Prefab Light Clay-Timber Elements for Net Zero Whole-Life Carbon Buildings
Prefab Light Clay-Timber Elements for Net Zero Whole-Life Carbon Buildings

Summary/Abstract: “Net zero whole life carbon” is an ambitious climate target that refers to neutralizing and offsetting the entire LCA-based carbon footprint of a building, including both operational and embodied greenhouse gas emissions. Especially in the Northern climate, viable building envelope structures must, therefore, provide good thermal insulation and low embodied emissions. Carbon offset is typically based on excess on-site renewable energy or purchased carbon offsets disconnected from the building and the site. Viable strategies for carbon neutrality start by minimizing material-related and energy-related CO2e emissions. As a result, new kinds of building envelope structures have been recently introduced in the academic literature and experimental building projects. Traditional construction materials, such as timber and clay, have been sourced locally and processed manually, providing good results for the embodied emissions in life cycle assessment. Recent studies on clay-based construction materials have concluded that more research on clay as a construction material is needed, in particular considering its environmental performance. One specific concern in the Northern climate is that the weather conditions limit clay construction outdoors and prevent industrial-scale application of these solutions. The methods of prefabrication can address these issues. This study introduces the critical technical and environmental properties of a new prefabricated wall element based on a combination of light timber frame and light clay. In a hybrid light clay-timber structure, a mixture of clay and hemp shives is cast between the timber studs. On the one hand, the novelty of this wall structure is the prefabrication that enables industrial applications and upscaling without the limitations of weather conditions. On the other hand, the study assesses the climate impact of a light clay-timber wall element : cradle-to-gate emissions, thermal insulation, and the climate benefits outside the system boundary (carbon handprint) reported in the D-module of the LCA framework. The study also shows that natural materials require a different approach than synthetic materials from industrial processes. There may be variations in the properties of hemp and clay, especially when local sourcing is prioritized for better environmental performance. Moreover, the mixing and installation processes have a significant impact on the final properties and the performance. We show that constructing a light clay wall is a knowledge-intensive process that may result in very different technical properties. The authors argue that the case study demonstrates a paradigm shift in developing building envelope solutions. The future low-carbon building solutions are knowledge-intensive but prioritize local, natural materials and minimize the processing of these materials. The quality control could be based on the grading of natural raw materials, similar to the grading of sawn timber. In light of the results, we discuss the interpretation of the net zero targets and the viability of accounting for the benefits beyond the system boundary, reported in the LCA framework as module D impacts (also referred to as “carbon handprint”) as a carbon offset. The best specimen of prefabricated light clay-timber elements shows low carbon footprint and good thermal insulation while providing a superb carbon handprint.

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