By William Larson and Frank Came
There is an old adage that says, “nothing is more powerful than an idea whose time has come”. A none too subtle implication behind this adage is that the emerging idea is both good and inevitable.
But history is replete with bad ideas that have come and gone and there is nothing to suggest that every new idea is necessarily inevitable or unstoppable.
Perhaps one idea that has been vastly overstated as being worthy and inevitable pertains to the use of mass timber as a construction material suitable for very tall buildings. It’s a questionable assertion on many fronts and one for which more research and test data is required.
For those unfamiliar with the term, mass timber refers to a form of engineered wood where pieces of wood are glued together, often in a cross-laminated form to create a panel that is stronger than an ordinary wooden beam. Mass timber is a relatively new building product, but it is gaining popularity in North America in large part due to intense lobbying efforts by proponents in the architectural community and in the wood products sector.
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We have been building with wood since the dawn of civilization. Nothing new there. What is new is the drive to use mass timber as a substitute for concrete or steel for mid- and high-rise buildings based in part on as yet unproven claims that it is stronger than either of these two more traditional building products and that it is more resistant to fire. It is also claimed that mass timber has certain environmental or climate-related advantages.
A recent article celebrating the arrival of a mass timber revolution in North America stated “A main argument for the use of mass timber is its power to mitigate climate change. The structures can have a lifespan of hundreds of years, and contain the unique ability of effectively sequestering or removing carbon from the atmosphere, which can reverse climate change effects at a large scale.”
Now, this is where things become somewhat speculative. First. It is debatable whether a building product that has only been around for a few decades will have a useful life that will span centuries. Second, to state mass timber buildings will remove carbon from the atmosphere and will reverse climate change is fundamentally false.
Let’s be clear, living trees sequester carbon from the atmosphere! Harvested and manufactured timber actually retains only a small fraction of the carbon that was originally stored in a tree. Most of the tree’s carbon remains behind in the forest soil or is lost in the leaves and bark usually left to rot on the ground, or burned as biofuel. Far less than half the carbon of a living tree ends up as a long-lasting building product. The rest is emitted back into the atmosphere.
It also is argued that carbon lost from harvesting sustainably managed forests is balanced by the absorption of carbon from new forest growth. Indeed, this is the basis of the biogenic carbon neutrality rule in international carbon accounting metrics that says carbon losses from harvesting trees are not counted in emissions statistics because somehow, they will be offset by new forest growth elsewhere.
The problem is that not all forests are sustainably managed, and even then, research indicates it can take over a hundred years before new forest growth will replace even half of the original biogenic carbon lost.
The second key point to note is that wood buildings do not absorb carbon from the atmosphere. As noted, they store only a fraction of the carbon that was sequestered in living trees. Consequently, it is completely false to say that building more wood buildings will reverse climate change.
Counter that point with the facts that are becoming more evident, concrete actually reverses a portion of the CO2 emissions from the manufacture of cement and permanently sequesters carbon through a process called carbonation. This may well be the only building material that permanently absorbs CO2 and sequesters carbon throughout its useful and life and beyond. Certainly, more research on this phenomenon is required to determine the true value of CO2 emissions from concrete.
Cutting down more trees – the only effective natural means for absorbing atmospheric carbon – to make more mass timber structures is not a sound adaptive strategy for dealing with climate change.
In a recent paper by the Sierra Club, the author points out that “Without great advances in forest protection and stewardship, the answer is almost certainly not. Increased wood use that leads to significant increases in deforestation and forest degradation would only deepen our climate problems.”
There are many other advantages touted in the popular press in favour of mass timber use for high rise structures. It is purported to be less expensive than concrete or steel, easier to assemble, more resistant to seismic disturbances, and more fire-resistant. More serious peer-reviewed studies have challenged most of these claims. In most cases, definitive answers are yet to be proven.
For example, one study suggests that the production cost for a panel of cross laminated timber is greater than the cost for a comparable slab or pre-cast concrete. So too, while building with wood may be faster than using ready mixed concrete on site, there are so many other variables such as location, design, size and adjacent environment, etc., that make categorical statements largely speculative. Clearly, more definitive and comparable studies are needed. We cannot and must not base investment decisions on speculative assertions.
We will leave aside discussing issues such as vulnerability to moisture damage, mould or rot, and risks from termites, not because they are unimportant, but simply because they are so self-evident.
One thing that is rather bothersome pertains to the claim of resistance to fire. The implication, however subtle, that a mass timber building offers greater fire protection than a comparable building made of concrete or steel is wrong.
No one is safe in a burning building and to suggest otherwise is dangerous and should be censored accordingly. There is much conflicting evidence regarding such claims.
Such evidence was certainly apparent to the National Association Of State Fire Marshals in its official position statement at the hearings for the advancement of Cross-Laminated Timber for the Construction of Tall Wood Buildings (TWB), G75-18, G80-18, and G84-18.
Regarding the effectiveness of fire suppression systems in such structures, the statement reads “No live fire testing has been conducted in buildings constructed to the limits being proposed, and the limited application of external influences to fire behavior. It is extremely difficult to accept that these proposals meet the committee’s own stated objectives. We are left with “professional judgment” as the only substantiation. The reason statements for these proposals places an over reliance on the presence of fire sprinklers. NASFM steadfastly supports the use of fire sprinklers, however, we are cognizant of the fact that sprinklers can never be 100% effective given the impact of human behavior on design, installation, maintenance, and intentional disabling.”
The positioning statement goes on the say, “The term “highly reliable”, as used by the TWB committee, is subjective at best. While it is agreed that sprinklers provide a valuable life-saving service, it is speculative to base a major part of justification on this one item. Code committees, fire service organizations, and fire safety advocates have rightly demanded data to support decisions related to code changes. NASFM feels the limited testing, in conjunction with a proposed commitment to conduct additional tests, is insufficient currently to warrant changes of this magnitude.”
There are many other points that can be made, both pros and cons to the use of mass timber. Clearly, wood use in low and mid-rise buildings is on the upswing across North America, fueled in part by rising demands for affordable housing, increased urban densification, and changing building codes largely due to intensive lobbying efforts on legislators.
It is also worth noting, that concrete still plays an important role in such structures and adds elements of resiliency that wooden structures alone cannot provide.
Extending the logic to creating whole cities or subdivisions comprised of skyscrapers composed entirely of mass timber does not appear to be an idea whose time has come based on a groundswell of necessity.
But then again, time will tell.
About the Authors
William Larson is Vice President Marketing at CalPortland. He also serves as Chairman, of the Steering Committee coordinating the development of the Pacific Northwest Building Resilience Coalition since its inception in October 2016.
Frank Came is Managing Director of Globe Advisors. He was the Project Director for the Globe Advisors study on Carbon Sequestration of Forest-Based Wood Building Products completed in 2017 for the Pacific Northwest Building Resilience Coalition.
