Timber and Carbon
In recent years there has been a great deal of excitement about the advent of new timber construction methods. These methods, referred to collectively as “mass timber,” often involve laminating small pieces of dimensional lumber into large timber slabs, most typically into cross-laminated timber (CLT). The emergence of this technology is coupled with building code revisions that permit timber construction for larger buildings. Often times the use of CLT, instead of other more “conventional” construction methods is seen as a key solution to reducing embodied carbon. Wood is an aesthetically pleasing material which, combined with it being a “green,” naturally-produced, carbon-sequestering material, leads to good public perception of these buildings. However, while it is true that trees sequester carbon during the growth phase, there are many other phases of the supply chain that must be understood.
Wood used in mass timber buildings is presently harvested in select regions of the country. These regions are in areas with high concentrations of affected resources, ranging from waterways to forest wildlife. An environmental product declaration (EPD) must consider not only the carbon sequestration of the wood, but the impacts that come with harvesting, milling, and shipping this product. Not all forests, forestry techniques and manufacturing processes are identical; thus, it is difficult to come to consensus agreement on how best to quantify these impacts. Additionally, shipping wood products far from their source may actually negate the benefits of using this product over concrete or steel.
Significant pushes have been made for sustainable forestry practices which include harvesting at optimal frequencies, thinning forests in a manner that supports longevity, minimizing impacts to waterways and wildlife habitats, and minimizing the use of harmful chemicals. A tree’s growth (and carbon sequestration) follows a sigmoid curve; accordingly, there is an ideal time at which to harvest a specific species of tree in a specific location. This decision can be optimized to provide the best ratio of material benefit versus environmental impact.
The process of harvesting trees and converting them to functional building materials is not overly complex, but varies widely amongst companies and across different geographies.
As life-cycle assessments (LCAs) are performed involving these products, the user should consider the comprehensiveness of the inputs defining this product’s environmental impact. More than 50% of a tree is lost as waste during harvesting and much of this is left behind in the forest, where it releases its sequestered CO2. Are the environmental impacts of the waste properly considered? When both primary timber pieces and waste products are converted to building materials, are the impacts of miscellaneous materials such as glues and fasteners properly accounted for? The accuracy of an LCA involving these products correlates directly with the accuracy of the inputs in the product-specific EPD.
In many locales, a wood building is, in fact, the most ideal solution from an embodied carbon perspective. However, all mass timber buildings still utilize concrete and steel in some capacity. Additionally, a mass timber building in location A is not the same building (carbon-wise) as it would be in location B. In fact, if not thoroughly evaluated, a wood building might actually be a less ideal solution than an optimized steel or concrete building. The ideal approach is to be as efficient as possible with each building material rather than forcing a wood building solution that, on the whole, may have more embodied carbon than a concrete or steel solution.