MASS TIMBER END-OF-LIFE SCENARIOS

Best Practices In Designing for Reuse

Allison Peitz 2022-2023

How should end-of-life scenarios for mass timber influence the way we design buildings?

Abstract

Mass timber construction has been recognized as a significant tool for combating climate change and supporting the global push for circular economies by creating building stock that also serves as a carbon sink. Beyond these initial benefits, as an inherently lightweight and modular system, mass timber also has high potential for reuse to extend these carbon benefits. This research paper catalogs examples of wood and mass timber reuse in historic civilizations to inform best practices for designing mass timber structures to maximize reuse potential. Additionally, this paper considers methods for accounting for biogenic carbon as part of a Life Cycle Assessment (LCA) and provides recommendations for supplementary tools that can be used to account for reuse scenarios.

Keywords

Mass Timber, End-of-Life Scenarios, Cradle-toCradle, Circular Economy, Life Cycle Assessment, Tally, Dynamic Carbon Accounting, Biogenic Carbon, Carbon Storage

Introduction

Mass timber has a critical role to play in addressing climate change and meeting goals to avoid critical tipping points. This method of construction has been called, “The quintessential carbon negative technology for climate change mitigation,” noting that the material’s longevity in use drives the magnitude of this impact (Lippke et al., 2021). The term ‘Carbon’ is often used as shorthand for global warming potential (GWP) and is measured in kilograms of carbon dioxide equivalent (kgCO2eq). Mass timber provides opportunities for climate change mitigation because wood can extract carbon from the atmosphere as it grows and retains this carbon, termed biogenic carbon, within its fibers throughout its lifespan. This ability to store carbon counteracts the typical flows of emissions into the atmosphere to delay the impacts of global warming. These beneficial impacts are optimized if mass timber elements can be reused or re-purposed, therefore retaining carbon for a longer period.

In 2021, the Intergovernmental Panel on Climate Change (IPCC) warned that a 45% reduction in carbon emissions is needed by 2030, followed by a 100% reduction in emission by 2050, to avoid an increase in global warming that will exceed 1.5° C (IPCC, Summary for Policymakers). The building sector alone accounts for 39% of energy and process-related carbon dioxide emissions globally (Toth et al., 2021). The energy that a building consumes while operating is referred to as operational carbon (Lewis et al., 2023). Buildings have become increasingly efficient over time as technology has become more advanced, reducing operational carbon impacts. Efforts have since turned to reducing building’s embodied carbon, or the emissions associated with the manufacturing, transportation, installation, maintenance, and disposal of construction materials used to produce a building (Lewis et al., 2023).

Within the building and construction sector, the most effective ways to minimize carbon emissions are through adaptive reuse or the deconstruction and reuse of building components, versus recycling (Lippke et al., 2021). This is especially true when it comes to mass timber construction. Mass timber, due to its modularity, lightweight, and durability, has excellent reuse potential.

Every new building constructed with mass timber has potential to serve as a carbon sink, delaying the release of CO2 into the atmosphere. This temporary carbon storage is sequestered for the life of the product, potentially through the lives of multiple buildings. Planning for deconstruction may seem like a concept for the distant future. However, given that the average commercial building has a life expectancy of 50-100 years, the way we choose to design mass timber buildings today will facilitate or impede our ability to reuse, deconstruct, or recycle buildings and their components for the next 50-100+ years (O’Connor, 2014).

This paper is a literature review that compiles relevant research and case studies to answer the question:

How should end-of-life scenarios for mass timber influence the way we design buildings?

This paper will provide an overview of international efforts promoting timber circular economies, reinforcing the need for clear guidelines that can be applied to promote adaptive reuse and deconstruction for reuse in mass timber buildings and of mass timber products. Second, the paper will dive into the history of wood and mass timber reuse to demonstrate the potential longevity and durability of wood and mass timber products. Third, Life Cycle Assessment (LCA) calculation methods and challenges will be discussed as they pertain to end-of-life scenarios for mass timber and biogenic carbon accounting. Finally, this paper compiles design recommendations that can serve as a reference for design professionals, to promote adaptive reuse and deconstruction for reuse in mass timber buildings. These recommendations will cover structural topics, including grid layouts and connection design, moisture management strategies, systems distribution approaches, and documentation.

Definitions And Abbreviations

Mass Timber: Engineered wood products including glue-laminated timber, cross laminated timber (CLT), and dowel laminated timber (DLT)

Biogenic Carbon: Carbon produced in natural processes by living organisms but not fossilized or derived from geological fossil resources. In the building industry, this most commonly occurs in wood products (The Carbon Leadership Forum, 2019).

Operational Carbon: Emissions generated by fuel consumption for heating/cooling, supplying, fresh water, ventilation, and power over the course of a building’s lifetime (Lewis et al., 2023).

Embodied Carbon: Emissions generated by the manufacturing, transportation, installation, maintenance, and disposal of construction materials used in buildings, roads, and other infrastructure (Lewis et al., 2023).

Carbon Sink: Anything that absorbs more carbon from the atmosphere than it releases through carbon sequestration (Churkina et al., 2020).

Carbon Storage: The sequestration of carbon in products for a period, resulting in a (temporary) reduction of the CO2 concentration in the atmosphere

CIRCULARITY: A GROWING INTERNATIONAL PRIORITY

To meet carbon emission reduction goals put forth by the IPCC, it is important to challenge the traditional linear model of consumption -- where products are made, used, and disposed of – and instead pursue a circular economy approach – where materials are designed to be reused, repaired over time, and/or recycled.

A key concept behind circular economies is that materials should be upcycled: reuse that maintains or increases the previous value of the material (Heinrich & Lang, 2019). For mass timber, this means that structural elements will be reused as structural components. This contrasts with downcycling: reuse or recycling that reduces the value of the material (Heinrich & Lang, 2019). For mass timber, or any other type of wood, downcycling typically involves reuse in furniture or as wall/ceiling finishes, processing for use in wood composite materials, or mulching (Love, 2007). While both upcycling and downcycling avoid raw material inputs, upcycling is strongly preferred because it requires less energy to process and requires a smaller embodied carbon input.

Cascading is a process that acknowledges that maintaining a material at its highest and best use is not always possible as material ages, and instead promotes the use of the waste material in its highest value alternative for as long as possible (Holgmeier et al., 2013).

As of 2021, only 8.6% of our world is circular (The Circularity Gap Report, 2022). Within the building sector, only 20-30% of construction and demolition waste is currently reused or recycled, leaving the remaining 70-80% to be landfilled (Building a world free from waste and pollution, 2021; Rypkema et al., 2021).

To challenge this trend, deconstruction programs have been implemented in several cities across the nation focusing on single-family homes. These programs, still in their infancy, have established flourishing wood salvage materials markets. Wood is the single most valuable salvage material yielded from the deconstruction process in older homes and according to David Greenhill, a deconstructor in Portland, “Selling salvaged wood is the only way to actually make money in this business (Allison Peitz, personal communication, April 13, 2022).”