MASS(ive) Timber

MASS(ive) Timber

By Aurora Jensen (MDes ’19); advised by Jonathan Grinham

In the face of climate change, designers are uniquely positioned to reduce operational loads of buildings through passive means, and reduce embodied impacts of buildings through material selections. Unfortunately, designers often compromise, selecting materials with high embodied impacts in order to improve operational performance. Mass timber products, like cross-laminated timber (CLT), offer a low embodied carbon alternative to concrete, but little research examines the operational performance of mass timber buildings. Passive operational strategies, like natural ventilation and thermal mass, have the potential to diminish cooling loads and delay investments in air-conditioning in places that are growing warmer and wealthier.[1] Internal thermal mass, in particular, refers to the ability of a material to store heat and thereby dampen extreme temperature fluctuations in an interior environment.

[1] Hacker, J. N., De Saulles, T. P., Minson, A. J., & Holmes, M. J. (2008). Embodied and operational carbon dioxide emissions from housing: A case study on the effects of thermal mass and climate change. Energy and Buildings, 40(3), 375–384. https://doi.org/10.1016/j.enbuild.2007.03.005

Figure 1 - Claims of massiveness. This graphic collages the various claims across the industry that timber is thermally massive. Oddly these claims do not refer to prior substantive literature as support.

Numerous claims across the building industry – by engineers, architects, manufacturers, and researchers – refer to mass timber products as thermally massive, however this claim has not been quantified in the literature. Taking the form of a comparative study of construction types, my thesis addresses the following interrelated questions: Is there a difference in operational energy use between lightweight timber, concrete, and mass timber construction systems? How do climate and control logic (mixed-mode v. conditioned) affect this?  What role does thermal mass play in shaving peak temperatures and delaying investment in air-conditioning? What is the lifecycle carbon tradeoff between these construction systems?

Figure 2 - Abstract schematic. This graphic visually describes the system of relationships that guided the development of the thesis. The study intervenes into this system by introducing various construction types on the left and traces how this intervention propagates through this system by engaging three types of analyses: transient dynamics, operational energy, and lifecycle GHG emissions.

My thesis offers three novel contributions. First, the thesis demonstrates a method of studying thermally massive behavior across three temporal scales: daily, annual and lifecycle through its quantification of the daily decrement, annual overheating hours/energy use, and lifecycle carbon to provide insight into the behavior of these construction types. Second, this work provides comparative and quantitative insight of the extent to which mass timber is thermally massive, in comparison to known construction types. Third, this study’s integration of embodied and operational GHG emissions into a cumulative timeline, provides critical insight into the impacts of an emerging practice – mass timber construction.

Figure 3 - Comparative Calibration. This work sought to approximately quantify the relative variation in massive behavior between the construction systems within the study.