Sustainability and the urban built environmentAcross the bustling cityscapes of Bangkok, Phnom Penh or Ho Chi Minh City, ubiquitous building cranes, scaffolding, drilling sounds and hard-helmeted construction workers are manifestations of the ongoing growth of cities across Southeast Asia. While a major driver of regional economic growth and employment, the sector has also seen dramatically increased levels of resource consumption and greenhouse gas emissions. Buildings now account for around 37% of global CO2 emissions (UNEP 2023). Considering the existing building stock and forecasted future floorspace demand in Southeast Asia, cities need to find more sustainable ways of constructing the human settlements of the future. Driven by institutions such as the World Green Building Council or the United Nations Environment Programme (UNEP), policy makers are increasingly acknowledging the pivotal role of the sector in achieving global sustainability goals and the Paris climate agreement. The built environment hence became one of the key themes of the recent COP28 in the United Arab Emirates where 28 states signed the Buildings Breakthrough Initiative. This initiative, operating under the auspices of UNEP and its Global ABC program (Global Alliance for Building & Construction), aims to normalize the construction of near-zero emission and resilient buildings.Across Southeast Asia the main sustainability concern within the sector has been on the proliferation of green building certifications and the construction of certified building projects. This includes global certification programmes such as LEED, developed by the US Green Building Council, EDGE, developed by the International Finance Corporation of the World Bank Group, or regional certifications such as BCA Green Mark (Singapore), Lotus (Vietnam), or TREES (Thailand). While these certifications have positively impacted sustainable practices in certain segments, they remain largely confined to specific markets, such as grade A offices, transnational firms' production sites, and luxury condominiums. Mainstream residential buildings, constituting the bulk of urban building stocks, remain uncertified, revealing a major gap in sustainable building practices. Many studies in the region on green building practices meanwhile exhibit a “certification-bias”, as they only analyse the application of such certifications and by associating “sustainability” in the building sector with these certifications. Many developments occurring below the certification level and that are part of the diversity of necessary approaches to transition the built environment sector towards sustainability are hence missed. Building Material Transitions To transcend certification-based transition processes, broader transformations are imperative. This includes the mainstreaming of bio-based, energy-efficient and circular building materials that might or might not be used in certified building projects. This is particularly relevant, as progress in building sustainability has so far foremost been in operational efficiency, but less so in terms of embodied carbon, i.e. primarily the production of building materials. Recognizing the pivotal role of building materials, a recent UNEP flagship report underscores that “[b]uilding materials are set to dominate climate change” (UNEP, 2023, p. 9). Transitioning the building material sector generally involves strategies to avoid or reduce the use of raw materials (build less, use of circular materials), to shift towards bio-based and earth-based materials where possible, and to improve conventional materials where shifting is not (yet) feasible. Such building material innovations can involve bio-based and earth-based materials such as rammed earth, boards made of ply bamboo, wood fibre, or agro-waste products like rice husks or rice straw. Key approaches are the reduction of required energy in the production process, for example through the use of non-fired products, or the reduction of resource quantities in the production process. Another avenue lies in increasing the share of by-products in building materials, for example such as fly ash, a waste product of coal-fired power stations. The case of Vietnam shows how a national policy that requires certain construction projects to use unfired bricks (regardless of certification) can influence the building material sector: The share of unburnt material production capacities increased from 5-8% of overall national capacities in 2010 to 30% in 2020 (Nguyen et al., 2022). Other commercially rather successful approaches include the increased use of autoclaved aerated concrete products, INSEE Ecocycle that uses waste to power cement kilns in Southeast Asia, or circular approaches like Holcim’s ECOCycle® that requires building materials to contain at least 10% of demolition waste. At the same time, start-ups like Cambodian “Eco-bricks” or “My Dream Home” are experimenting with non-fired bricks that consist of up to 30% plastic waste. Nevertheless, while local innovation take place at the grassroots level, many struggle to gain widespread adoption. Regional strategies, or how to materialise regional change?To understand these sustainable building material innovation and diffusion processes, it is crucial to study the regional preconditions for diverse transition pathways. Research projects like REBUMAT & CAMARSEC look into the transformative potential of different material innovations and their diffusion (dena 2023). Besides testing the physical characteristics of bio-based, resource efficient and circular material innovations and their appropriateness for local climatic contexts of Southeast Asia, the regional and material-specific innovation and diffusion conditions need to be understood. This knowledge can then be the baseline for tailored, regional building material transition strategies. To this end, the Rebumat team developed a three pillared approach that considers (a) regional material flows, (b) existing cooperation patterns, as well as (c) local market networks. The team is currently implementing a study that compares the regional conditions in Da Nang, Ho Chi Minh City and Hanoi.Following this approach, diverse actors such as national and sub-national governments, firms, unions and industry bodies can develop innovation policies that support a wider transition in the building sector, while at the same time driving regional economic development, green industrial paths and the creation of green jobs.Furthermore, these diverse actors can be brought together to form transformative, regional multi-stakeholder coalitions that can play a major role in driving just transitions in the sector (Jayaweera et al 2023). Convening innovators and frontrunners in protected spaces to co-create problem assessments, visions, and transition strategies is a promising means of empowering frontrunners, supporting innovations, and driving transformative change towards sustainability. As the clock ticks away and vast amounts of conventional, high-emission and resource-intensive building materials are daily incorporated into the fabric of Southeast Asian cities, stakeholders need to move quickly for a Building Breakthrough and better cities of the future.LiteratureDeutsche Energie-Agentur [dena] (2023). Renewable Building Materials in Vietnam: A study on sustainable construction material potential for the Vietnamese building sector, https://www.dena.de/fileadmin/dena/Publikationen/PDFs/2023/Studie_Renewable_Building_Materials_in_Vietnam.pdf.Frantzeskaki N, Castán Broto V, Coenen L, Loorbach D (eds.) (2017). Urban Sustainability Transitions. New York: Routledge.Nguyen, Xuan Hung, Nguyen, Khanh Linh, Nguyen, Thi Van Ha; Nguyen, Thi Thanh Huyen (2022). The Green Innovation of Construction Enterprises in Vietnam. In: Journal of Economics & Sustainable Development 13(8): 1–8, https://doi.org/10.7176/JESD/13-8-01.Jayaweera, R., Becker, A., Rohracher, H., Nop, S., Waibel, M. (2023). Urban Transition Interventions in the Global South: Creating empowering environments in disempowering contexts? In: Energy Research and Social Sciences (106), https://doi.org/10.1016/j.erss.2023.103312.United Nations Environment Programme [UNEP] (2022). 2022 Global Status Report for Buildings and Construction: Towards a Zero‑emission, Efficient and Resilient Buildings and Construction Sector. Nairobi. https://www.unep.org/resources/publication/2022-global-status-report-buildings-and-construction.United Nations Environment Programme [UNEP] (2023). Building Materials and the Climate: Constructing a New Future. Nairobi. https://www.unep.org/resources/report/building-materials-and-climate-constructing-new-future.