Much has been written lately about the environmental and climate-related benefits of high-rise buildings constructed mainly from wood. Too often, these articles fail to address serious risk factors associated with such structures or misrepresent their actual impacts on the environment.
One article (Tall buildings out of timber? In the face of climate change, Seattle encourages it), extolling the virtues of tall timber buildings, noted that structures built using cross-laminated wood are an environmentally sustainable alternative to concrete and steel, which generate large quantities of greenhouse gases in their production.
Like many articles on this subject, this piece claims construction of a cross-laminated timber high-rise emits roughly 25% less carbon dioxide than concrete, and such buildings store atmospheric carbon locked in the trees used to build them, which over time will be replaced by new forest growth that will absorb carbon dioxide.
For those unfamiliar with the term, cross-laminated timber is a form of engineered wood where pieces of wood are glued together to create a panel that is stronger than an ordinary wooden beam. This is a relatively new building product, but it is gaining popularity in North America due to intense lobbying efforts by proponents in the architectural community and the wood products sector.
The use of mass timber for tall buildings is often cited for its power to mitigate climate change because they remove carbon from the atmosphere. This is a fundamentally false assertion.
Let’s be clear; living trees sequester carbon from the atmosphere! Harvested and manufactured timber retains only a tiny fraction of the carbon initially 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.
What little carbon remains in long-lasting wood-based building products remains stored over the asset’s lifetime. Still, far too often, when wood buildings are deconstructed, much of that wood ends up in landfills, where it decays and emits methane, a far more noxious greenhouse gas than carbon dioxide.
It is also argued that carbon lost from harvesting sustainably managed forests is balanced by absorbing carbon from new forest growth. Indeed, this is the basis of the carbon neutrality rule in international carbon accounting metrics that says carbon losses from harvesting trees are not counted in emissions statistics. They will somehow 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 carbon lost. The critical point is that wood buildings do not absorb carbon from the atmosphere, so it is false to say that building more wood buildings will reverse climate change. 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.
A recent paper by the Sierra Club notes that without significant advances in forest protection and stewardship, increased wood use that leads to substantial increases in deforestation and forest degradation would only deepen our climate problems.
(Interestingly, a growing body of research indicates exposed concrete reabsorbs CO2 from the atmosphere and permanently sequesters it through carbonation. Indeed, more research on this phenomenon is required to determine the actual value of CO2 emissions from concrete.) See Carbonation – The New Face of Concrete.
Many other advantages are touted in the popular press, favouring mass timber used for high-rise structures. It is 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 similar slab or pre-cast concrete. So too, while building with wood may be faster than using ready mixed concrete on site, so many variables such as location, design, size and adjacent environment, etc., make categorical statements largely speculative. More definitive and comparable studies are needed. We cannot and must not base investment decisions on theoretical assertions.
What never seems to be discussed in these articles is the vulnerability of wood-based buildings to moisture damage, mold, and risks from termites. These are important risk factors that new home buyers are often unaware of until too late.
One aggravating risk factor pertains to mass timber is the claim of its more excellent resistance to fire. However subtle, the implication that mass timber buildings offer more fire protection than comparable buildings 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.
This 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.
As with all high-rise buildings, design features to enable safe access and egress in times of emergency are of paramount importance. That is why stairwells and elevator shafts are usually constructed with concrete in most cases.
The risk factors associated with wood construction are tangible and can prove costly. Recent research has shown that course-of-construction insurance and post-construction insurance rates for wood buildings can be five to ten times higher than comparable buildings constructed from non-combustible materials.
Many other points can be made, both for and against the use of mass timber. 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 mainly due to intensive lobbying efforts on legislators. It is also worth noting that concrete still plays a vital role in such structures and adds elements of resiliency that wooden frames alone cannot provide.
Learn more in the articles on this subject on the Pacific Northwest Building Resilience Coalition website.
