Strategies for Applying the Circular Economy to Light Timber Framing

The building and construction industry is the world’s largest consumer of raw virgin materials while also being the largest producer of solid waste. The implementation of a circular economy in this sector represents an opportunity to mitigate the negative environmental impacts associated with such levels of consumption and waste creation. The circular economy is ideally suited to addressing the construction sector’s sustainability issues as it deals with consumption and waste issues concurrently. Although the benefits of circularity are clear, the implementation of relevant building methods embodying circular economy ideals is difficult. Economic pressures, regulated conventional building practices, buildings’ life expectancy and architectural design requirements all limit the adoption of circular building solutions. It is the ambition of this research to propose, implement and evaluate viable circular building systems to overcome existing barriers. Several full-scale built prototypes formed the core of this research. Prototypes were developed to purposely interact with real-world factors that would conventionally limit or restrict the efficacy of circular economy solutions. Research actions centred on finding solutions to these issues that balanced circularity performance against economic and compliance factors. Through prototyping iterations, and in line with circular theory, it was confirmed that the type of relationship formed between different building layers is fundamental to achieving built circularity. It was found that the type of connection between adjacent layers not only dictated circularity performance but also informed economic and compliance performance characteristics. Experimentation validated that conventional building layer dependency could be eliminated by introducing spatially defined mechanical multi-use fixing technologies. Layer relationships through the building’s external wall, and in particular the external weather, moisture and air control layers of that external wall, proved the most difficult to resolve from a viable circular standpoint. Gasket-based junction details have been identified and validated as the most effective approach for achieving viable circularity in the weather control layers of timber-framed wall systems. The relationship between the geometric conditions of the building system and geometric requirements established by the building’s designer was explored with respect to the associated circular performance. Standardisation of the building system’s components’ sizes to increase recovery and reuse utility value underpinned the proposed circular building methods. The effectiveness of such standardisation from the perspectives of energy, effort, carbon, economic and reuse performance was substantiated. The exact specification of that standardised module was challenged through multiple project-specific installations. Adherence to the established building industry module of 0.6m (2 ft) proved most successful from both a material efficiency and planning standpoint. Centreline grid planning emerged alongside this module as a suitable approach for circular design providing components could self-intersect without requiring additional members. Tartan (or off-set) grid planning superseded this approach in later prototypes however as it offered greater standardisation of lining elements (both externally and internally) and improved material utilisation. Multiple cycles of design, prototyping and evaluation identified a systems-based construction approach highly conducive to facilitating viable material circularity. A standardised self-braced structural frame made from engineered timber and capable of both vertical and horizontal modularisation formed the basis of this system. The modular frame natively facilitated reuse across three scales (parts, panels and sections) and supported the integration of custom members that did not compromise long-term reuse flexibility. Integral to the frame's functionality was a pre-defined scalable fixing interface to support the reversible connection of cladding, air control, thermal control and internal lining layers. Through-wall modularity was established as a way of increasing reuse utility value and the sizes of adjoining elements were adjusted to match the base module. An engineered timber rigid air barrier with gasket-sealed seams working in conjunction with a bolt-fixed cavity batten was identified as the most circular exterior air control build-up. Economic pressures suggested that the first circular feature to be substituted in the build-up would be this gasket-based air-seal. Throughout the prototyping process the methodology for evaluating the viability and effectiveness of circular building solutions matured. At the outset of prototyping processes, it was identified that there was no clear existing best practice method to evaluate the relative circular performance of one construction approach over another. In response to this constraint, the circular assessment methodology adopted was based on an index of multiple well-regarded circular assessment frameworks. Over the course of the prototyping activities, a category-based assessment approach evolved that better captured the true circular performance of a given construction approach. The final circular assessment framework split the analysis by functional building layer (frame, cladding, cavity, air control, thermal control, interior finish) and clustered evaluations around material performance, fixing design and modular conditions. The resulting assessment framework is specifically devised to support designers in identifying circular weaknesses in a construction system.