**1. Introduction**

*1.1. Carbon Emissions and Buildings in High-Rise, High-Density Cities*

Buildings significantly contribute to global climate change. As a worldwide average, buildings consume 35–40% of the annual energy produced and emit 30–40% of annual carbon emissions [1,2] Ahmed et al. [3] emphasised that embodied carbon emissions of buildings account for about 11%, while operational carbon emissions from buildings account for about 28% of global carbon emissions annually. Accordingly, 'building stock' can be identified as an important focal point for initiating energy management and carbon emission reduction programmes [4].

Highly urbanised countries/regions with high-rise, high-density cities should implement effective policies and procedures to reduce energy consumption and carbon emissions from buildings since these highly dense cities consume about 80% of energy and emit about 75% of carbon in the country/region [5,6]. Nevertheless, more cities will grow vertically and become highly dense in future due to the rising building demand and migration of the population to urban areas [7]. UN DESA [8] predicted that about 68% of the global population will live in cities and urban areas by 2050.

**Citation:** Kumaraswamy, M.M.; Hewa Welege, N.M.; Pan, W. Accelerating the Delivery of Low-Carbon Buildings by Addressing Common Constraints: Perspectives from High-Rise, High-Density Cities. *Buildings* **2023**, *13*, 1455. https://doi.org/10.3390/ buildings13061455

Academic Editors: Simon P. Philbin, Yongjian Ke and Jingxiao Zhang

Received: 20 March 2023 Revised: 24 May 2023 Accepted: 26 May 2023 Published: 2 June 2023

**Copyright:** © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

Having identified the environmental threat from building stock due to the high energy consumption and carbon emissions, a majority of the countries with high-rise, high-density cities are continuously attempting to reduce the carbon emissions from their building stock [9,10].

Taking a few examples, Hong Kong initiated a 'climate action plan 2030+' to achieve the goals of the 'Paris Agreement' [11]. The UK government has initiated plans to achieve an 80% reduction in carbon emissions by 2050 compared to the emission levels in 1990 [12]. In Australia, several states have initiated plans to achieve the certifications of the Commercial Building Disclosure (CBD) programme and the National Australian Building Energy Rating System (NABRES) [13]. The 'national climate change plan of the UAE 2017–2050' also includes targets for reducing emissions from buildings [14]. 'Singapore green building masterplan' [15] targets to increase the percentage of green buildings up to 80% by 2030.

Despite the above initiatives, notable carbon emissions reduction from the buildings sector is not evident [16]. Moreover, about three billion m<sup>2</sup> of building floor area is being constructed yearly without complying with energy/carbon policies and procedures [17]. Indeed, databases clearly indicate, and scholars further highlight, how the rapid increase in carbon emissions and energy consumption from buildings in highly dense regions constitute a significant threat to the environment. This should raise a clarion call for the immediate attention of responsible stakeholders [10,16,18]. This situation demonstrates that the evolution of building carbon and energy reduction programmes is not keeping pace with the rapid growth in high-rise, high-density cities.

#### *1.2. Low-Carbon Buildings (LCBs)*

With increased rates of urbanisation, 'environmental sustainability' aspects are frequently incorporated into building construction, operations, and management activities throughout the world [19,20]. Accordingly, many scholars have emphasised that delivering LCBs is one of the highly supportive strategies to ensure the sustainability of buildings and the built environment [9,21].

LCBs are buildings which are specifically engineered with GHG reduction in mind. By definition, an LCB is a building which emits significantly less GHG than regular buildings [22]. An LCB typically includes energy-efficient features. However, a building with energy-efficient features does not necessarily mean that the building is an LCB [23]. The low-carbon concept covers a wide spectrum that transcends being just energy efficient. Accordingly, building orientation, structure, building envelope materials, window size, location and glazing, the efficiency of heating, ventilation and air conditioning (HVAC) systems, usage of materials with minimum GHG emissions while production, usage of onsite renewable energy, emission reduction in building systems, adapting low-carbon behaviour, reusing or recycling at the end of the lifecycle, compliance, minimising overall carbon footprint, etc. contribute to delivering an LCB.

To align with global carbon reduction efforts, all countries should implement efficient programmes to deliver LCBs. Further, yearly low-carbon intensive new building construction should improve up to 4 billion m2 of floor area (the current level is around 250 million m2 per year). Moreover, at least a 30–50% improvement in energy performance should be achieved through effective policies and technology adoption when renovating existing buildings [17].

Pan and Pan [9] emphasised the importance of delivering LCBs in high-rise, highdensity cities/regions in order to minimise the carbon emissions from the building stock. Various action plans are being proposed and implemented for carbon emission reduction from buildings in most of the high-rise, high-density cities throughout the world, especially in countries with stable/developed economies [24]. However, these attempts could not achieve a noteworthy reduction in carbon emissions /energy usage in these cities [16,25]. This may arise from a number of constraints (related to policies and regulations, technologies, social conditions, geographical conditions, and financial status) that hinder the delivery of LCBs [26,27].

#### *1.3. Constraints to Delivering LCBs and Strategies to Accelerate the Delivery of LCBs*

Figure 1 shows a summary of constraints reported in the previous literature between 2011 and 2020. Accordingly, it is evident through the blue coloured lines that more studies reported the constraints related to financial level, policy/regulatory level, technology level, and knowledge. The number of reported constraints is also high for these categories.

**Figure 1.** Constraint categories.

Many of these constraints show interdependencies and interrelationships among each other [28]. A clear identification and analysis of these interdependencies among constraints would be beneficial for relevant stakeholders to determine effective strategies to overcome the constraints [29]. Accordingly, developing strategies to address the driving constraints (significant constraints which generate other constraints) should be given a higher priority. Addressing the driving constraints upfront would ease the efforts in mitigating the dependent constraints [10,29].

Government involvement and policy intervention are regarded as essential and effective ways to promote building energy efficiency and low-carbon measures by overcoming common constraints [30–32]. Formulating policies for developing standards and designs of building materials as well as buildings, is crucial in accelerating the adoption of low energy/carbon buildings [31,33]. Design criteria should be duly supported by innovative and creative research methodologies and advanced technological support [33]. All future designs of buildings should ideally include the use of sustainable and energyefficient materials. The development of such sustainable, eco-friendly, and energy-efficient building materials should be supported and promoted by the government, related institutions and departments [34,35]. Furthermore, effective collaboration among the industry, regulatory bodies, and academia should be maintained to facilitate the customised and effective research and development of feasible technological advancements [36]. Strict regulations should also be imposed on energy and carbon compliance documentation and

reporting [34,37]. Furthermore, market-based incentive schemes are thought to be effective and cost-efficient instruments to support and enhance the low carbon and energy-efficient building investments [38,39].

Furthermore, carbon emissions and energy usage should be assessed throughout the life cycle of buildings [40]. Life cycle assessment should be supported by providing suitable calculation tools, technology, expertise, and training [33,39,41]. Relevant institutions, including government departments, should promote the new energy-efficient and lowcarbon building development by setting examples by implementing pilot projects with measurable benefits [33]. Awareness raising, training, and skill development of the building professionals and the community, in general, should be considered as a long-term strategic approach to drive towards a sustainable built environment [42,43].

#### *1.4. Research Gap and the Aim of the Present Study*

Although many previous studies have identified the constraints to delivering LCBs from different dimensions and explored the strategies to accelerate the delivery of LCBs, a detailed exploration of strategies to address common constraints to delivering LCBs in high-rise, high-density cities could not be found in the previous literature. Although the constraints to delivering LCBs show complex interdependencies, none of the previous studies attempted to analyse these interdependencies and explore innovative methods to accelerate the delivery of LCBs by synergising with these interdependencies. Moreover, less attention has been paid in the literature to identifying and exploring the constraints and strategies for delivering LCBs by focusing on high-rise, high-density cities.

Therefore, this study aimed to identify the potential strategies to accelerate the delivery of LCBs by addressing the common and significant constraints to delivering LCBs in high-rise, high-density cities. The list of common and significant constraints to delivering LCBs in high-rise, high-density cities was adopted from a precursor study by the authors of this paper [29]. This precursor study explored the common constraints to Hong Kong, Singapore, Australia (Sydney and Melbourne), UAE (Dubai and Abu Dhabi), and Qatar (Doha). The present study summarised the findings of Madhusanka et al. [29] and identified suitable strategies to accelerate the delivery of LCBs by addressing the constraints. Moreover, the present study mapped and analysed the connections between constraints and strategies using an Interpretive Structural Modelling (ISM) based structure and a Social Network Analysis (SNA) based on a two-mode network. These analyses provided a clear view of the significance, centrality, and interdependencies of the identified strategies. Furthermore, the necessary involvement and collaboration of different stakeholders in implementing the identified strategies are also identified and discussed in the present study.

#### **2. Methods**

This section discusses the research methods chosen to suit this study. Accordingly, methods followed to identify and analyse the constraints, methods of identifying strategies and mapping with constraints, and the SNA approach utilised in this study are elaborated in Figure 2 and the sub-sections below. Some inputs to the present study were adopted from the precursor studies of the same authors of this paper. In Figure 2, the specific tasks covered in the present study are indicated by a dotted line.

#### *2.1. Methods Used for Identifying and Analysing the Constraints*

The list of common and significant constraints to delivering LCBs in high-rise, highdensity cities and the interdependencies among these constraints were published in precursor studies by the authors of this paper [10,29]. The present study adopted a summary of these precursor studies and explored suitable strategies to accelerate the delivery of LCBs by addressing the constraints identified in the precursor studies. Madhusanka et al. [10] and Madhusanka et al. [29] identified the common and significant constraints to delivering LCBs from the perspectives of five high-rise, high-density regions through a comprehensive literature review followed by a questionnaire survey. Subsequently, the interdependencies among the identified constraints were analysed by using ISM and Matriced' Impacts Croise's Multiplication Appliquee a UN Classement (MICMAC) analysis approaches.

**Figure 2.** Research methods.

ISM approach is used to systematically reveal the relational links and interdependencies among a set of factors by placing the considered factors in a hierarchical model [44]. MICMAC analysis is used to categorise and prioritise a set of factors based on their driving/dependence nature [45]. Accordingly, these factors ('constraints' in the present study) can be categorised into four groups [46]: (1) autonomous barriers (weak driving power and weak dependence); (2) dependent barriers (weak driving potential but strong dependence); (3) linkage barriers (strong driving power and dependence); and (4) independent barriers (strong driving power but weak dependence).
