Talk about climate change caused by carbon emissions, and people call out the usual suspects: transportation, electric power, heating. But a giant source of carbon in our atmosphere often goes unmentioned: construction using concrete and steel. According to The Economist, cement-making alone produces 6 percent of carbon emissions.

The numbers are driving a renewed interest in using wood as a substitute in construction — specifically, construction using a manufactured wood product called mass timber, and particularly in large panels known as cross-laminated timber (CLT for short). Unlike the production of steel and concrete, which use fossil fuels (mostly coal) to heat raw materials to temperatures over 2,500°F, wood is a product of sunshine. Rather than releasing vast amounts of carbon into the atmosphere during production for concrete and steel, CLT buildings actually sequester carbon. And thanks to modern technology, wood can substitute for concrete and steel in a surprisingly wide range of construction applications. 

Consider, for example, the 18-story Brock Commons Tallwood House dormitory at the University of British Columbia in Vancouver, the tallest CLT building in North America. Wood construction means that this building stores over 1,900 metric tons of carbon dioxide, and will store it indefinitely. And while scaling a vast technologically based change is not a simple matter, a widespread shift to wood construction combined with sustainable forestry practices (i.e., replacing harvested trees) could cut carbon by as much as all current renewable energy sources combined.

In Washington state, CLT is being hailed for both its environmental benefits and potential to revive distressed rural counties. “This is just one example of how we can create jobs, transition to a cleaner economy and build new infrastructure with a lower carbon footprint through innovation,” Washington’s governor, Jay Inslee, crowed at the opening of a CLT manufacturing facility in Spokane last September. “Our state leads in so many ways,” he added, “and now Washington will lead the nation in manufacturing cross-laminated timber for buildings. This is a leap forward for sustainable construction and economic development in our state.”

Oregon and New England are also hotbeds of CLT innovation and advocacy, but they are hardly alone: there are some 600 mass-timber construction projects across the country in states as diverse as Wisconsin and Arkansas. Microsoft says its Silicon Valley CLT project will be the largest mass wood structure built to date in the U.S. when completed this summer. Google’s parent company has even bigger plans for timber construction in its role as master developer for a 12-acre site in Toronto.

Made in the USA, Conceived in Europe

CLT may be the new new thing in the Pacific Northwest and New England, but it has been employed in construction in Austria and Germany since the early 1990s. By the 2000s, CLT was widely used in Europe in both single-family and multi-story housing. One reason: unlike the United States, where inexpensive stick-built homes have long been the rule, European housing was more likely to be built from masonry — and CLT was a cost-effective alternative.

Europe also led the way in both regulating the carbon footprint of new buildings and taxing carbon in manufacturing, giving CLT construction a leg up in the marketplace. “Part of what motivated the spread of CLT in Europe is that European codes require carbon sequestration in the building,” said Frank Lowenstein, the chief conservation officer of the New England Forestry Foundation. “I think it will be the climate angle that will really drive this model to take over the construction sector. Twenty years from now, buildings of steel and concrete heated by fossil fuels will seem nonsensical.”

Building with CLT in the United States only dates to 2015, when the material was incorporated into the National Design Specification for wood construction, which was used as a reference for the 2015 International Building Code. These code changes permitted CLT to be used in the assembly of exterior walls, floors, partition walls and roofs.

“For the first time you had a direct link from the product standard to the building code; then anybody could start using it,” explained Bill Parsons, vice president for operations at Woodworks, an industry trade group based in Washington, DC. “The reason we’re seeing growth is it’s starting to pencil out more and more,” he said. Contractors are “finding that if you involve the design team early in the process, although the materials cost more up front, the time you save on how fast you build the building is pretty remarkable. You can be cost-effective or better.”

Choose Your Externality

Of course, CLT has its own set of what economists call negative externalities. Chainsaws are not exactly green, and logging remains the most dangerous occupation in the United States. The adhesives used to glue the layers of lumber together to make super-strong laminates are petrochemical based. And then the big drawback: would not constructing new buildings out of wood spur more rapid deforestation, itself a major contributor to climate change?

“Proponents of mass timber maintain that emissions can be reduced by substituting wood for other building materials because trees capture and store atmospheric carbon,” the Sierra Club, Natural Resources Defense Council, Greenpeace and 45 other environmental groups and companies, wrote in a letter last year protesting a California effort to boost mass timber construction. “Too often, however, the carbon benefit of such substitution is exaggerated because the impacts of timber extraction on forests are left out of the equation.”

But CLT advocates, who span a remarkably broad cross-section of interests, from forest-products boosters to edgy architects and city planners, say CLT construction might actually help reduce deforestation. They say that harvesting for wood products is far down the list of causes of deforestation, which is driven more by activities like cattle grazing in the Amazon and suburban sprawl in the U.S.

CLT and related products are typically harvested from managed forests that have already been in production for a long time, they point out. They’re not the product of old-growth timber stands. Moreover, some mass-timber products can even be made from so-called waste wood, the small stuff that litters forest floors and catches fire easily.

“People tend to lump all forests together, as in cutting a tree is ‘bad,’” said Alan Organschi, a partner at Gray Organschi Architecture and a member of the faculty of the Yale School of Architecture. “Deforestation comes primarily from forces outside the harvested woods products industry. A lot of it comes from mining. We strip forests for concrete, mountains for coal, forests for grazing — and we suburbanize,” he said. 

Forests do not die of old age. Organschi goes on: “They go through all sorts of disturbances that are not anthropogenic, and they are incredibly good at regenerating. The steel and cement industries are using images of clear cuts to frighten people about wood construction. Take a look at the tar sands or copper mines. Once you strip a mountain-top for coal, there’s no chance that is going to regenerate.”

Urban Funk

One other factor fueling interest in new construction methods is urbanization. The United Nations predicts that by 2050, 6.3 billion people will live in cities, about two-thirds of the total population. Since the global rural population is expected to decrease in that time frame, the world will need to build enough urban dwellings to accommodate 2.9 billion people. Doing things the old way, that’s a lot of concrete and steel — and a lot of CO₂ emissions.

Housing all those new urbanites will require changes in both public policy and construction technology. Last year, Minneapolis banned new construction of single-family homes throughout the city because they crowd out more affordable housing. Similar legislation for cities in Oregon failed to pass the state senate, but it remains under consideration. CLT boosters say the housing solution — more medium-rise apartments and condominiums — falls right in the sweet spot for mass timber, which is widely seen as up to 12 stories with the wood exposed, up to 18 with it encapsulated.

Some movers and shakers are not waiting for regulations on land use and outmoded limitations on the height of wood buildings to catch up. “Climate change, jobs and affordability; all three of those ideas allow us to do a project that doesn’t meet the building code, but does meet the intentions of the code,” said Michael Green, an architect in Vancouver, B.C. “You can’t innovate based on a code because a code takes a long time to change and the code had never imagined this kind of building.”

The 18-story building at the University of British Columbia was only possible because the school had the leeway to write its own code, noted Green, who now has a 35-story project in the works. “The 12-story and 18-story numbers are not based on science; they’re based on emotions. There should be no height limit on wood buildings, as there is none on steel and concrete. If we’re going to deal with the climate crisis, we can’t put up with that nonsense.”

Green said a new generation will demand more wood buildings, both for their environmental benefit and for the aesthetic attraction. “If you design simply, these buildings are always beautiful,” he said. “People love them. People are inherently attracted to living, working and going to school in wood buildings. Why don’t we start over? Why don’t we do buildings that have the texture people are craving?”