Is the road to a low-carbon future paved with … wood? Maybe so. One lane of the
Framing walls with 2x4s and building single-family houses largely of wood is routine.
But using wood for walls and floors and structural support in office buildings, apartments, and dormitories is still relatively new. This is not the familiar lumber. It is “mass timber,” much of it large, rigid, factory-assembled panels of “cross-laminated timber” or CLT. Each pane of CLT is basically a three-layer board sandwich (though it can have more layers.) The boards in each layer run parallel to each other and perpendicular to the boards in the adjacent layer or layers. They can arrive at the job site with attachment holes and window openings pre-cut.
Why bother? The use of CLT and the more familiar “glu-lam” columns and beams – small boards combined to make heavy timbers of great structural strength – keeps carbon out of the atmosphere in two ways.
First, carbon gets locked up in the wood itself, since trees store carbon and the lumber made from trees does, too. The more wood embodied in a building, the more carbon that building stores. If you assume the building will stand a century or so, that means keeping carbon out of the atmosphere for a long time.
One might get still more carbon savings out of the wood, explains Indroneil Ganguly, associate professor at the University of Washington’s School of Environmental and Forest Sciences, by using lower-quality boards in the middle layer of the board sandwich. Europeans do that, Ganguly says, and it doesn’t make the panel any weaker. One advantage is that low-end wood is generally used in short-term applications, so that it doesn’t sequester carbon for very long, so putting it in a sandwich would keep the carbon out of the atmosphere much longer.
Also, the market for low-end wood doesn’t usually generate enough profit to pay for the thinning that improves forest health and reduces the likelihood of big, carbon-spewing wildfires.
Second advantage: the wood replaces construction materials with much larger carbon footprints. Builders use mass timber instead of reinforced concrete. Producing the steel for rebar puts a lot of carbon dioxide into the atmosphere, and producing cement (the crucial
ingredient in concrete) is responsible for some 7 percent of all human-caused emissions of CO2. Less concrete means less greenhouse gas.
Building with mass timber no longer lies far outside the mainstream. Building codes permit t. And people are building large structures out of wood. Look in Seattle at the north end of Lake Union or on Capitol Hill. Look north to Vancouver. Or look across an ocean. People in Austria, Scandinavia, and elsewhere in northern Europe have been building with mass timber for years. For a while, the world’s tallest timber-framed building stood in Norway. Now, the world’s tallest timber-framed building, a 25-story structure completed last year, stands in Milwaukee.
When a University of British Columbia residence hall, Brock Common Tallwood House, was completed in 2017, it was the tallest modern timber-framed building in the world, says UBC professor of wood design and construction Frank Lam. Brock Commons “was designed and built outside the building code,” Lam explains. It didn’t meet the code. But the university had “a strong interest in building with wood,” Lam says, and in using this structure as a demonstration project. So it pursued approval as a site-specific alternative project, one that didn’t meet the letter of the code but met its intent. That workaround worked, but “it’s a one-time deal,” Lam says. Build an identical building somewhere else, and you’d need another site-specific approval.
Needless to say, building a one-off structure, one unlike anything done in Canada before, came with some extra cost. UBC “had a budget in mind,” Lam says. When the university estimated the cost, it “aimed for a concrete building.” But everyone realized that there would be extra costs. Code approval took time. So did the practice exercise of putting up a smaller timber building. Lam says that for building a first-of-its-kind structure, the university paid a premium of $4.4 million – only about 10 percent over the original estimate. He doubts that more 18-story mass timber buildings are the immediate future. “The real lower-hanging fruit,” Lam says, are buildings of six to ten stories. There are lots of them, and they’re virtually always made of reinforced concrete.
Lesser structures are springing up on college campuses in the U. S. The University of Washington has just completed Founders Hall in the business school complex, a six- story building with glu-lam columns and beams, and CLT walls and floors. To monetize some of the savings at Founders Hall, the UW has worked with Boulder-based Aureus Earth, which has quantified the carbon sequestered in the mass timber and sold carbon offsets for it.
Elsewhere, Lisa Prevost has reported in The New York Times, mass timber “has become increasingly prominent at colleges across the country, where it is included not only as a concept in the curriculum but also as a material in campus buildings.” Peter B. MacKeith, the dean of the University of Arkansas Fay Jones School of Architecture, told Prevost that ‘“[b]y using mass timber in a library storage annex, completed in 2018, the university saved more than $1 million over the original steel-and-concrete plan.” MacKeith added, “That’s when people start to sit up and say, Well, maybe we should at least be looking at this as an alternative form of construction.”
Outside American colleges, an increasing number of people are doing just that. And have been for a while. When Seattle’s Bullitt Center opened in 2007 as “the world’s greenest commercial building,” it justly drew a lot of attention, some of it for structural use of wood. Above the first two floors, which are made of concrete, the building incorporates heavy glu-lam columns and beams. According to the Bullitt Center’s website, “545 metric tons of CO2 are sequestered in the wood.”
Walk a block south of the Bullitt Center then a block west on East Union, and you reach the 8-story Heartwood Apartments, where a sign on a construction fence says, ”this building is a climate solution.” Heartwood, with 126 units of “workforce” housing and owned by Community Roots Housing, is scheduled to be finished in a couple of months. The building’s architect, Susan Jones, says “it’s the first Type IV-C building in the United States.” That is, although other mass-timber buildings have been permitted under a jumble of code provisions and workarounds, as far as she knows, this is the first one permitted under a specific provision of the International Building Code. (The code also covers taller mass timber buildings under provisions for Type IV-A and Type IV-B. The IBC provisions have been incorporated into the building codes for Washington state and Seattle.) Jones should know. She played a key role in getting tall mass-timber buildings included in the International Building Code.
Less than a 15-minute drive from that Capitol Hill neighborhood, across Latona Avenue from Dunn Lumber, the mass-timber Northlake Commons is taking shape above the former site of the Dunn lumber yard. The concrete first story will hold a Dunn warehouse and distribution center. (At this point, there’s no way to escape reinforced concrete entirely. Concrete foundations are still the norm. Above a certain height, codes require non-lammable materials for fire exits. And here in the Northwest, earthquake safety requires a certain amount of steel and concrete.)
Increasingly, you can manufacture cement with a lower carbon footprint. Cody Lodi, a principal at Weber Thompson, the architects for Northlake Commons, says that method has been used there, and it can be done in buildings that contain no mass timber at all. The floors above will hold offices and labs for life sciences companies. There’s not a lot of new office construction going on right now, but Lodi notes that most people can’t do laboratory science working from home.
Above that concrete first floor, it’s all mass timber. Lodi says that the wood will be exposed in offices and hallways, but he assumes that in the labs, the wood will be covered, the walls painted white.
There’s nothing revolutionary about building with wood. People have virtually always done it. In a very interesting book, The Age of Wood, the author, Roland Ennos, argues that basically, every age has been an age of wood. Copper, bronze, iron may be the substance that survives the centuries, but perishable wood is what sheltered people and formed the handles for the stone or metal that we associate with a period of time. Even the medieval cathedrals, in addition to their stone walls, relied on wood: the roof of Notre Dame was supported by oak beams cut in the 1200s. Until the beams caught fire in 2019 and threatened to bring the whole cathedral down.
When people remain skeptical about the use of wood in modern multi-family or commercial buildings, it’s fire they worry about. The fire risk is “kind of a myth,” Lodi says. But it is grounded in everyday experience, which for most people doesn’t include mass timber. “We always think about wood as 2x4s,” Gangully explains. Two-by-fours are, of course, staple elements of single-family home construction. And of fire starting: You want to get a campfire or a wood stove going in a hurry? Throw in some 2×4 scraps; the dried wood blazes up in a hurry.
But, Gangully says, “mass timber behaves very differently.” The outer layer — a “sacrificial layer,” if you like, doesn’t go up in flames. It chars. The charred layer doesn’t ignite. Instead, it insulates the layers below it. In fact Gangully says, “charred wood is one of the best insulators.” In addition, he says, “in a traditional building, as the fire gets really hot, steel . . . conducts heat up and down. [That’s the] type of info you have to convey to the local firefighters.”
Nevertheless, that fear of fire is the main obstacle that advocates of mass-timber construction must overcome. Which they’ve been doing. Still, just explaining the theory of CLT’s fire resistance may not be entirely convincing. Seattle architect Susan Jones, who has designed Heartwood — as well as her own mass-timber house in the Madison Valley — says that when she served on the 18-member international Code Council (ICC) committee that considered revising the international building code to allow tall mass timber buildings, the char theory obviously wouldn’t cut it. (Jones has explained in a Bloomberg interview that when she was encouraged to join the committee, “[t]hey wanted me for my mass timber building experience. At that time , I had built two [timber] projects in the West of the U.S. And that was about two more than almost anybody else at the time.”)
Instead, the committee had five mass-timber structures built and torched to see if those CLT panels would really resist fire. By the time they burned the second one, Jones says, most committee members realized that the panels really did char without catching fire. The committee vote still wasn’t unanimous – three members, including representatives of the steel and concrete industries, still didn’t buy it – but in 2019, everyone else voted to incorporate mass timber into the new code that came out in 2021.
Washington had already become the first state to incorporate it into a state building code. The Washington State Building Code Council okayed mass timber after the legislature told it to do so in 2018. The Seattle City Council followed suit. (Oregon was quicker, but Oregon accepted it only as an alternate solution.) Jones says that the Seattle-based conservation group Forterra played a key role in bringing together people who wanted to move mass timber forward and in lobbying the code change through the legislature. (Local fire departments have to approve, though, so each municipality can accept mass timber or not.)
Is building with mass timber faster? Yes, Gangully says. There is “no question.” Once the foundation is done, “you are pouring less concrete,” which in a traditional tall building “is always time consuming.” Installing mass timber panels isn’t a process that most construction workers have encountered, and it naturally takes a while to develop expertise. Lam says that before the building at Brock Commons went up, workers built a two-story structure at a different site for practice.
Once the workers get the process down, it goes quickly. Much more quickly than conventional construction. Frank Lam says that at Brock Commons, it all went together “like a Lego.” Mass timber reduced the construction time by an estimated four months.
Once the people building Heartwood got the hang of it, Jones says, you’d see them “just dancing there on that mass timber.”
Does the fact that a building goes up faster mean it will cost less? The University of Arkansas evidently thinks so, but “I would be more inclined to say it would not cost significantly more,” Gangully cautions. Labor costs are lower, but material costs are higher, and “I haven’t seen any convincing evidence that it would be cheaper.” On the other hand, Jones said in the Bloomberg interview, if you look beyond construction cost per se, if finishing a building sooner means collecting extra months of rent, the overall project number will look better.
CLT comes in big panels, and staging them on, say, a small lot on Capitol Hill or in the congested strip along the northern shore of Lake Union would be difficult. Consequently, both Heartwood and Northlake Commons have been built with just-in-time deliveries: A truckload of panels arrives from British Columbia, a crane picks a panel off the truck and delivers it to whichever floor is being assembled. A very small crew or workers puts it in place. Both projects have gotten their CLT from British Columbia. And both have got glu-lam beams assembled in Oregon.
Not in Washington? Nope. Governments both north and south of the state have done more to promote the use of mass timber, Lodi explains. The Brock Commons building grew out of a Tall Wood Demonstration Initiative, sponsored by Natural Resource
Canada and the Canadian Wood Council. On this side of the border, Portland has become “kind of an epicenter,” Lodi says.
But historically, no place has depended more on – or prospered more from – forest products than Washington. And Lodi suggests that using a lot of wood is a great historical fit with Northlake Commons’ Fremont location. The building site became Dunn’s lumber yard in 1931. Even earlier, mills had cut lumber and shingles in that part of the city for generations.
And, of course, it wasn’t just Seattle: At the turn of the last century, western Washington had some 340 mills There’s no going back to the good old days in which people earned livings (and often lost body parts) in those mills, and their wages sustained small towns all over the Northwest. Some people see the manufacture of CLT as a way to revive – or re-create – some of those historic rural economies. Mass timber production won’t revive all or even most of them, but it could “absolutely” help some, Lam says. “The game is adding value,” and “if you build with mass timber (instead of 2x4s), it is a several-fold increase in value.”
Gangully would also like to see new businesses that add value to basically low-value trees and worthless scraps. Since the turn of the last century, forest products companies have tried to prevent wildfires by getting rid of the branches and other slash left over from logging, using controlled burns. He suggests it doesn’t make a whole lot of sense for a company to burn the slash merely to dispose of it and then turn around and burn natural gas to fuel its mill. He’d like to see the mills using biomass as a fuel and manufacturing wood pellets that could be exported to Europe.
Where do we go from here? Acceptance of mass timber is obviously growing. The state Department of Commerce says that “cross-laminated timber is one of the fastest-growing segments of [Washington’s forest products] industry.” But it has a long way to go. It’s hard to imagine mass timber replacing reinforced concrete in a big way. But who knows? The status quo is always the easiest option to imagine.
How could one accelerate the move to wood? Jones points to existing U.S. Department of Agriculture Wood Innovation Grant programs, “which give out approximately $250,000 to teams on a yearly competitive basis. Heartwood won a grant in 2019, and it became the seed money to start the early feasibility study design work. It mobilized the team and allowed the non-profit Community Roots Housing to take the risk more easily.”
“However,” Jones says, “I think there are ways to accelerate the progress further, by tying [low-income housing tax credit funding] to more sustainable building technologies.” Or, she says, governments acting as builders or purchasers could try “sanctioning the use of mass timber for all or almost all public buildings.” In addition, “building prototypes is a hugely important way to get the private sector to move forward. [The federal General Services Administration and Department of Defense] could make excellent use of their facilities funds to accelerate changing ‘business as usual’ 20th-century construction methodologies.”
Why not? The current century isn’t a bad time to be building as if carbon mattered.