management, CLT could be one of the most exciting opportunities the industry embraces.