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 the use of CLT, instead of more “conventional” construction methods, is seen as a key solution to reducing embodied carbon. Wood is aesthetically pleasing, naturally produced, and carbon-sequestering, leading to a positive public perception of mass timber buildings. However, while it is true that trees do sequester carbon during the growth phase, the whole story can’t be told without understanding the other phases of the supply chain.

The wood used in mass timber buildings is 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 also 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 an agreement on how best to quantify these impacts. Additionally, shipping wood products far from their source may negate the benefits of using this product over concrete or steel.

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. Optimized harvesting will 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 an 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 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.