*Perspective Chapter: Promoting Circular Design Strategies in Housing Delivery in Nigeria DOI: http://dx.doi.org/10.5772/intechopen.110656*

In a linear economy, goods are designed for a single lifetime and disposed of at the end of their useful life (cradle-to-death). In contrast, a circular economy aims to eliminate the concept of waste altogether through continuous use of materials. The idea is that the material from the end of one product's life cycle acts as the input for another product's life cycle (cradle-to-cradle). As a result, demand pressure for virgin material is greatly reduced, thus leading to resource optimization. CE slows down the depletion of natural resources and reduces environmental damage resulting from material extraction and processing of virgin materials [18].

Circular design is at the service of a circular economy. Design is the basis of all innovations in products, services, and systems. Circular design is the application of circular economy principles at the design stage of any product, service, or system. It has been estimated that about 80% of a product's environmental impact is determined by the design process [19]. Hence, designing products for reuse can reduce materials and environmental costs. In addition, it has been estimated that over 70% of a product's life cycle costs and environmental footprint are determined at the design stage [20].

Circular design is of particular interest to the built environment. This is so because the built environment is known to be a heavy consumer of resources and energy, as well as a heavy emitter of carbon dioxide and the attendant consequences to the environment. Specifically, the built environment consumes about 50 percent of global materials resources, 50 percent of energy resources, 40 percent of global water use, and 60 percent of prime land, as well as 70 percent of global timber products [21]. More recent estimates put resource use and waste generation by the built environment at about 40% [22]. In addition, the United Nations Environment Programme [23] estimated that the building and construction sector accounted for 35% of global energy use and 38% of all energy-related carbon dioxide emissions in the year 2019. Hence, the built environment needs strategies to reduce its environmental impact in terms of resource utilization, energy use, and carbon dioxide emissions.

Sustainability has been adopted as a preferred development paradigm to ensure efficient resource use while minimizing waste generation in the built environment. In addition, the metrics deployed in the assessment of sustainability have also evolved and can be grouped into three main categories as enunciated by Forsberg and Von-Malmborg [24]. As a result of these metrics, sustainable buildings have evolved, resulting in the reduction of the environmental impact of buildings and the built environment as a whole. However, sustainable buildings have limitations in the sense that they are based on the linear model, which follows the life cycle path of design, construction, use and disposal [25].

Similarly, uptake of circular buildings can be facilitated through adoption of appropriate metrics. In broad terms, Attia and Al-Obaidy [26] identified four primary criteria for assessing the circularity of buildings namely: carbon footprint of building materials used, reused content of the building materials, disassembly potential and longevity of the building, and building design flexibility and longtime use.

Circularity metrics can be applied at micro-, meso-, and macro-levels. At the micro- or product level, one of the popular metrics for measuring circularity is the material circularity indicator (MCI) as articulated by the Ellen MacArthur Foundation [27]. Other indicators include Material Efficiency Metric, Circular Economy Indicator Prototype, and Circularity Potential Indicator [28]. Meanwhile, Drager et al. [29] referred to six circularity metrics aimed at actualizing the major objectives of circular economy as enunciated by the European Environment Agency. These objectives of circularity metrics were further summarized into three categories namely: protection of materials stock, protection of the environment, and value retention [30].

One thing that is clear with respect to circularity metrics is the plethora of methods available. It has been observed that this multiplicity of metrics can be conflicting and even confusing, and may sometimes lack clarity [31]. In response to the foregoing, the World Business Council for Sustainable Development (WBCSD) developed a comprehensive indicator-based metric for measuring all aspects of circular economy [32, 33]. The WBCSD framework also referred to as circular transition indicators (CTI) comprises a suite of indicators grouped into three broad categories. The first category (close the loop) measures the effectiveness of closing the material loop, while the second category (optimize the loop) demonstrates how material recovery strategies are optimized. The third category (value the loop) demonstrates the business value derivable from applying circular strategies. The CTI framework aligns very well with the major principles of circular economy as enunciated by EMF as follows: design out waste and pollution, keep products and materials in use, and regenerate natural systems.

Given the differences between linear and circular approaches, it would appear that the metrics are parallel. However, it has been shown that LCA, which is the most scientific metric for linear systems, has some usefulness in circularity metrics. Brandstrom and Saidani [28] indicated that material-based circularity metrics align very well with LCA measures in some specific instances. Also, Saade et al. [34] underscored the complementary roles of LCA and circularity indicators in measuring sustainability, especially in relation to early design of urban projects. Similarly, Weidemann et al. [35] demonstrated the complementary roles of LCA and circularity indicators in measuring sustainability, especially in an industrial production context. Realizing that the closed-loop concept of CE does not always ensure environmental benefits, Mannan and Al-Ghamdi [36] demonstrated that LCA can be beneficial for assessing CE options in product design. Very importantly, Van Stijn et al. [37] proposed and successfully tested an LCA-based CE model for the assessment of circular building products.
